feat: faster startup by lazy-loading main window panels on idle or first use (#15811)

docs/HLSD/deferred-page-construction.md

@@ -0,0 +1,220 @@
+# Deferred Page Construction: High Level Design
+
+## Why it exists
+
+The main window is a notebook of tabs. When the first frame appears, one tab is on
+screen and the others are not; some of them are opened later in the session, some
+never, and some only exist for certain printers. Building a tab before the first frame
+adds its cost to every startup, whether or not the tab is used.
+
+This subsystem builds a tab's panel the first time the tab is shown. It also builds the
+remaining tabs while the user is idle after startup, in units of tens of milliseconds,
+so a click that lands in the middle of one waits for that unit and no longer. Startup
+pays only for what the first frame shows, and the other tabs are usually built before
+anyone opens them.
+
+## The parts
+
+`src/slic3r/GUI/Lazy.hpp`, `LazyPage.hpp`, `StagedBuild.hpp` and `IdleScheduler.hpp/.cpp`
+(with its wx-free `PrebuildQueue.hpp`) are independent. A holder can hold anything a
+factory makes, a page is a holder with a placeholder widget, a staged object can live
+outside a holder, and the scheduler knows none of them; it runs tasks, which `MainFrame`
+makes from the holders.
+
+### Lazy<T>: the holder
+
+Holds an object that a factory makes on first use, under a name for the log and a place
+in the idle queue, both given by the owner. The header has no wx dependency; the busy
+cursor for an on-demand build lives in `Lazy.cpp` and is skipped when there is no app,
+so the holder is unit-tested.
+
+- `get()` is null until the object is completely built; `built()` says the same.
+- `ensure()` builds whatever is left now, under a busy cursor, and returns the object, or
+  null in the two cases below. A click on an unbuilt tab or a first open of a dialog goes
+  through this, and it logs the units and time it took.
+- `build_step()` runs one unit of construction and returns true while more remain. The
+  first unit is the factory call, and each later unit is one `StagedBuild` step if the
+  type has them.
+- `when_built(fn)` runs `fn(object)` now if it exists, otherwise once it is built.
+- `pending()` says whether the idle prebuild has work here: not built, and the factory
+  has not returned null. A null factory result is logged and the holder stays unbuilt.
+- A unit that pumps the event loop cannot re-enter the holder; a nested `build_step()`
+  does nothing and a nested `ensure()` returns null.
+- After a unit throws, the holder and the scheduler still run the next one.
+
+The holder does not own the object; its wx parent does, as for any window. A type with
+one instance in the app derives from `LazyInstance<T>`, which points at that instance's
+holder (the holder registers itself, and a recreated `MainFrame`'s holder replaces the
+old frame's) and gives the type the static entry points the rest of the app uses,
+`T::if_built()`, `T::ensure()` and `T::when_built(fn)`. They return null, or do
+nothing, while no holder exists, so a caller needs no `mainframe` check.
+
+`built()` is an atomic flag, since a job worker reads it through the statics
+(`MainFrame::get_calibration_curr_tab()`); it is set after the object is complete.
+
+`LazyBase` is the holder's type-free interface (`name()`, `built()`, `pending()`,
+`build_step()`, `prebuild_order()`) and is what the scheduler side sees.
+
+### LazyPage<Panel>: the placeholder
+
+A `wxPanel` placed in the parent in place of the real panel, and a `Lazy<Panel>` whose
+factory makes the panel inside it (by default `new Panel(parent)`). The notebook needs a
+page object for the tab to exist and for `show_device()` to insert and remove tabs by
+pointer, and the placeholder is that object. `MainFrame` creates every page once, named
+after its `TAB_ID_*`, and keeps it for the frame's life; `show_device()` only moves
+pages in and out of the book.
+
+- `Show()` is forwarded to the panel, so a panel's own `Show()` override stays its
+  activation hook (refresh timers, machine sync) and `SelectPageByName()` works
+  unchanged. A show builds the panel unless the frame itself is still hidden, because
+  the book selects its first page as it is inserted; `MainFrame::Show()` shows the
+  current page again when the frame becomes visible, which builds it.
+- `in_book()` says whether the parent notebook currently lists the page, and
+  `pending()` is that and not built, so a tab `show_device()` has taken out of the book
+  is not prebuilt.
+- A panel built while its page is hidden stays hidden, and a completed panel gets the
+  dark-mode pass the frame ran before it existed.
+
+The Compare presets dialog is a holder without a page. `MainFrame` keeps a
+`Lazy<DiffPresetDialog>` whose factory constructs the dialog and binds its events, the
+dialog derives from `LazyInstance`, and its callers use
+`DiffPresetDialog::ensure()->show()` and `DiffPresetDialog::if_built()` like a tab's
+callers do. Saving a preset refreshes the dialog only while it is shown, since `show()`
+reloads the presets.
+
+### StagedBuild: construction in units
+
+A mixin for a panel whose constructor is too big to be one unit. The constructor builds
+the skeleton (sizers, and the parts other code may touch) and queues the rest with
+`add_build_step()`. `build_step()` runs one step, and `add_build_steps_of(child)`
+forwards a child's steps so a nested panel is spread the same way; the parent is built
+only once the child is, including steps the child queues later. Steps run in order, on
+the main thread. A `Lazy<T>` recognises a staged type at compile time and runs its steps
+one per unit.
+
+Nothing may touch what a step builds before the last step has run. In practice:
+
+- nothing paints the panel before it is complete, since a panel built at idle is hidden
+  with its page and one built on demand finishes inside `ensure()` before the event loop
+  runs again;
+- members created in steps are initialised to null in the header, so a partially built
+  panel can be destroyed;
+- timers and event handlers that use step content check `built()` first
+  (`MonitorPanel::update_all()`, `CalibrationPanel::update_all()`), and a child's steps
+  are queued before anything in the constructor can fire such a handler;
+- a destructor that disconnects from step content checks `built()` first;
+- a constructor or step does not take focus while the panel is off screen
+  (`IsShownOnScreen()` before `SetFocus()`), since it may run while the user is typing
+  elsewhere;
+- where a step's widgets must keep their place in a sizer that later steps also fill, the
+  constructor adds an empty slot sizer in that position and the step fills the slot
+  (`StatusBasePanel`).
+
+### IdleScheduler: when to build
+
+A task is any `LazyBase`: a name for the log, `pending()`, `build_step()` and
+`prebuild_order()`. `PrebuildQueue` holds the tasks by order (equal order in the order
+added) and runs a slice, the units of the first pending task until it completes, the
+budget is spent on the clock it is given, or the interrupt predicate says input arrived.
+It has no wx dependency and is unit-tested with a fake clock. A task whose work is gone
+is passed over and stays in the queue, so a tab that `show_device()` removes and later
+re-inserts is pending again.
+
+`IdleScheduler` drives the queue with the real clock, the app's input timestamp and the
+Windows queue check, and logs each unit and slice. Each slice is a timer message: a
+period of 250 ms while waiting for the user to go quiet, and a one-shot of zero after a
+slice that left work, so the event loop dispatches whatever it has queued (paint,
+timers, input) before the next slice runs. Chaining slices with `CallAfter` would not do
+this: wx drains pending events fully before the next native message, on every platform.
+On GTK the one-shot is 5 ms, because a due GLib timeout runs ahead of the redraw and
+idle sources that paint and deliver posted events.
+A slice runs only once the user has been idle for the quiet time, and the timer stops
+itself once no task is pending. The tick period, quiet time and slice length are
+constants in `IdleScheduler.cpp`. A unit cannot be interrupted once started, so the
+largest unit bounds click latency on Windows; on the other platforms a click also waits
+for the rest of the slice. A unit that pumps the event loop lets the timer fire inside
+its own slice, and that tick does nothing.
+
+The tasks are made by their owners. `MainFrame::prebuild_pages_when_idle()` registers
+every `LazyPage` the frame created, in or out of the book (`pending()` is false for a
+page out of the book), the Compare presets holder, and the Prepare sidebar's settings
+page from `ParamsPanel::settings_page_prebuild()`, whose first unit selects the default
+tab if none is selected yet and whose later units build one option group each.
+`show_device()` only restarts the timer. `MainFrame` owns the scheduler because it owns
+everything the tasks build, and clearing the queue with the frame is what keeps a task
+from outliving its object.
+
+Idle time comes from `GUI_App::FilterEvent`, which timestamps mouse and keyboard events
+(`wxEVT_CATEGORY_USER_INPUT` minus command events, which all claim that category), and
+`GUI_App::input_idle_ms()` reports it. On Windows a slice additionally refuses to start
+when the message queue holds keyboard, button, touch or pen input. Mouse moves are
+excluded because Windows synthesises one whenever a window appears under the cursor,
+which every unit does. Other platforms use the timestamp alone.
+
+Once every registered page is built the timer is stopped and the subsystem costs
+nothing.
+
+## Rules
+
+**What builds before the first frame.** The Prepare tab's plater, because `post_init()`
+needs its GL canvas on screen to initialise OpenGL in every startup state, and the start
+page the user configured. Home is built by the first `MainFrame::Show()`, Prepare's
+settings page by selecting the tab. `post_init()` passes through the Prepare tab for GL
+init under `Freeze()` with `MainFrame::select_prepare_for_gl_init()`, which changes the
+selection without the page-changed event, so nothing else is built for that pass.
+
+**Reaching a lazy object.** A caller uses the type's own statics. `T::if_built()` may
+return null and is for telling the object something it can live without (a rescale, a
+colour change, a status update). `T::ensure()` builds the object and is for navigating
+to it or for a caller that is about to show it. A caller that tells the object something
+it would not fetch for itself on construction uses `T::when_built()`, which keeps the
+message until the object exists. A panel that pulls its state when constructed (the Home
+page requests the recent list on load, the Device tab reads the device manager on show)
+is reached with `if_built()`; one that cannot pull gets `when_built()`.
+
+**Unit size.** A unit cannot be interrupted, so it should stay within the slice length on
+a fast machine. A constructor above that is staged. A single widget above it is the
+floor unless the widget itself is split.
+
+**Order.** Cheapest and most likely to be opened first, given where `MainFrame` creates
+each holder. The settings page is 0 (the Prepare tab's own content), Home 10, Device 20
+(a Bambu user's usual second stop), Calibration 30, Multi-device 40, the web Device
+view 50, Project 60 (a second WebView2 instance) and the Compare presets dialog 100;
+steps of ten so a new tab takes a value between its neighbours without renumbering
+them. `MainFrame::prebuild_pages_when_idle()` registers the tasks once, from
+`post_init()`.
+
+**Never prebuilt.** A holder with a negative order, for a tab that few sessions open
+and that costs more to build unasked than it saves (the Design tab), and plugin-provided
+tabs, which are Python-side and not lazy pages.
+
+## Adopting it
+
+A lazy tab needs:
+
+1. The panel derived from `LazyInstance<Panel>`, since a tab's panel has one instance.
+2. A `LazyPage<Panel>*` member in `MainFrame`, created once in `init_tabpanel()` with
+   its `TAB_ID_*` as the name, its place by the order rule (and a factory if
+   `new Panel(parent)` is not enough), added to `m_lazy_pages`, and used wherever the
+   tab is added to or looked up in the notebook.
+3. Every use of the panel outside `MainFrame` going through one of the panel's statics,
+   chosen by the rule above. When converting an existing member, `grep` for it; the
+   compiler finds the rest.
+4. A constructor that copes with the frame already existing and the user being busy
+   elsewhere, since `wxGetApp().mainframe` is set, the frame may be shown, and the user
+   may be typing when a lazy panel is built: no `SetFocus()` while off screen, and
+   whatever the constructor did through a `MainFrame` accessor before (a mode update, a
+   deferred URL) done in the constructor itself.
+
+To stage a heavy constructor, inherit `StagedBuild`, keep the skeleton in the constructor,
+move the rest into `add_build_step()` lambdas in the original order, and follow the
+staged-panel rules above. Measure the units. A step that is still one big widget has to
+be split inside that widget or accepted as the floor.
+
+Verification is by log. `MainFrame::prebuild_pages_when_idle` lists the queue it
+registered, `IdleScheduler::tick` reports each completed task by name at info level and
+each slice and unit at debug level, and `Lazy::ensure` reports an object a user built
+on demand with the units and time it took. A run from the configured start
+page shows every registered page complete, in order, with no unit longer than intended.
+A click on a tab mid-prebuild shows the finished panel with an `ensure` line for what was
+left, and the slices resume for the remaining tasks once the user is idle again.

src/slic3r/CMakeLists.txt

@@ -286,6 +286,8 @@ set(SLIC3R_GUI_SOURCES
     GUI/FilamentMapDialog.hpp
     GUI/IconManager.cpp
     GUI/IconManager.hpp
+    GUI/IdleScheduler.cpp
+    GUI/IdleScheduler.hpp
     GUI/ImageGrid.cpp
     GUI/ImageGrid.h
     GUI/ImGuiWrapper.cpp
@@ -347,6 +349,10 @@ set(SLIC3R_GUI_SOURCES
     GUI/KBShortcutsDialog.hpp
     GUI/KeyChord.cpp
     GUI/KeyChord.hpp
+    GUI/Lazy.cpp
+    GUI/Lazy.hpp
+    GUI/LazyPage.cpp
+    GUI/LazyPage.hpp
     GUI/LibVGCode/LibVGCodeWrapper.hpp
     GUI/LibVGCode/LibVGCodeWrapper.cpp
     GUI/LinuxDisplayBackend.cpp
@@ -433,6 +439,7 @@ set(SLIC3R_GUI_SOURCES
     GUI/Plater.hpp
     GUI/PlateSettingsDialog.cpp
     GUI/PlateSettingsDialog.hpp
+    GUI/PrebuildQueue.hpp
     GUI/Preferences.cpp
     GUI/Preferences.hpp
     GUI/PresetBundleDialog.cpp
@@ -512,6 +519,7 @@ set(SLIC3R_GUI_SOURCES
     GUI/SliceInfoPanel.hpp
     GUI/SlicingProgressNotification.cpp
     GUI/SlicingProgressNotification.hpp
+    GUI/StagedBuild.hpp
     GUI/StatusPanel.cpp
     GUI/StatusPanel.hpp
     GUI/StepMeshDialog.cpp

src/slic3r/GUI/Auxiliary.cpp

@@ -863,17 +863,27 @@ void AuxiliaryPanel::init_tabpanel()
     m_tabpanel->SetBackgroundColour(wxColour("#FEFFFF"));
     m_tabpanel->Bind(wxEVT_BOOKCTRL_PAGE_CHANGED, [](wxBookCtrlEvent &e) { /* Event handling */ });
 
-    m_designer_panel          = new DesignerPanel(m_tabpanel, AuxiliaryFolderType::DESIGNER);
-    m_pictures_panel          = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::MODEL_PICTURE);
-    m_bill_of_materials_panel = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::BILL_OF_MATERIALS);
-    m_assembly_panel          = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::ASSEMBLY_GUIDE);
-    m_others_panel            = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::OTHERS);
-
-    m_tabpanel->AddPage(m_designer_panel, _L("Basic Info"), true);
-    m_tabpanel->AddPage(m_pictures_panel, _L("Pictures"), false);
-    m_tabpanel->AddPage(m_bill_of_materials_panel, _L("Bill of Materials"), false);
-    m_tabpanel->AddPage(m_assembly_panel, _L("Assembly Guide"), false);
-    m_tabpanel->AddPage(m_others_panel, _L("Others"), false);
+    add_build_step([this] {
+        m_designer_panel = new DesignerPanel(m_tabpanel, AuxiliaryFolderType::DESIGNER);
+        m_tabpanel->AddPage(m_designer_panel, _L("Basic Info"), true);
+    });
+    add_build_step([this] {
+        m_pictures_panel = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::MODEL_PICTURE);
+        m_tabpanel->AddPage(m_pictures_panel, _L("Pictures"), false);
+    });
+    add_build_step([this] {
+        m_bill_of_materials_panel = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::BILL_OF_MATERIALS);
+        m_tabpanel->AddPage(m_bill_of_materials_panel, _L("Bill of Materials"), false);
+    });
+    add_build_step([this] {
+        m_assembly_panel = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::ASSEMBLY_GUIDE);
+        m_tabpanel->AddPage(m_assembly_panel, _L("Assembly Guide"), false);
+    });
+    add_build_step([this] {
+        m_others_panel = new AuFolderPanel(m_tabpanel, AuxiliaryFolderType::OTHERS);
+        m_tabpanel->AddPage(m_others_panel, _L("Others"), false);
+        Layout();
+    });
 }
 
 wxWindow *AuxiliaryPanel::create_side_tools()

src/slic3r/GUI/Auxiliary.hpp

@@ -46,6 +46,7 @@
 #include "slic3r/GUI/UpgradePanel.hpp"
 #include "slic3r/GUI/AmsWidgets.hpp"
 #include "Widgets/SideTools.hpp"
+#include "StagedBuild.hpp"
 
 #define AUFILE_GREY700 wxColour(107, 107, 107)
 #define AUFILE_GREY500 wxColour(158, 158, 158)
@@ -194,7 +195,7 @@ public:
 };
 
 
-class AuxiliaryPanel : public wxPanel
+class AuxiliaryPanel : public wxPanel, public StagedBuild
 {
 private:
     Tabbook *m_tabpanel = {nullptr};

src/slic3r/GUI/CAD/DesignPanel.hpp

@@ -12,6 +12,7 @@
 
 #include "libslic3r/CAD/CadDocument.hpp"
 #include "slic3r/GUI/CAD/DesignInteraction.hpp"   // CadLevel: what one Esc press means
+#include "slic3r/GUI/Lazy.hpp"
 
 class ComboBox;    // Orca dropdown (Widgets/ComboBox.hpp) — replaces wxChoice everywhere here
 class StaticBox;   // Orca rounded card frame (Widgets/StaticBox.hpp)
@@ -46,7 +47,7 @@ class DesignCanvas;
 // Design (CAD) tab: a sketch-first, Onshape-style form-driven CAD panel.
 // Sketch and Extrude are independent tools: the user creates a Sketch first,
 // then selects it and Extrudes to produce a solid.
-class DesignPanel : public wxPanel
+class DesignPanel : public wxPanel, public LazyInstance<DesignPanel>
 {
 public:
     explicit DesignPanel(wxWindow* parent);

src/slic3r/GUI/CAD/McpControl.cpp

@@ -1989,11 +1989,10 @@ json action_set_feature_expr(DesignPanel* panel, const json& params)
 // Dispatch one parsed request ON THE MAIN THREAD. Returns a JSON-RPC reply string.
 std::string handle_on_main(const std::string& method, const json& params, const json& id)
 {
-    MainFrame* mf = wxGetApp().mainframe;
     // The panel is built on first use, and in a headless session nobody clicks the tab that
     // would build it -- so build it here rather than refusing. Safe: this runs on the main
     // thread (see the CallAfter that dispatches us).
-    DesignPanel* panel = mf ? mf->ensure_design_panel() : nullptr;
+    DesignPanel* panel = DesignPanel::ensure();
     if (!panel)
         return rpc_error(id, -32001, "Design panel not ready");
 

src/slic3r/GUI/CalibrationPanel.cpp

@@ -491,14 +491,15 @@ void CalibrationPanel::init_tabpanel() {
             selected);
     }
 
+    for (int i = 0; i < (int)CALI_MODE_COUNT; i++)
+        add_build_steps_of(*m_cali_panels[i]);
+
     // ORCA use standard paddings and keep arrow icon for consistent look between sidebars
     //for (int i = 0; i < (int)CALI_MODE_COUNT; i++)
     //    m_tabpanel->SetPageImage(i, "");
 
     //auto padding_size = m_tabpanel->GetBtnsListCtrl()->GetPaddingSize(0);
     //m_tabpanel->GetBtnsListCtrl()->SetPaddingSize({ FromDIP(15), padding_size.y });
-
-    m_initialized = true;
 }
 
 void CalibrationPanel::init_timer()
@@ -534,6 +535,8 @@ void CalibrationPanel::update_print_error_info(int code, std::string msg, std::s
 }
 
 void CalibrationPanel::update_all() {
+    // Every wizard's pages exist once the last build step has run.
+    if (!built()) return;
 
     NetworkAgent* m_agent = wxGetApp().getAgent();
     Slic3r::DeviceManager* dev = Slic3r::GUI::wxGetApp().getDeviceManager();
@@ -597,7 +600,7 @@ void CalibrationPanel::update_all() {
 
 void CalibrationPanel::show_status(int status)
 {
-    if (!m_initialized) return;
+    if (!built()) return;
     if (last_status == status)return;
     last_status = status;
 

src/slic3r/GUI/CalibrationPanel.hpp

@@ -3,6 +3,7 @@
 
 #include "CalibrationWizard.hpp"
 #include "Tabbook.hpp"
+#include "Lazy.hpp"
 //#include "Widgets/SideTools.hpp"
 
 namespace Slic3r { namespace GUI {
@@ -88,7 +89,7 @@ private:
 };
 
 
-class CalibrationPanel : public wxPanel
+class CalibrationPanel : public wxPanel, public StagedBuild, public LazyInstance<CalibrationPanel>
 {
 public:
     CalibrationPanel(wxWindow* parent, wxWindowID id = wxID_ANY, const wxPoint& pos = wxDefaultPosition, const wxSize& size = wxDefaultSize, long style = wxTAB_TRAVERSAL);
@@ -109,7 +110,6 @@ protected:
 
 
     int                     last_status;
-    bool                    m_initialized { false };
     std::string             last_conn_type = "undedefined";
     MachineObject*          obj{ nullptr };
     MachineObject*          last_obj { nullptr };

src/slic3r/GUI/CalibrationWizard.cpp

@@ -130,6 +130,15 @@ CalibrationWizard::~CalibrationWizard()
     ;
 }
 
+void CalibrationWizard::add_page_step(CalibrationWizardPageStep*& step, std::function<CalibrationWizardPage*()> make)
+{
+    add_build_step([this, &step, make = std::move(make)] {
+        step = new CalibrationWizardPageStep(make());
+        m_all_pages_sizer->Add(step->page, 1, wxEXPAND | wxALL, FromDIP(25));
+        step->page->Hide();
+    });
+}
+
 void CalibrationWizard::on_cali_job_finished(wxCommandEvent& event)
 {
     this->on_cali_job_finished(event.GetString());
@@ -520,33 +529,28 @@ void PressureAdvanceWizard::on_cali_job_finished(wxString evt_data)
 
 void PressureAdvanceWizard::create_pages()
 {
-    start_step  = new CalibrationWizardPageStep(new CalibrationPAStartPage(m_scrolledWindow));
-    preset_step = new CalibrationWizardPageStep(new CalibrationPresetPage(m_scrolledWindow, m_mode, false));
-    cali_step   = new CalibrationWizardPageStep(new CalibrationCaliPage(m_scrolledWindow, m_mode));
-    save_step   = new CalibrationWizardPageStep(new CalibrationPASavePage(m_scrolledWindow));
+    add_page_step(start_step, [this] { return new CalibrationPAStartPage(m_scrolledWindow); });
+    add_page_step(preset_step, [this] { return new CalibrationPresetPage(m_scrolledWindow, m_mode, false); });
+    add_page_step(cali_step, [this] { return new CalibrationCaliPage(m_scrolledWindow, m_mode); });
+    add_page_step(save_step, [this] { return new CalibrationPASavePage(m_scrolledWindow); });
 
-    m_all_pages_sizer->Add(start_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(preset_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(cali_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(save_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
+    add_build_step([this] {
+        m_page_steps.push_back(start_step);
+        m_page_steps.push_back(preset_step);
+        m_page_steps.push_back(cali_step);
+        m_page_steps.push_back(save_step);
 
+        for (int i = 0; i < m_page_steps.size() -1; i++) {
+            m_page_steps[i]->chain(m_page_steps[i+1]);
+        }
 
-    m_page_steps.push_back(start_step);
-    m_page_steps.push_back(preset_step);
-    m_page_steps.push_back(cali_step);
-    m_page_steps.push_back(save_step);
+        for (int i = 0; i < m_page_steps.size(); i++) {
+            m_page_steps[i]->page->Bind(EVT_CALI_ACTION, &PressureAdvanceWizard::on_cali_action, this);
+        }
 
-    for (int i = 0; i < m_page_steps.size() -1; i++) {
-        m_page_steps[i]->chain(m_page_steps[i+1]);
-    }
-
-    for (int i = 0; i < m_page_steps.size(); i++) {
-        m_page_steps[i]->page->Hide();
-        m_page_steps[i]->page->Bind(EVT_CALI_ACTION, &PressureAdvanceWizard::on_cali_action, this);
-    }
-
-    if (!m_page_steps.empty())
-        show_step(m_page_steps.front());
+        if (!m_page_steps.empty())
+            show_step(m_page_steps.front());
+    });
 }
 
 void PressureAdvanceWizard::on_cali_action(wxCommandEvent& evt)
@@ -1053,59 +1057,46 @@ FlowRateWizard::FlowRateWizard(wxWindow* parent, wxWindowID id, const wxPoint& p
 
 void FlowRateWizard::create_pages()
 {
-    start_step = new CalibrationWizardPageStep(new CalibrationFlowRateStartPage(m_scrolledWindow));
-    preset_step = new CalibrationWizardPageStep(new CalibrationPresetPage(m_scrolledWindow, m_mode, false));
+    add_page_step(start_step, [this] { return new CalibrationFlowRateStartPage(m_scrolledWindow); });
+    add_page_step(preset_step, [this] { return new CalibrationPresetPage(m_scrolledWindow, m_mode, false); });
 
     // manual
-    cali_coarse_step = new CalibrationWizardPageStep(new CalibrationCaliPage(m_scrolledWindow, m_mode, CaliPageType::CALI_PAGE_CALI));
-    coarse_save_step = new CalibrationWizardPageStep(new CalibrationFlowCoarseSavePage(m_scrolledWindow));
-    cali_fine_step = new CalibrationWizardPageStep(new CalibrationCaliPage(m_scrolledWindow, m_mode, CaliPageType::CALI_PAGE_FINE_CALI));
-    fine_save_step = new CalibrationWizardPageStep(new CalibrationFlowFineSavePage(m_scrolledWindow));
+    add_page_step(cali_coarse_step, [this] { return new CalibrationCaliPage(m_scrolledWindow, m_mode, CaliPageType::CALI_PAGE_CALI); });
+    add_page_step(coarse_save_step, [this] { return new CalibrationFlowCoarseSavePage(m_scrolledWindow); });
+    add_page_step(cali_fine_step, [this] { return new CalibrationCaliPage(m_scrolledWindow, m_mode, CaliPageType::CALI_PAGE_FINE_CALI); });
+    add_page_step(fine_save_step, [this] { return new CalibrationFlowFineSavePage(m_scrolledWindow); });
 
     // auto
-    cali_step = new CalibrationWizardPageStep(new CalibrationCaliPage(m_scrolledWindow, m_mode));
-    save_step = new CalibrationWizardPageStep(new CalibrationFlowX1SavePage(m_scrolledWindow));
+    add_page_step(cali_step, [this] { return new CalibrationCaliPage(m_scrolledWindow, m_mode); });
+    add_page_step(save_step, [this] { return new CalibrationFlowX1SavePage(m_scrolledWindow); });
 
-    m_all_pages_sizer->Add(start_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(preset_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(cali_coarse_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(coarse_save_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(cali_fine_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(fine_save_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
+    add_build_step([this] {
+        m_page_steps.push_back(start_step);
+        m_page_steps.push_back(preset_step);
+        m_page_steps.push_back(cali_coarse_step);
+        m_page_steps.push_back(coarse_save_step);
+        m_page_steps.push_back(cali_fine_step);
+        m_page_steps.push_back(fine_save_step);
 
-    m_all_pages_sizer->Add(cali_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
-    m_all_pages_sizer->Add(save_step->page, 1, wxEXPAND | wxALL, FromDIP(25));
+        //m_page_steps.push_back(cali_step);
+        //m_page_steps.push_back(save_step);
 
-    m_page_steps.push_back(start_step);
-    m_page_steps.push_back(preset_step);
-    m_page_steps.push_back(cali_coarse_step);
-    m_page_steps.push_back(coarse_save_step);
-    m_page_steps.push_back(cali_fine_step);
-    m_page_steps.push_back(fine_save_step);
+        for (int i = 0; i < m_page_steps.size() - 1; i++) {
+            m_page_steps[i]->chain(m_page_steps[i + 1]);
+        }
 
-    //m_page_steps.push_back(cali_step);
-    //m_page_steps.push_back(save_step);
+        for (int i = 0; i < m_page_steps.size(); i++) {
+            m_page_steps[i]->page->Bind(EVT_CALI_ACTION, &FlowRateWizard::on_cali_action, this);
+        }
 
-    for (int i = 0; i < m_page_steps.size() - 1; i++) {
-        m_page_steps[i]->chain(m_page_steps[i + 1]);
-    }
+        cali_step->page->Bind(EVT_CALI_ACTION, &FlowRateWizard::on_cali_action, this);
+        save_step->page->Bind(EVT_CALI_ACTION, &FlowRateWizard::on_cali_action, this);
 
-    // hide all pages
-    cali_step->page->Hide();
-    save_step->page->Hide();
-    for (int i = 0; i < m_page_steps.size(); i++) {
-        m_page_steps[i]->page->Hide();
-        m_page_steps[i]->page->Bind(EVT_CALI_ACTION, &FlowRateWizard::on_cali_action, this);
-    }
+        if (!m_page_steps.empty())
+            show_step(m_page_steps.front());
 
-
-    cali_step->page->Bind(EVT_CALI_ACTION, &FlowRateWizard::on_cali_action, this);
-    save_step->page->Bind(EVT_CALI_ACTION, &FlowRateWizard::on_cali_action, this);
-
-    if (!m_page_steps.empty())
-        show_step(m_page_steps.front());
-
-    set_cali_method(CalibrationMethod::CALI_METHOD_MANUAL);
+        set_cali_method(CalibrationMethod::CALI_METHOD_MANUAL);
+    });
 }
 
 void FlowRateWizard::on_cali_action(wxCommandEvent& evt)

src/slic3r/GUI/CalibrationWizard.hpp

@@ -9,6 +9,7 @@
 #include "CalibrationWizardPresetPage.hpp"
 #include "CalibrationWizardCaliPage.hpp"
 #include "CalibrationWizardSavePage.hpp"
+#include "StagedBuild.hpp"
 
 namespace Slic3r { namespace GUI {
 
@@ -36,7 +37,7 @@ struct ConfigIndexValue
     int   index{0};
 };
 
-class CalibrationWizard : public wxPanel {
+class CalibrationWizard : public wxPanel, public StagedBuild {
 public:
     CalibrationWizard(wxWindow* parent, CalibMode mode,
         wxWindowID id = wxID_ANY,
@@ -79,6 +80,9 @@ public:
 protected:
     void on_cali_go_home();
 
+    // Queues a page as a build step, created hidden and added to the pages sizer.
+    void add_page_step(CalibrationWizardPageStep*& step, std::function<CalibrationWizardPage*()> make);
+
 protected:
     /* wx widgets*/
     wxScrolledWindow* m_scrolledWindow;

src/slic3r/GUI/CalibrationWizardCaliPage.cpp

@@ -192,8 +192,10 @@ void CalibrationCaliPage::update(MachineObject* obj)
             set_cali_img();
         }
 
+        // A calibration can run before the Device tab is ever opened, and only its status
+        // panel shows a print error.
         if (obj->print_error > 0) {
-            StatusPanel* status_panel = Slic3r::GUI::wxGetApp().mainframe->m_monitor->get_status_panel();
+            StatusPanel* status_panel = MonitorPanel::ensure()->get_status_panel();
             status_panel->obj = obj;
             status_panel->update_error_message();
         }

src/slic3r/GUI/DeviceErrorDialog.cpp

@@ -538,7 +538,8 @@ void DeviceErrorDialog::on_button_click(ActionButton btn_id)
         break;
     }
     case DeviceErrorDialog::CHECK_ASSISTANT: {
-        wxGetApp().mainframe->m_monitor->jump_to_HMS(); // go to assistant page
+        if (MonitorPanel* monitor = MonitorPanel::if_built())
+            monitor->jump_to_HMS(); // go to assistant page
         break;
     }
     case DeviceErrorDialog::FILAMENT_EXTRUDED: {
@@ -568,7 +569,8 @@ void DeviceErrorDialog::on_button_click(ActionButton btn_id)
     }
     case DeviceErrorDialog::JUMP_TO_LIVEVIEW: {
         Slic3r::GUI::wxGetApp().mainframe->jump_to_monitor();
-        Slic3r::GUI::wxGetApp().mainframe->m_monitor->jump_to_LiveView();
+        if (MonitorPanel* monitor = MonitorPanel::if_built())
+            monitor->jump_to_LiveView();
         break;
     }
     case DeviceErrorDialog::NO_REMINDER_NEXT_TIME: {
@@ -618,7 +620,8 @@ void DeviceErrorDialog::on_button_click(ActionButton btn_id)
     }
     case DeviceErrorDialog::OK_JUMP_RACK: {
         Slic3r::GUI::wxGetApp().mainframe->jump_to_monitor();
-        Slic3r::GUI::wxGetApp().mainframe->m_monitor->jump_to_Rack();
+        if (MonitorPanel* monitor = MonitorPanel::if_built())
+            monitor->jump_to_Rack();
         break;
     }
     case DeviceErrorDialog::ABORT: {

src/slic3r/GUI/DeviceTab/wgtDeviceNozzleRack.cpp

@@ -934,7 +934,8 @@ void wgtDeviceNozzleRackNozzleItem::OnBtnNozzleStatus(wxMouseEvent& evt)
         dlg.AddButton(wxID_OK,_L("Jump to the upgrade page"), true);
 
         if (dlg.ShowModal() == wxID_OK) {
-            wxGetApp().mainframe->m_monitor->jump_to_Upgrade();
+            if (MonitorPanel* monitor = MonitorPanel::if_built())
+                monitor->jump_to_Upgrade();
         };
     }
 }

src/slic3r/GUI/DeviceTab/wgtDeviceNozzleRackUpdate.cpp

@@ -344,7 +344,8 @@ void wgtDeviceNozzleRackHotendUpdate::OnStatusIconClick(wxMouseEvent& event)
 
             if (dlg.ShowModal() == wxID_OK) 
             {
-                wxGetApp().mainframe->m_monitor->jump_to_Upgrade();
+                if (MonitorPanel* monitor = MonitorPanel::if_built())
+                    monitor->jump_to_Upgrade();
 
                 wxCommandEvent evt(wxEVT_NOZZLE_JUMP_UPGRADE, GetId());
                 evt.SetEventObject(this);

src/slic3r/GUI/GUI_App.cpp

@@ -863,7 +863,7 @@ void GUI_App::post_init()
         mainframe->Freeze();
 #endif
         plater_->canvas3D()->enable_render(false);
-        mainframe->select_tab(TAB_ID_PREPARE);
+        mainframe->select_prepare_for_gl_init();
         plater_->select_view_3D("3D");
         // The first render happens before the queued new_project() sets the same view.
         plater_->get_camera().select_view("topfront");
@@ -903,10 +903,10 @@ void GUI_App::post_init()
                 BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ", finished rendering a first frame for test";
             }
         }
-        if (is_editor())
-            mainframe->select_tab(TAB_ID_HOME);
-        if (app_config->get("default_page") == "1")
+        if (starts_on_prepare())
             mainframe->select_tab(TAB_ID_PREPARE);
+        else if (is_editor())
+            mainframe->select_tab(TAB_ID_HOME);
 #ifndef __linux__
         mainframe->Thaw();
 #endif
@@ -915,6 +915,7 @@ void GUI_App::post_init()
 
     plater_->trigger_restore_project(1);
     //#endif
+    mainframe->prebuild_pages_when_idle();
 
     //BBS: remove GCodeViewer as seperate APP logic
     /*if (this->init_params->start_as_gcodeviewer) {
@@ -1929,9 +1930,9 @@ bool GUI_App::hot_reload_network_plugin()
         m_device_manager->add_user_subscribe();
     }
 
-    if (mainframe && mainframe->m_monitor) {
-        mainframe->m_monitor->update_network_version_footer();
-        mainframe->m_monitor->set_default();
+    if (MonitorPanel* monitor = MonitorPanel::if_built()) {
+        monitor->update_network_version_footer();
+        monitor->set_default();
         BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": reset monitor panel";
     }
 
@@ -3422,14 +3423,16 @@ bool GUI_App::on_init_inner()
     }
     BOOST_LOG_TRIVIAL(info) << "create the main window";
     mainframe = new MainFrame();
-    // hide settings tabs after first Layout
     if (is_editor()) {
-        mainframe->select_tab(TAB_ID_HOME);
+        if (starts_on_prepare()) {
+            mainframe->select_tab(TAB_ID_PREPARE);
+        } else {
+            mainframe->select_tab(TAB_ID_HOME);
+        }
     }
 
     sidebar().obj_list()->init();
     //sidebar().aux_list()->init_auxiliary();
-    mainframe->m_project->init_auxiliary();
 
 //     update_mode(); // !!! do that later
     SetTopWindow(mainframe);
@@ -4140,13 +4143,13 @@ void GUI_App::select_machine(const std::string& agent_id)
 
     // Use MonitorPanel::select_machine() to trigger full selection flow
     // This reuses existing logic for machine switching (UI updates, callbacks, etc.)
-    if (mainframe && mainframe->m_monitor) {
-        mainframe->m_monitor->select_machine(dev_id);
+    if (MonitorPanel* monitor = MonitorPanel::if_built()) {
+        monitor->select_machine(dev_id);
         BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": triggered select_machine for dev_id=" << dev_id;
-    } else {
-        // Fallback if MonitorPanel not available
-        m_device_manager->set_selected_machine(dev_id);
-        BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": fallback set_selected_machine dev_id=" << dev_id;
+    } else if (m_device_manager->set_selected_machine(dev_id)) {
+        // The Device tab's own state is set when the tab is built.
+        MonitorPanel::on_machine_selected(m_device_manager->get_selected_machine());
+        BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": set_selected_machine dev_id=" << dev_id;
     }
 }
 
@@ -4670,6 +4673,7 @@ void GUI_App::recreate_GUI(const wxString &msg_name)
 
     //BBS: trigger restore project logic here, and skip confirm
     plater_->trigger_restore_project(1);
+    mainframe->prebuild_pages_when_idle();
 
     // #ys_FIXME_delete_after_testing  Do we still need this  ?
 //     CallAfter([]() {
@@ -5011,7 +5015,8 @@ void GUI_App::get_login_info(const std::string& provider/* = ORCA_CLOUD_PROVIDER
             wxString    strJS      = wxString::Format("window.postMessage(%s)", from_u8(logout_cmd));
             GUI::wxGetApp().run_script(strJS);
         }
-        mainframe->m_webview->SetLoginPanelVisibility(true);
+        if (WebViewPanel* home = WebViewPanel::if_built())
+            home->SetLoginPanelVisibility(true);
     }
 }
 
@@ -5143,9 +5148,9 @@ std::string GUI_App::handle_web_request(std::string cmd)
                 "homepage_bambu_login_or_register",
             };
             if (app_config->get_stealth_mode() && stealth_blocked_info_commands.count(command_str)) {
-                CallAfter([this] {
-                    if (mainframe && mainframe->m_webview)
-                        mainframe->m_webview->SendCloudProvidersInfo();
+                CallAfter([] {
+                    if (WebViewPanel* home = WebViewPanel::if_built())
+                        home->SendCloudProvidersInfo();
                 });
                 return "";
             }
@@ -5159,8 +5164,8 @@ std::string GUI_App::handle_web_request(std::string cmd)
                     if (dlg.ShowModal() == wxID_OK) {
                         app_config->set_bool("stealth_mode", false);
                         app_config->save();
-                        if (mainframe && mainframe->m_webview)
-                            mainframe->m_webview->SendCloudProvidersInfo();
+                        if (WebViewPanel* home = WebViewPanel::if_built())
+                            home->SendCloudProvidersInfo();
                         // Continue with login
                         if (command_str == "homepage_login_or_register")
                             this->request_login(true);
@@ -5241,8 +5246,8 @@ std::string GUI_App::handle_web_request(std::string cmd)
             }
             else if (command_str.compare("get_recent_projects") == 0) {
                 if (mainframe) {
-                    if (mainframe->m_webview) {
-                        mainframe->m_webview->SendRecentList(INT_MAX);
+                    if (WebViewPanel* home = WebViewPanel::if_built()) {
+                        home->SendRecentList(INT_MAX);
                     }
                 }
             }
@@ -7775,8 +7780,8 @@ void GUI_App::on_stealth_mode_enter()
     BOOST_LOG_TRIVIAL(info) << "logout: on_stealth_mode_enter";
     request_user_logout(ORCA_CLOUD_PROVIDER);
     request_user_logout(BBL_CLOUD_PROVIDER);
-    if (mainframe && mainframe->m_webview) {
-        mainframe->m_webview->SendCloudProvidersInfo();
+    if (WebViewPanel* home = WebViewPanel::if_built()) {
+        home->SendCloudProvidersInfo();
     }
 }
 
@@ -8186,6 +8191,24 @@ ConfigOptionMode GUI_App::get_saved_mode()
     return saved_mode_from_string(app_config->get("user_mode"));
 }
 
+bool GUI_App::starts_on_prepare() const
+{
+    return app_config->get("default_page") == "1";
+}
+
+int GUI_App::input_idle_ms() const
+{
+    return int(std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::steady_clock::now() - m_last_input).count());
+}
+
+// Every wxCommandEvent claims the user-input category, so only real mouse and key events count.
+int GUI_App::FilterEvent(wxEvent& event)
+{
+    if (!event.IsCommandEvent() && (event.GetEventCategory() & wxEVT_CATEGORY_USER_INPUT))
+        m_last_input = std::chrono::steady_clock::now();
+    return Event_Skip;
+}
+
 ConfigOptionMode GUI_App::get_mode()
 {
     return app_config->get_bool("developer_mode") ? comDevelop : get_saved_mode();
@@ -8237,9 +8260,10 @@ void GUI_App::update_mode()
         mainframe->m_param_panel->update_mode();
     if (mainframe->m_param_dialog)
         mainframe->m_param_dialog->panel()->update_mode();
-    if (mainframe->m_printer_view)
-        mainframe->m_printer_view->update_mode();
-    mainframe->m_webview->update_mode();
+    if (PrinterWebView* view = PrinterWebView::if_built())
+        view->update_mode();
+    if (WebViewPanel* home = WebViewPanel::if_built())
+        home->update_mode();
 
 #ifdef _MSW_DARK_MODE
     if (!wxGetApp().tabs_as_menu())
@@ -8257,9 +8281,10 @@ void GUI_App::update_mode()
 }
 
 void GUI_App::update_internal_development() {
-    mainframe->m_webview->update_mode();
-    if (mainframe->m_printer_view)
-        mainframe->m_printer_view->update_mode();
+    if (WebViewPanel* home = WebViewPanel::if_built())
+        home->update_mode();
+    if (PrinterWebView* view = PrinterWebView::if_built())
+        view->update_mode();
 }
 
 void GUI_App::show_ip_address_enter_dialog(wxString title)

src/slic3r/GUI/GUI_App.hpp

@@ -1,6 +1,7 @@
 #ifndef slic3r_GUI_App_hpp_
 #define slic3r_GUI_App_hpp_
 
+#include <chrono>
 #include <functional>
 #include <memory>
 #include <string>
@@ -248,6 +249,7 @@ private:
     bool            m_app_conf_exists{ false };
     EAppMode        m_app_mode{ EAppMode::Editor };
     bool            m_is_recreating_gui{ false };
+    std::chrono::steady_clock::time_point m_last_input{ std::chrono::steady_clock::now() };
 #ifdef __linux__
     bool            m_opengl_initialized{ false };
 #endif
@@ -387,6 +389,11 @@ public:
     bool is_editor() const { return m_app_mode == EAppMode::Editor; }
     bool is_gcode_viewer() const { return m_app_mode == EAppMode::GCodeViewer; }
     bool is_recreating_gui() const { return m_is_recreating_gui; }
+    // Milliseconds since the last mouse or keyboard event the app processed.
+    int  input_idle_ms() const;
+    int  FilterEvent(wxEvent& event) override;
+    // The Preferences "Default page" choice, stored as its index: 0 Home, 1 Prepare.
+    bool starts_on_prepare() const;
     std::string logo_name() const { return is_editor() ? "OrcaSlicer" : "OrcaSlicer-gcodeviewer"; }
 
     bool is_closing() const { return m_is_closing.load(std::memory_order_acquire); }

src/slic3r/GUI/HMSPanel.cpp

@@ -122,7 +122,8 @@ HMSNotifyItem::HMSNotifyItem(const std::string& dev_id, wxWindow *parent, DevHMS
     m_hms_content->Bind(wxEVT_LEFT_UP, [this](wxMouseEvent& e) {
             wxCommandEvent evt(EVT_ALREADY_READ_HMS);
             evt.SetString(long_error_code);
-            wxPostEvent(wxGetApp().mainframe->m_monitor, evt);
+            if (MonitorPanel* monitor = MonitorPanel::if_built())
+                wxPostEvent(monitor, evt);
 
             if (!m_url.empty()) wxLaunchDefaultBrowser(m_url);
         });

src/slic3r/GUI/IdleScheduler.cpp

@@ -0,0 +1,100 @@
+#include "IdleScheduler.hpp"
+
+#include <chrono>
+
+#include <boost/log/trivial.hpp>
+
+#include "libslic3r/Utils.hpp"
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+namespace Slic3r { namespace GUI {
+
+namespace {
+
+// The timer's period while waiting for the user to go quiet.
+constexpr int tick_ms  = 250;
+// Input-free time before a slice may start, about a double-click interval, so a user
+// mid-gesture is left alone.
+constexpr int quiet_ms = 500;
+// Budget of one slice. The next slice is a timer message, so paint, timers and input
+// queued meanwhile are handled first; a click waits at most a slice plus the unit that
+// overran it.
+constexpr int slice_ms = 40;
+// On GTK a due timer runs ahead of repaints and posted events, so the next slice waits a few ms.
+#ifdef __WXGTK__
+constexpr int next_slice_ms = 5;
+#else
+constexpr int next_slice_ms = 0;
+#endif
+
+// True when unhandled keyboard, button, touch or pen input is queued; only Windows can ask.
+bool input_pending()
+{
+#ifdef _WIN32
+    // Windows synthesizes a mouse move whenever a window appears under the cursor, so those
+    // do not count.
+    return (GetQueueStatus(QS_INPUT & ~QS_MOUSEMOVE) >> 16) != 0;
+#else
+    return false;
+#endif
+}
+
+} // namespace
+
+IdleScheduler::IdleScheduler(std::function<int()> input_idle_ms) : m_input_idle_ms(std::move(input_idle_ms))
+{
+    m_timer.Bind(wxEVT_TIMER, [this](wxTimerEvent&) { tick(); });
+}
+
+void IdleScheduler::start()
+{
+    if (!m_timer.IsRunning())
+        m_timer.Start(tick_ms);
+}
+
+void IdleScheduler::stop()
+{
+    m_timer.Stop();
+}
+
+void IdleScheduler::tick()
+{
+    // A unit that pumps the event loop lets the timer fire inside its own slice.
+    if (m_in_slice)
+        return;
+    if (!m_queue.pending()) {
+        stop();
+        return;
+    }
+    if (m_input_idle_ms() < quiet_ms || input_pending()) {
+        start();
+        return;
+    }
+    const auto now_ms = [] {
+        return std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::steady_clock::now().time_since_epoch()).count();
+    };
+    const char* const fn = __FUNCTION__;
+    m_in_slice           = true;
+    PrebuildQueue::Slice slice;
+    {
+        ScopeGuard in_slice([this] { m_in_slice = false; });
+        slice = m_queue.run_slice(slice_ms, now_ms, input_pending, [fn](const std::string& name, long long ms) {
+            BOOST_LOG_TRIVIAL(debug) << fn << ": " << name << ": unit took " << ms << " ms";
+        });
+    }
+    if (slice.completed)
+        BOOST_LOG_TRIVIAL(info) << fn << ": task complete: " << slice.name << ", " << slice.units << " unit(s) in " << slice.ms << " ms this slice";
+    else
+        BOOST_LOG_TRIVIAL(debug) << fn << ": " << slice.name << ": " << slice.units << " unit(s) in " << slice.ms << " ms, yielding";
+    if (!slice.remaining)
+        stop();
+    else if (input_pending())
+        m_timer.Start(tick_ms);
+    else
+        m_timer.StartOnce(next_slice_ms);
+}
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/IdleScheduler.hpp

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <functional>
+#include <string>
+
+#include <wx/event.h>
+#include <wx/timer.h>
+
+#include "PrebuildQueue.hpp"
+
+namespace Slic3r { namespace GUI {
+
+// Runs the queue's units while the user is idle. Once the user has been idle long enough,
+// slices run one per timer message until the work is done or input arrives, so the event
+// loop handles what it has between slices. Main thread only.
+class IdleScheduler
+{
+public:
+    // input_idle_ms reports how long ago the user last touched the mouse or keyboard.
+    explicit IdleScheduler(std::function<int()> input_idle_ms);
+
+    void add(LazyBase& task) { m_queue.add(task); }
+    void clear() { m_queue.clear(); }
+    // The task names in queue order, for a log line.
+    std::string names() const { return m_queue.names(); }
+
+    // Arms the timer; it stops itself once no task is pending, so call again when a task
+    // appears.
+    void start();
+    void stop();
+
+private:
+    void tick();
+
+    PrebuildQueue        m_queue;
+    wxTimer              m_timer;
+    std::function<int()> m_input_idle_ms;
+    bool                 m_in_slice{ false };
+};
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/Lazy.cpp

@@ -0,0 +1,27 @@
+#include "Lazy.hpp"
+
+#include <boost/log/trivial.hpp>
+#include <wx/app.h>
+#include <wx/utils.h>
+
+namespace Slic3r { namespace GUI {
+
+LazyBase::OnDemandBuild::OnDemandBuild(const LazyBase& lazy) : m_lazy(lazy), m_started(std::chrono::steady_clock::now())
+{
+    // The unit tests have no app.
+    if (wxTheApp != nullptr)
+        m_busy = std::make_unique<wxBusyCursor>();
+}
+
+LazyBase::OnDemandBuild::~OnDemandBuild()
+{
+    BOOST_LOG_TRIVIAL(info) << "Lazy::ensure: built " << m_lazy.name() << " on demand, " << m_units << " unit(s) in "
+                            << std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::steady_clock::now() - m_started).count() << " ms";
+}
+
+void LazyBase::log_null_factory(const std::string& name)
+{
+    BOOST_LOG_TRIVIAL(error) << "Lazy::build_step: the factory for " << name << " returned null";
+}
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/Lazy.hpp

@@ -0,0 +1,198 @@
+#pragma once
+
+#include <atomic>
+#include <chrono>
+#include <functional>
+#include <memory>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "StagedBuild.hpp"
+
+class wxBusyCursor;
+
+// Deferred construction of one object. Lazy<T> holds the factory and builds the object on
+// demand or, unit by unit, from an IdleScheduler; LazyBase is what the scheduler sees of
+// it; LazyInstance<Self> gives a type with one such object app-wide static access to it.
+// docs/HLSD/deferred-page-construction.md covers the design.
+
+namespace Slic3r { namespace GUI {
+
+template <class Self> class LazyInstance;
+
+// A prebuild task: what PrebuildQueue sees of a Lazy<T>. A type with its own unit sequence
+// implements it directly.
+class LazyBase
+{
+public:
+    virtual ~LazyBase() = default;
+
+    // Labels the object in logs.
+    virtual const std::string& name() const = 0;
+    virtual bool               built() const = 0;
+    // Whether the idle prebuild has work here.
+    virtual bool pending() const { return !built(); }
+    // Runs one unit and returns true while more remain.
+    virtual bool build_step() = 0;
+    // Position in the idle queue: lower builds first, negative is never prebuilt.
+    virtual int prebuild_order() const = 0;
+
+protected:
+    // Busy cursor around a build the user is waiting on, and a log line after it.
+    class OnDemandBuild
+    {
+    public:
+        explicit OnDemandBuild(const LazyBase& lazy);
+        ~OnDemandBuild();
+        void unit() { ++m_units; }
+
+    private:
+        const LazyBase&               m_lazy;
+        std::unique_ptr<wxBusyCursor> m_busy;
+        std::chrono::steady_clock::time_point m_started;
+        int                           m_units{ 0 };
+    };
+
+    static void log_null_factory(const std::string& name);
+};
+
+// Holds an object a factory makes on the first build_step() or ensure(), followed by one
+// StagedBuild step per unit if the type has them, and keeps callbacks until the object is
+// complete. The object's parent owns it, not the holder. A LazyInstance type is registered
+// for its statics; any other type is reached only through the holder.
+template <class T>
+class Lazy : public LazyBase
+{
+public:
+    using Factory = std::function<T*()>;
+
+    Lazy(std::string name, int order, Factory make) : m_name(std::move(name)), m_order(order), m_make(std::move(make))
+    {
+        if constexpr (registered())
+            LazyInstance<T>::s_lazy = this;
+    }
+    ~Lazy() override
+    {
+        if constexpr (registered())
+            if (LazyInstance<T>::s_lazy == this)
+                LazyInstance<T>::s_lazy = nullptr;
+    }
+    Lazy(const Lazy&) = delete;
+    Lazy& operator=(const Lazy&) = delete;
+
+    // Null until completely built.
+    T* get() const { return built() ? m_object : nullptr; }
+
+    // Builds whatever is left now and returns the object, null if the factory returned null
+    // or a nested call finds the object mid-build.
+    T* ensure()
+    {
+        if (!built() && !m_building) {
+            OnDemandBuild build(*this);
+            do
+                build.unit();
+            while (build_step());
+        }
+        return get();
+    }
+
+    // Runs fn on the object now if it is built, otherwise once it is.
+    void when_built(std::function<void(T&)> fn)
+    {
+        if (built())
+            fn(*m_object);
+        else
+            m_deferred.push_back(std::move(fn));
+    }
+
+    const std::string& name() const override { return m_name; }
+    // Read by a worker thread through the statics; the last write in build_step() releases it.
+    bool built() const override { return m_complete.load(std::memory_order_acquire); }
+    bool pending() const override { return !built() && !m_failed; }
+    int  prebuild_order() const override { return m_order; }
+
+    // The factory is the first unit, then one StagedBuild step each. A unit that pumps the
+    // event loop cannot re-enter; a nested call does nothing.
+    bool build_step() override
+    {
+        if (built() || m_building || m_failed)
+            return false;
+        m_building = true;
+        try {
+            if (m_object == nullptr)
+                m_object = m_make();
+            else
+                step();
+        } catch (...) {
+            m_building = false;
+            throw;
+        }
+        m_building = false;
+        if (m_object == nullptr) {
+            m_failed = true;
+            log_null_factory(m_name);
+            return false;
+        }
+        if (steps_remain())
+            return true;
+        m_complete.store(true, std::memory_order_release);
+        for (auto& fn : m_deferred)
+            fn(*m_object);
+        m_deferred.clear();
+        return false;
+    }
+
+private:
+    static constexpr bool registered() { return std::is_base_of_v<LazyInstance<T>, T>; }
+    static constexpr bool staged() { return std::is_base_of_v<StagedBuild, T>; }
+
+    bool steps_remain() const
+    {
+        if constexpr (staged())
+            return !m_object->built();
+        else
+            return false;
+    }
+    void step()
+    {
+        if constexpr (staged())
+            m_object->build_step();
+    }
+
+    std::string                          m_name;
+    int                                  m_order;
+    Factory                              m_make;
+    T*                                   m_object{ nullptr };
+    std::atomic<bool>                    m_complete{ false };
+    bool                                 m_building{ false };
+    bool                                 m_failed{ false };
+    std::vector<std::function<void(T&)>> m_deferred;
+};
+
+// Mixin for a type with one lazily built instance in the app. The statics reach that
+// instance through its Lazy holder (a recreated MainFrame's holder replaces the old
+// frame's).
+template <class Self>
+class LazyInstance
+{
+public:
+    // Null until completely built.
+    static Self* if_built() { return s_lazy ? s_lazy->get() : nullptr; }
+    // Builds the object if needed; null while no holder exists or the holder has no
+    // object.
+    static Self* ensure() { return s_lazy ? s_lazy->ensure() : nullptr; }
+    // Runs fn on the object now if it is built, otherwise once it is.
+    static void when_built(std::function<void(Self&)> fn)
+    {
+        if (s_lazy)
+            s_lazy->when_built(std::move(fn));
+    }
+
+private:
+    friend class Lazy<Self>;
+    inline static Lazy<Self>* s_lazy{ nullptr };
+};
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/LazyPage.cpp

@@ -0,0 +1,14 @@
+#include "LazyPage.hpp"
+
+#include "GUI_App.hpp"
+
+namespace Slic3r { namespace GUI {
+
+void apply_dark_ui_to_lazy_panel(wxWindow* panel)
+{
+#ifdef _MSW_DARK_MODE
+    wxGetApp().UpdateDarkUIWin(panel);
+#endif
+}
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/LazyPage.hpp

@@ -0,0 +1,77 @@
+#pragma once
+
+#include <functional>
+#include <string>
+#include <utility>
+
+#include <wx/bookctrl.h>
+#include <wx/panel.h>
+#include <wx/sizer.h>
+
+#include "Lazy.hpp"
+
+namespace Slic3r { namespace GUI {
+
+// Applies the app's dark-mode pass to a panel built after the frame's own pass ran.
+void apply_dark_ui_to_lazy_panel(wxWindow* panel);
+
+// A page whose panel is built the first time the page is shown, or earlier by an
+// IdleScheduler; nothing builds while the frame is still hidden. The Lazy side holds the
+// panel and, for a LazyInstance panel type, registers it for the type's statics.
+template <class Panel>
+class LazyPage : public wxPanel, public Lazy<Panel>
+{
+public:
+    using Factory = std::function<Panel*(wxWindow* parent)>;
+
+    LazyPage(wxWindow* parent, std::string name, int order, Factory make = [](wxWindow* parent) { return new Panel(parent); })
+        : wxPanel(parent), Lazy<Panel>(std::move(name), order, [this, make = std::move(make)] {
+            Panel* panel = make(this);
+            GetSizer()->Add(panel, 1, wxEXPAND);
+            // Hidden while its page is, as an inserted page would be.
+            if (!IsShown())
+                panel->Hide();
+            return panel;
+        })
+    {
+        SetSizer(new wxBoxSizer(wxVERTICAL));
+        // Shown by the book when its tab is selected.
+        Hide();
+        this->when_built([this](Panel& panel) {
+            apply_dark_ui_to_lazy_panel(&panel);
+            Layout();
+        });
+    }
+
+    // The parent book currently lists this page.
+    bool in_book() const
+    {
+        auto* book = dynamic_cast<wxBookCtrlBase*>(GetParent());
+        return book != nullptr && book->FindPage(this) != wxNOT_FOUND;
+    }
+
+    // Pending only while the tab is in the book.
+    bool pending() const override { return in_book() && Lazy<Panel>::pending(); }
+
+    // Forwarded so the panel's own Show() override stays its activation hook; wx calls this
+    // virtual from the book's ShowWithEffect() only for wxSHOW_EFFECT_NONE, the default.
+    bool Show(bool show = true) override
+    {
+        const bool changed = wxPanel::Show(show);
+        if (show) {
+            // The book shows its first page as it is inserted, before startup has chosen the
+            // start page, so a hidden frame builds nothing; MainFrame::Show() completes it.
+            if (this->built() || wxGetTopLevelParent(this)->IsShown()) {
+                if (Panel* panel = this->ensure())
+                    panel->Show(true);
+                // The sizer skipped the panel while the book kept it hidden.
+                Layout();
+            }
+        } else if (Panel* panel = this->get()) {
+            panel->Show(false);
+        }
+        return changed;
+    }
+};
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/MainFrame.cpp

@@ -317,7 +317,8 @@ DPIFrame(NULL, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, BORDERLESS_FRAME_
     // BBS
     , m_recent_projects(18)
     , m_settings_dialog(this)
-    , diff_dialog(this)
+    , m_idle([] { return wxGetApp().input_idle_ms(); })
+    , m_diff_dialog("compare_presets", 100, [this] { return make_diff_dialog(); })
 {
 #ifdef __WXOSX__
     set_miniaturizable(GetHandle());
@@ -737,9 +738,6 @@ DPIFrame(NULL, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, BORDERLESS_FRAME_
 
     wxGetApp().persist_window_geometry(this, true);
     wxGetApp().persist_window_geometry(&m_settings_dialog, true);
-    // bind events from DiffDlg
-
-    bind_diff_dialog();
 }
 
 bool MainFrame::handle_global_shortcut(const KeyChord& chord)
@@ -832,8 +830,10 @@ bool MainFrame::handle_global_shortcut(const KeyChord& chord)
     return true;
 }
 
-void MainFrame::bind_diff_dialog()
+DiffPresetDialog* MainFrame::make_diff_dialog()
 {
+    auto* dialog = new DiffPresetDialog(this);
+
     auto get_tab = [](Preset::Type type) {
         Tab* null_tab = nullptr;
         for (Tab* tab : wxGetApp().tabs_list)
@@ -842,23 +842,24 @@ void MainFrame::bind_diff_dialog()
         return null_tab;
     };
 
-    auto transfer = [this, get_tab](Preset::Type type) {
-        get_tab(type)->transfer_options(diff_dialog.get_left_preset_name(type),
-                                        diff_dialog.get_right_preset_name(type),
-                                        diff_dialog.get_selected_options(type));
+    auto transfer = [dialog, get_tab](Preset::Type type) {
+        get_tab(type)->transfer_options(dialog->get_left_preset_name(type),
+                                        dialog->get_right_preset_name(type),
+                                        dialog->get_selected_options(type));
     };
 
-    auto process_options = [this](std::function<void(Preset::Type)> process) {
-        const Preset::Type diff_dlg_type = diff_dialog.view_type();
+    auto process_options = [dialog](std::function<void(Preset::Type)> process) {
+        const Preset::Type diff_dlg_type = dialog->view_type();
         if (diff_dlg_type == Preset::TYPE_INVALID) {
-            for (const Preset::Type& type : diff_dialog.types_list() )
+            for (const Preset::Type& type : dialog->types_list() )
                 process(type);
         }
         else
             process(diff_dlg_type);
     };
 
-    diff_dialog.Bind(EVT_DIFF_DIALOG_TRANSFER,      [process_options, transfer](SimpleEvent&)         { process_options(transfer); });
+    dialog->Bind(EVT_DIFF_DIALOG_TRANSFER, [process_options, transfer](SimpleEvent&) { process_options(transfer); });
+    return dialog;
 }
 
 
@@ -1186,8 +1187,9 @@ void MainFrame::update_edge_panels()
 void MainFrame::shutdown()
 {
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << "MainFrame::shutdown enter";
-    if (m_project != nullptr)
-        m_project->shutdown();
+    m_idle.stop();
+    if (ProjectPanel* project = ProjectPanel::if_built())
+        project->shutdown();
     m_plugin_pages.shutdown();
     if (m_plater != nullptr)
         m_plater->remove_dock_panes();
@@ -1314,19 +1316,6 @@ void MainFrame::show_option(bool show)
     }
 }
 
-#ifdef SLIC3R_CAD
-DesignPanel* MainFrame::ensure_design_panel()
-{
-    if (m_design_panel == nullptr && m_design_page != nullptr) {
-        wxBusyCursor busy;
-        m_design_panel = new DesignPanel(m_design_page);
-        m_design_page->GetSizer()->Add(m_design_panel, 1, wxEXPAND);
-        m_design_page->Layout();
-    }
-    return m_design_panel;
-}
-#endif
-
 void MainFrame::init_tabpanel() {
     // wxNB_NOPAGETHEME: Disable Windows Vista theme for the Notebook background. The theme performance is terrible on
     // Windows 10 with multiple high resolution displays connected.
@@ -1369,23 +1358,20 @@ void MainFrame::init_tabpanel() {
         //    m_param_panel->OnActivate();
 #ifdef SLIC3R_CAD
         else if (m_design_page != nullptr && panel == m_design_page) {
-            // Built on first activation, never at startup: the panel creates several hundred
-            // controls and its own GL canvas, which a user who does not open the tab should
-            // not pay for.
-            ensure_design_panel();
             // Re-sync the Design bed to the active printer: the panel is built before the
             // printer profile is fully applied, so its bed must refresh on activation or the
             // grid (true bed) spills past the stale default bed quad.
-            m_design_panel->on_tab_shown();
+            DesignPanel::ensure()->on_tab_shown();
         }
 #endif
-        else if (panel == m_monitor) {
+        else if (panel == m_monitor_page) {
             //monitor
         }
 #ifdef SLIC3R_CAD
         // Any page that is not Design takes the Design status line down with it — see
         // DesignPanel::on_tab_hidden for why the popup does not follow the page on its own.
-        if (m_design_panel != nullptr && panel != m_design_page) m_design_panel->on_tab_hidden();
+        if (DesignPanel* design = DesignPanel::if_built(); design != nullptr && panel != m_design_page)
+            design->on_tab_hidden();
 #endif
 #ifndef __APPLE__
         if (m_last_selected_tab == TAB_ID_PREPARE) {
@@ -1401,13 +1387,14 @@ void MainFrame::init_tabpanel() {
     });
 
     if (wxGetApp().is_editor()) {
-        m_webview         = new WebViewPanel(m_tabpanel);
+        m_home_page = new LazyPage<WebViewPanel>(m_tabpanel, TAB_ID_HOME, 10);
+        m_lazy_pages.push_back(m_home_page);
         Bind(EVT_LOAD_URL, [this](wxCommandEvent &evt) {
             wxString url = evt.GetString();
             select_tab(TAB_ID_HOME);
-            m_webview->load_url(url);
+            WebViewPanel::ensure()->load_url(url);
         });
-        m_tabpanel->AddPage(TAB_ID_HOME, m_webview, "", "tab_home_active");
+        m_tabpanel->AddPage(TAB_ID_HOME, m_home_page, "", "tab_home_active");
         m_param_panel = new ParamsPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxBK_LEFT | wxTAB_TRAVERSAL);
     }
 
@@ -1418,15 +1405,13 @@ void MainFrame::init_tabpanel() {
     wxGetApp().plater_ = m_plater;
 
 #ifdef SLIC3R_CAD
-    // Stand-in page for the Design tab. The real DesignPanel is built into it the first time
-    // the tab is selected (see the page-changed handler above), so nothing it constructs sits
-    // on the startup path. The experimental feature is off by default, and when it is off the
-    // page is never created, so the tab does not appear at all (the preference takes effect on
-    // the next start, like the other feature toggles).
+    // The experimental feature is off by default, and when it is off the page is never
+    // created, so the tab does not appear at all (the preference takes effect on the next
+    // start, like the other feature toggles).
     if (wxGetApp().is_enable_cad_feature()) {
-        m_design_page = new wxPanel(this);
-        m_design_page->SetSizer(new wxBoxSizer(wxVERTICAL));
-        m_design_page->Hide();
+        // Experimental and heavy enough that building it unasked would cost more than it saves.
+        m_design_page = new LazyPage<DesignPanel>(this, TAB_ID_DESIGN, -1);
+        m_lazy_pages.push_back(m_design_page);
         start_mcp_control_if_enabled();   // opens the MCP socket iff ORCA_CAD_MCP is set
     }
 #endif
@@ -1434,33 +1419,41 @@ void MainFrame::init_tabpanel() {
     create_preset_tabs();
 
         //BBS add pages
-    m_monitor = new MonitorPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-    m_monitor->SetBackgroundColour(*wxWHITE);
-    m_tabpanel->AddPage(TAB_ID_MONITOR, m_monitor, _L("Device"), "tab_monitor_active");
+    m_monitor_page = new LazyPage<MonitorPanel>(m_tabpanel, TAB_ID_MONITOR, 20);
+    m_lazy_pages.push_back(m_monitor_page);
+    m_tabpanel->AddPage(TAB_ID_MONITOR, m_monitor_page, _L("Device"), "tab_monitor_active");
 
-    m_printer_view = new PrinterWebView(m_tabpanel);
+    m_printer_view_page = new LazyPage<PrinterWebView>(m_tabpanel, TAB_ID_MONITOR_WEB, 50, [this](wxWindow* parent) {
+        auto* view = new PrinterWebView(parent);
+        if (!m_printer_url.empty())
+            view->load_url(m_printer_url, m_printer_api_key);
+        return view;
+    });
+    m_lazy_pages.push_back(m_printer_view_page);
     Bind(EVT_LOAD_PRINTER_URL, [this](LoadPrinterViewEvent &evt) {
-        wxString url = evt.GetString();
-        wxString key = evt.GetAPIkey();
         //select_tab(MainFrame::tpMonitor);
-        m_printer_view->load_url(url, key);
+        m_printer_url     = evt.GetString();
+        m_printer_api_key = evt.GetAPIkey();
+        if (PrinterWebView* view = PrinterWebView::if_built())
+            view->load_url(m_printer_url, m_printer_api_key);
     });
-    m_printer_view->Hide();
 
+    m_multi_machine_page = new LazyPage<MultiMachinePage>(m_tabpanel, TAB_ID_MULTI_DEVICE, 40);
+    m_lazy_pages.push_back(m_multi_machine_page);
     if (wxGetApp().is_enable_multi_machine()) {
-        m_multi_machine = new MultiMachinePage(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-        m_multi_machine->SetBackgroundColour(*wxWHITE);
         // TODO: change the bitmap
-        m_tabpanel->AddPage(TAB_ID_MULTI_DEVICE, m_multi_machine, _L("Multi-device"), "tab_multi_active");
+        m_tabpanel->AddPage(TAB_ID_MULTI_DEVICE, m_multi_machine_page, _L("Multi-device"), "tab_multi_active");
     }
 
-    m_project = new ProjectPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-    m_project->SetBackgroundColour(*wxWHITE);
-    m_tabpanel->AddPage(TAB_ID_PROJECT, m_project, _L("Project"), "tab_auxiliary_active");
+    m_project_page = new LazyPage<ProjectPanel>(m_tabpanel, TAB_ID_PROJECT, 60);
+    m_lazy_pages.push_back(m_project_page);
+    m_tabpanel->AddPage(TAB_ID_PROJECT, m_project_page, _L("Project"), "tab_auxiliary_active");
 
-    m_calibration = new CalibrationPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-    m_calibration->SetBackgroundColour(*wxWHITE);
-    m_tabpanel->AddPage(TAB_ID_CALIBRATION, m_calibration, _L("Calibration"), "tab_calibration_active");
+    // show_device() removes this tab for printers without the Bambu device tab, and the page
+    // then never builds its panel.
+    m_calibration_page = new LazyPage<CalibrationPanel>(m_tabpanel, TAB_ID_CALIBRATION, 30);
+    m_lazy_pages.push_back(m_calibration_page);
+    m_tabpanel->AddPage(TAB_ID_CALIBRATION, m_calibration_page, _L("Calibration"), "tab_calibration_active");
 
     // Plugin pages are appended after the built-in tabs; their ids are namespaced
     // (plugin.<plugin_key>.<name>) so they can't collide with the built-in TAB_ID_* constants.
@@ -1495,67 +1488,44 @@ void MainFrame::show_device(bool should_use_native) {
     // Remove the extra page before switching to any layout that shouldn't have it.
     if (!want_web_device_tab) {
         if ((idx = m_tabpanel->FindPageByName(TAB_ID_MONITOR_WEB)) != wxNOT_FOUND) {
-            m_printer_view->Show(false);
+            m_printer_view_page->Show(false);
             m_tabpanel->RemovePage(idx);
         }
     }
 
     if (use_printer_agents) {
-        if (!m_monitor) {
-            m_monitor = new MonitorPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-            m_monitor->SetBackgroundColour(*wxWHITE);
-        }
-
-        if (m_tabpanel->FindPage(m_monitor) == wxNOT_FOUND) {
-            if ((idx = m_tabpanel->FindPage(m_printer_view)) != wxNOT_FOUND) {
-                m_printer_view->Show(false);
+        if (!m_monitor_page->in_book()) {
+            if ((idx = m_tabpanel->FindPage(m_printer_view_page)) != wxNOT_FOUND) {
+                m_printer_view_page->Show(false);
                 m_tabpanel->RemovePage(idx);
             }
-            m_monitor->Show(false);
-            m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PREVIEW}), TAB_ID_MONITOR, m_monitor,
+            m_monitor_page->Show(false);
+            m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PREVIEW}), TAB_ID_MONITOR, m_monitor_page,
                                    _L("Device"), "tab_monitor_active");
         }
 
-        if (m_printer_view == nullptr) {
-            m_printer_view = new PrinterWebView(m_tabpanel);
-            Bind(EVT_LOAD_PRINTER_URL, [this](LoadPrinterViewEvent& evt) {
-                wxString url = evt.GetString();
-                wxString key = evt.GetAPIkey();
-                // select_tab(MainFrame::tpMonitor);
-                m_printer_view->load_url(url, key);
-            });
-        }
-
         if (wxGetApp().is_enable_multi_machine()) {
-            if (!m_multi_machine) {
-                m_multi_machine = new MultiMachinePage(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-                m_multi_machine->SetBackgroundColour(*wxWHITE);
-            }
             // TODO: change the bitmap
-            if (m_tabpanel->FindPage(m_multi_machine) == wxNOT_FOUND) {
-                m_multi_machine->Show(false);
+            if (!m_multi_machine_page->in_book()) {
+                m_multi_machine_page->Show(false);
                 // Past the web Device tab when it is already there, so enabling multi-machine
                 // later can't wedge this page between the two Device tabs.
                 m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_MONITOR_WEB, TAB_ID_MONITOR}),
-                                       TAB_ID_MULTI_DEVICE, m_multi_machine, _L("Multi-device"), "tab_multi_active");
+                                       TAB_ID_MULTI_DEVICE, m_multi_machine_page, _L("Multi-device"), "tab_multi_active");
             }
         }
-        if (!m_calibration) {
-            m_calibration = new CalibrationPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-            m_calibration->SetBackgroundColour(*wxWHITE);
-        }
-        if (m_tabpanel->FindPage(m_calibration) == wxNOT_FOUND) {
-            m_calibration->Show(false);
-            m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PROJECT}), TAB_ID_CALIBRATION, m_calibration,
+        if (!m_calibration_page->in_book()) {
+            m_calibration_page->Show(false);
+            m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PROJECT}), TAB_ID_CALIBRATION, m_calibration_page,
                                    _L("Calibration"), "tab_calibration_active");
         }
 
         if (want_web_device_tab) {
-            if ((idx = m_tabpanel->FindPage(m_printer_view)) == wxNOT_FOUND) {
-                m_printer_view->Show(false);
+            if ((idx = m_tabpanel->FindPage(m_printer_view_page)) == wxNOT_FOUND) {
+                m_printer_view_page->Show(false);
                 // Immediately right of the native Device tab, not at the end of the tab bar.
                 m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_MONITOR}), TAB_ID_MONITOR_WEB,
-                                       m_printer_view, _L("Device (Web)"), "tab_monitor_active");
+                                       m_printer_view_page, _L("Device (Web)"), "tab_monitor_active");
             } else {
                 m_tabpanel->SetPageText(idx, _L("Device (Web)"));
             }
@@ -1567,48 +1537,37 @@ void MainFrame::show_device(bool should_use_native) {
 
         fit_tab_labels(); // ORCA on printer change
         m_plugin_pages.relayout(); // re-sync plugin tabs against the native tabs just mutated above
-
+        if (m_prebuild_started)
+            m_idle.start();
         return;
     }
 
     if (should_use_native) {
-        if (m_tabpanel->FindPage(m_monitor) != wxNOT_FOUND) {
+        if (m_monitor_page->in_book()) {
             fit_tab_labels(); // ORCA on printer change - same button layout
             return;
         }
         // Remove printer view
-        if ((idx = m_tabpanel->FindPage(m_printer_view)) != wxNOT_FOUND) {
-            m_printer_view->Show(false);
+        if ((idx = m_tabpanel->FindPage(m_printer_view_page)) != wxNOT_FOUND) {
+            m_printer_view_page->Show(false);
             m_tabpanel->RemovePage(idx);
         }
 
-        // Create/insert monitor page
-        if (!m_monitor) {
-            m_monitor = new MonitorPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-            m_monitor->SetBackgroundColour(*wxWHITE);
-        }
-        m_monitor->Show(false);
-        m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PREVIEW}), TAB_ID_MONITOR, m_monitor,
+        // Insert monitor page
+        m_monitor_page->Show(false);
+        m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PREVIEW}), TAB_ID_MONITOR, m_monitor_page,
                                _L("Device"), "tab_monitor_active");
 
         if (wxGetApp().is_enable_multi_machine()) {
-            if (!m_multi_machine) {
-                m_multi_machine = new MultiMachinePage(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-                m_multi_machine->SetBackgroundColour(*wxWHITE);
-            }
             // TODO: change the bitmap
-            m_multi_machine->Show(false);
-            m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_MONITOR}), TAB_ID_MULTI_DEVICE, m_multi_machine,
+            m_multi_machine_page->Show(false);
+            m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_MONITOR}), TAB_ID_MULTI_DEVICE, m_multi_machine_page,
                                    _L("Multi-device"), "tab_multi_active");
         }
-        if (!m_calibration) {
-            m_calibration = new CalibrationPanel(m_tabpanel, wxID_ANY, wxDefaultPosition, wxDefaultSize);
-            m_calibration->SetBackgroundColour(*wxWHITE);
-        }
-        m_calibration->Show(false);
+        m_calibration_page->Show(false);
         // Last of the built-in tabs, but plugin tabs already sit past it — anchor rather than
         // append, so its position doesn't depend on the relayout() below running afterwards.
-        m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PROJECT}), TAB_ID_CALIBRATION, m_calibration,
+        m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PROJECT}), TAB_ID_CALIBRATION, m_calibration_page,
                                _L("Calibration"), "tab_calibration_active");
 
 #ifdef _MSW_DARK_MODE
@@ -1616,37 +1575,30 @@ void MainFrame::show_device(bool should_use_native) {
 #endif // _MSW_DARK_MODE
 
     } else {
-        if (m_tabpanel->FindPage(m_printer_view) != wxNOT_FOUND) {
+        if (m_printer_view_page->in_book()) {
             fit_tab_labels(); // ORCA on printer change - same button layout
             return;
         }
-        if ((idx = m_tabpanel->FindPage(m_calibration)) != wxNOT_FOUND) {
-            m_calibration->Show(false);
+        if ((idx = m_tabpanel->FindPage(m_calibration_page)) != wxNOT_FOUND) {
+            m_calibration_page->Show(false);
             m_tabpanel->RemovePage(idx);
         }
-        if ((idx = m_tabpanel->FindPage(m_multi_machine)) != wxNOT_FOUND) {
-            m_multi_machine->Show(false);
+        if ((idx = m_tabpanel->FindPage(m_multi_machine_page)) != wxNOT_FOUND) {
+            m_multi_machine_page->Show(false);
             m_tabpanel->RemovePage(idx);
         }
-        if ((idx = m_tabpanel->FindPage(m_monitor)) != wxNOT_FOUND) {
-            m_monitor->Show(false);
+        if ((idx = m_tabpanel->FindPage(m_monitor_page)) != wxNOT_FOUND) {
+            m_monitor_page->Show(false);
             m_tabpanel->RemovePage(idx);
         }
-        if (m_printer_view == nullptr) {
-            m_printer_view = new PrinterWebView(m_tabpanel);
-            Bind(EVT_LOAD_PRINTER_URL, [this](LoadPrinterViewEvent& evt) {
-                wxString url = evt.GetString();
-                wxString key = evt.GetAPIkey();
-                // select_tab(MainFrame::tpMonitor);
-                m_printer_view->load_url(url, key);
-            });
-        }
-        m_printer_view->Show(false);
-        m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PREVIEW}), TAB_ID_MONITOR, m_printer_view,
+        m_printer_view_page->Show(false);
+        m_tabpanel->InsertPage(m_tabpanel->PositionAfter({TAB_ID_PREVIEW}), TAB_ID_MONITOR, m_printer_view_page,
                                _L("Device"), "tab_monitor_active");
     }
     fit_tab_labels(); // ORCA on printer change
     m_plugin_pages.relayout(); // re-sync plugin tabs against the native tabs just mutated above
+    if (m_prebuild_started)
+        m_idle.start();
 }
 
 bool MainFrame::is_prepare_or_preview_tab() const
@@ -2692,13 +2644,11 @@ void MainFrame::on_dpi_changed(const wxRect& suggested_rect)
     //BBS GUI refactor: remove unused layout new/dlg
     //if (m_layout != ESettingsLayout::Dlg) // Do not update tabs if the Settings are in the separated dialog
     m_param_panel->msw_rescale();
-    m_project->msw_rescale();
-    if(m_monitor)
-        m_monitor->msw_rescale();
-    if(m_multi_machine)
-        m_multi_machine->msw_rescale();
-    if(m_calibration)
-        m_calibration->msw_rescale();
+    // A panel mid-build gets the pass once it is complete.
+    ProjectPanel::when_built([](ProjectPanel& project) { project.msw_rescale(); });
+    MonitorPanel::when_built([](MonitorPanel& monitor) { monitor.msw_rescale(); });
+    MultiMachinePage::when_built([](MultiMachinePage& multi_machine) { multi_machine.msw_rescale(); });
+    CalibrationPanel::when_built([](CalibrationPanel& calibration) { calibration.msw_rescale(); });
 
     // BBS
 #if 0
@@ -2752,7 +2702,8 @@ void MainFrame::on_sys_color_changed()
 #endif
 #endif
 
-    diff_dialog.on_sys_color_changed();
+    if (DiffPresetDialog* dialog = DiffPresetDialog::if_built())
+        dialog->on_sys_color_changed();
 
     // BBS
     m_tabpanel->Rescale();
@@ -2760,10 +2711,8 @@ void MainFrame::on_sys_color_changed()
 
     // update Plater
     wxGetApp().plater()->sys_color_changed();
-    if(m_monitor)
-        m_monitor->on_sys_color_changed();
-    if(m_calibration)
-        m_calibration->on_sys_color_changed();
+    MonitorPanel::when_built([](MonitorPanel& monitor) { monitor.on_sys_color_changed(); });
+    CalibrationPanel::when_built([](CalibrationPanel& calibration) { calibration.on_sys_color_changed(); });
     // update Tabs
     for (auto tab : wxGetApp().tabs_list)
         tab->sys_color_changed();
@@ -3696,7 +3645,8 @@ void MainFrame::set_max_recent_count(int max)
         }
         wxGetApp().app_config->set_recent_projects(recent_projects);
         wxGetApp().app_config->save();
-        m_webview->SendRecentList(-1);
+        if (WebViewPanel* home = WebViewPanel::if_built())
+            home->SendRecentList(-1);
     }
 }
 
@@ -4040,23 +3990,56 @@ void MainFrame::select_tab(wxPanel* panel)
     select_tab(page_name);
 }
 
+// Selects the Prepare page without the page-changed event, so the GL canvas is on screen
+// and nothing else is built for the pass.
+void MainFrame::select_prepare_for_gl_init()
+{
+    m_tabpanel->ChangeSelection(m_tabpanel->FindPageByName(TAB_ID_PREPARE));
+}
+
+// The book shows its first page as it is inserted, while the frame is hidden and nothing
+// may build; the first show completes that page.
+bool MainFrame::Show(bool show)
+{
+    const bool changed = DPIFrame::Show(show);
+    if (show && changed && m_tabpanel != nullptr)
+        if (wxWindow* page = m_tabpanel->GetCurrentPage())
+            page->Show(true);
+    return changed;
+}
+
+// A page out of the book stays registered and is passed over; a negative order is never
+// registered.
+void MainFrame::prebuild_pages_when_idle()
+{
+    m_idle.clear();
+    if (m_param_panel)
+        m_idle.add(m_param_panel->settings_page_prebuild());
+    for (LazyBase* page : m_lazy_pages)
+        if (page->prebuild_order() >= 0)
+            m_idle.add(*page);
+    m_idle.add(m_diff_dialog);
+    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": prebuild queue: " << m_idle.names();
+    m_idle.start();
+    m_prebuild_started = true;
+}
+
 //BBS
 void MainFrame::jump_to_monitor(std::string dev_id)
 {
-    if(!m_monitor)
-        return;
     m_tabpanel->SelectPageByName(TAB_ID_MONITOR);
     if (!dev_id.empty()) {
-        ((MonitorPanel*)m_monitor)->select_machine(dev_id);
+        MonitorPanel::ensure()->select_machine(dev_id);
     }
 }
 
 void MainFrame::jump_to_multipage()
 {
-    if(!m_multi_machine)
+    if (!m_multi_machine_page->in_book())
         return;
     m_tabpanel->SelectPageByName(TAB_ID_MULTI_DEVICE);
-    ((MultiMachinePage*)m_multi_machine)->jump_to_send_page();
+    if (MultiMachinePage* page = m_multi_machine_page->ensure())
+        page->jump_to_send_page();
 }
 
 
@@ -4105,8 +4088,8 @@ void MainFrame::request_select_tab(const wxString& id)
 }
 
 int MainFrame::get_calibration_curr_tab() {
-    if (m_calibration)
-        return m_calibration->get_tabpanel()->GetSelection();
+    if (CalibrationPanel* calibration = CalibrationPanel::if_built())
+        return calibration->get_tabpanel()->GetSelection();
     return -1;
 }
 
@@ -4215,7 +4198,8 @@ void MainFrame::add_to_recent_projects(const wxString& filename)
             recent_projects.push_back(into_u8(m_recent_projects.GetHistoryFile(i)));
         }
         wxGetApp().app_config->set_recent_projects(recent_projects);
-        m_webview->SendRecentList(0);
+        if (WebViewPanel* home = WebViewPanel::if_built())
+            home->SendRecentList(0);
     }
 }
 
@@ -4331,7 +4315,8 @@ void MainFrame::open_recent_project(size_t file_id, wxString const & filename)
                 recent_projects.push_back(into_u8(m_recent_projects.GetHistoryFile(i)));
             }
             wxGetApp().app_config->set_recent_projects(recent_projects);
-            m_webview->SendRecentList(-1);
+            if (WebViewPanel* home = WebViewPanel::if_built())
+                home->SendRecentList(-1);
         }
     }
 }
@@ -4354,7 +4339,8 @@ void MainFrame::remove_recent_project(size_t file_id, wxString const &filename)
         recent_projects.push_back(into_u8(m_recent_projects.GetHistoryFile(i)));
     }
     wxGetApp().app_config->set_recent_projects(recent_projects);
-    m_webview->SendRecentList(-1);
+    if (WebViewPanel* home = WebViewPanel::if_built())
+        home->SendRecentList(-1);
 }
 
 void MainFrame::load_url(wxString url)
@@ -4408,14 +4394,14 @@ bool MainFrame::is_printer_view() const { return m_tabpanel->GetSelectedPageName
 
 void MainFrame::refresh_plugin_tips()
 {
-    if (m_webview != nullptr)
-        m_webview->ShowNetpluginTip();
+    if (WebViewPanel* home = WebViewPanel::if_built())
+        home->ShowNetpluginTip();
 }
 
 void MainFrame::RunScript(wxString js)
 {
-    if (m_webview != nullptr)
-        m_webview->RunScript(js);
+    if (WebViewPanel* home = WebViewPanel::if_built())
+        home->RunScript(js);
 }
 
 void MainFrame::technology_changed()
@@ -4524,7 +4510,8 @@ void MainFrame::update_side_preset_ui()
 
 
     //take off multi machine
-    if(m_multi_machine){m_multi_machine->clear_page();}
+    if (MultiMachinePage* multi_machine = MultiMachinePage::if_built())
+        multi_machine->clear_page();
 }
 
 void MainFrame::on_select_default_preset(SimpleEvent& evt)

src/slic3r/GUI/MainFrame.hpp

@@ -26,6 +26,8 @@
 #include "Widgets/SideButton.hpp"
 #include "Widgets/SideMenuPopup.hpp"
 #include "FilamentGroupPopup.hpp"
+#include "LazyPage.hpp"
+#include "IdleScheduler.hpp"
 
 
 #include <boost/property_tree/ptree_fwd.hpp>
@@ -136,6 +138,13 @@ class MainFrame : public DPIFrame
 #endif
     bool     m_loaded {false};
     wxTimer* m_reset_title_text_colour_timer{ nullptr };
+    IdleScheduler         m_idle;
+    bool                  m_prebuild_started{ false };
+    // Every LazyPage, in and out of the book; prebuild_pages_when_idle() registers them.
+    std::vector<LazyBase*> m_lazy_pages;
+    // The latest EVT_LOAD_PRINTER_URL, applied when the web Device view is built.
+    wxString              m_printer_url;
+    wxString              m_printer_api_key;
 
     wxString    m_qs_last_input_file = wxEmptyString;
     wxString    m_qs_last_output_file = wxEmptyString;
@@ -180,7 +189,7 @@ class MainFrame : public DPIFrame
     bool can_delete() const;
     bool can_delete_all() const;
     bool can_reslice() const;
-    void bind_diff_dialog();
+    DiffPresetDialog* make_diff_dialog();
 
     // BBS
     wxBoxSizer* create_side_tools();
@@ -377,6 +386,12 @@ public:
     void        select_tab(wxPanel* panel);
     void        select_tab(const wxString& id = wxString());
     void        request_select_tab(const wxString& id);
+    // post_init() needs the plater's canvas on screen to initialize OpenGL; this pass does not
+    // build the settings page.
+    void        select_prepare_for_gl_init();
+    // Builds the lazy tab pages while the user is idle; post_init() calls it once.
+    void        prebuild_pages_when_idle();
+    bool        Show(bool show = true) override;
     int         get_calibration_curr_tab();
     void        select_view(const std::string& direction);
     void        update_shortcut_labels();
@@ -435,27 +450,21 @@ public:
     BBLTopbar*            m_topbar{ nullptr };
     PrintHostQueueDialog* printhost_queue_dlg() { return m_printhost_queue_dlg; }
     Plater*               m_plater { nullptr };
+    // Lazy pages, created once and kept for the frame's life; their panels are reached
+    // through LazyInstance's statics, and show_device() only moves pages in and out of the book.
 #ifdef SLIC3R_CAD
-    // The tab page is the placeholder; m_design_panel stays null until the tab is first
-    // selected, so everything the Design panel builds stays off the startup path.
-    wxPanel*              m_design_page { nullptr };
-    DesignPanel*          m_design_panel { nullptr };
-    // Builds the Design panel if it does not exist yet and returns it (null only before the
-    // placeholder page itself exists). Main thread only -- it creates wx controls. Both the
-    // tab activation and the MCP socket go through this: the socket is driven headlessly,
-    // with nobody to click the tab, and without this every verb would answer "not ready".
-    DesignPanel*          ensure_design_panel();
+    LazyPage<DesignPanel>* m_design_page { nullptr };
 #endif
     //BBS: GUI refactor
-    MonitorPanel*         m_monitor{ nullptr };
+    LazyPage<MonitorPanel>* m_monitor_page{ nullptr };
 
     //AuxiliaryPanel*       m_auxiliary{ nullptr };
-    MultiMachinePage*     m_multi_machine{ nullptr };
-    ProjectPanel*         m_project{ nullptr };
+    LazyPage<MultiMachinePage>* m_multi_machine_page{ nullptr };
+    LazyPage<ProjectPanel>* m_project_page{ nullptr };
 
-    CalibrationPanel*     m_calibration{ nullptr };
-    WebViewPanel*         m_webview { nullptr };
-    PrinterWebView*       m_printer_view{nullptr};
+    LazyPage<CalibrationPanel>* m_calibration_page{ nullptr };
+    LazyPage<WebViewPanel>* m_home_page { nullptr };
+    LazyPage<PrinterWebView>* m_printer_view_page{nullptr};
     PluginPages           m_plugin_pages;
     wxLogWindow*          m_log_window { nullptr };
     // BBS
@@ -466,7 +475,8 @@ public:
     ParamsDialog*         m_param_dialog{ nullptr };
     //BBS
     SettingsDialog        m_settings_dialog;
-    DiffPresetDialog      diff_dialog;
+    // The Compare presets dialog, built on first use or at idle through its holder.
+    Lazy<DiffPresetDialog> m_diff_dialog;
     wxWindow*             m_plater_page{ nullptr };
     PrintHostQueueDialog* m_printhost_queue_dlg;
 

src/slic3r/GUI/Monitor.cpp

@@ -103,6 +103,7 @@ MonitorPanel::MonitorPanel(wxWindow* parent, wxWindowID id, const wxPoint& pos,
     : wxPanel(parent, id, pos, size, style),
     m_select_machine(SelectMachinePopup(this))
 {
+    SetBackgroundColour(*wxWHITE);
 #ifdef __WINDOWS__
     SetDoubleBuffered(true);
 #endif //__WINDOWS__
@@ -180,31 +181,36 @@ void MonitorPanel::init_tabpanel()
             auto title = m_tabpanel->GetPageText(m_tabpanel->GetSelection());
             m_media_file_panel->SwitchStorage(title == _L("Storage"));
         }
-        page->SetFocus();
+        // The first page is selected while the panel is built off screen.
+        if (page->IsShownOnScreen())
+            page->SetFocus();
         update_all();
         }, m_tabpanel->GetId());
 
-    //m_status_add_machine_panel = new AddMachinePanel(m_tabpanel);
-    m_status_info_panel        = new StatusPanel(m_tabpanel);
+    m_status_info_panel = new StatusPanel(m_tabpanel);
+    // Queued before the page is added, since adding it selects it and runs the handler above,
+    // where built() must already be false.
+    add_build_steps_of(*m_status_info_panel);
     m_tabpanel->AddPage(m_status_info_panel, _L("Status"), true);
+    add_build_step([this] {
+        m_media_file_panel = new MediaFilePanel(m_tabpanel);
+        m_tabpanel->AddPage(m_media_file_panel, _L("Storage"), false);
+    });
+    add_build_step([this] {
+        m_upgrade_panel = new UpgradePanel(m_tabpanel);
+        m_tabpanel->AddPage(m_upgrade_panel, _L_CONTEXT(L_CONTEXT("Update", "Firmware"), "Firmware"), false);
+    });
+    add_build_step([this] {
+        m_hms_panel = new HMSPanel(m_tabpanel);
+        m_tabpanel->AddPage(m_hms_panel, _L("Assistant(HMS)"), false);
 
-    m_media_file_panel = new MediaFilePanel(m_tabpanel);
-    m_tabpanel->AddPage(m_media_file_panel, _L("Storage"), false);
-    //m_tabpanel->AddPage(m_media_file_panel, _L("Internal Storage"), false);
+        std::string network_ver = Slic3r::NetworkAgent::get_version();
+        if (!network_ver.empty()) {
+            m_tabpanel->SetFooterText(wxString::Format(_L("Network plug-in v%s"), network_ver));
+        }
 
-    m_upgrade_panel = new UpgradePanel(m_tabpanel);
-    m_tabpanel->AddPage(m_upgrade_panel, _L_CONTEXT(L_CONTEXT("Update", "Firmware"), "Firmware"), false);
-
-    m_hms_panel = new HMSPanel(m_tabpanel);
-    m_tabpanel->AddPage(m_hms_panel, _L("Assistant(HMS)"),    false);
-
-    std::string network_ver = Slic3r::NetworkAgent::get_version();
-    if (!network_ver.empty()) {
-        m_tabpanel->SetFooterText(wxString::Format(_L("Network plug-in v%s"), network_ver));
-    }
-
-    m_initialized = true;
-    show_status((int)MonitorStatus::MONITOR_NO_PRINTER);
+        show_status((int)MonitorStatus::MONITOR_NO_PRINTER);
+    });
 }
 
 void MonitorPanel::set_default()
@@ -288,19 +294,23 @@ void MonitorPanel::on_select_printer(wxCommandEvent& event)
     set_default();
     update_all();
 
-    MachineObject *obj_ = dev->get_selected_machine();
-    if (obj_) {
-        obj_->last_cali_version = -1;
-        obj_->reset_pa_cali_history_result();
-        obj_->reset_pa_cali_result();
-        Sidebar &sidebar = GUI::wxGetApp().sidebar();
-        sidebar.update_sync_status(obj_);
-        sidebar.set_need_auto_sync_after_connect_printer(sidebar.need_auto_sync_extruder_list_after_connect_priner(obj_));
-    }
+    on_machine_selected(dev->get_selected_machine());
 
     Layout();
 }
 
+void MonitorPanel::on_machine_selected(MachineObject* obj)
+{
+    if (obj == nullptr)
+        return;
+    obj->last_cali_version = -1;
+    obj->reset_pa_cali_history_result();
+    obj->reset_pa_cali_result();
+    Sidebar& sidebar = GUI::wxGetApp().sidebar();
+    sidebar.update_sync_status(obj);
+    sidebar.set_need_auto_sync_after_connect_printer(sidebar.need_auto_sync_extruder_list_after_connect_priner(obj));
+}
+
 void MonitorPanel::on_printer_clicked(wxMouseEvent &event)
 {
     auto mouse_pos = ClientToScreen(event.GetPosition());
@@ -331,7 +341,8 @@ void MonitorPanel::on_size(wxSizeEvent &event)
 
 void MonitorPanel::update_all()
 {
-    if (!m_initialized)
+    // Every page exists once the last build step has run.
+    if (!built())
         return;
 
     NetworkAgent* m_agent = wxGetApp().getAgent();
@@ -412,16 +423,12 @@ void MonitorPanel::update_hms_tag()
 
 bool MonitorPanel::Show(bool show)
 {
-#ifdef __APPLE__
-    // Notebook::InsertPage() hides every page it appends, so this also runs while MainFrame is
-    // still constructing, before GUI_App::mainframe is assigned. Same guard as Plater::Show().
-    if (wxGetApp().mainframe)
-        wxGetApp().mainframe->SetMinSize(wxGetApp().plater()->GetMinSize());
-#endif
-
     NetworkAgent* m_agent = wxGetApp().getAgent();
     DeviceManager* dev = Slic3r::GUI::wxGetApp().getDeviceManager();
     if (show) {
+#ifdef __APPLE__
+        wxGetApp().mainframe->SetMinSize(wxGetApp().plater()->GetMinSize());
+#endif
         start_update();
         update_network_version_footer();
 
@@ -448,7 +455,7 @@ bool MonitorPanel::Show(bool show)
 
 void MonitorPanel::show_status(int status)
 {
-    if (!m_initialized) return;
+    if (!built()) return;
     if (last_status == status)return;
     if ((last_status & (int)MonitorStatus::MONITOR_CONNECTING) != 0) {
         NetworkAgent* agent = wxGetApp().getAgent();

src/slic3r/GUI/Monitor.hpp

@@ -49,6 +49,7 @@
 #include "slic3r/GUI/AmsWidgets.hpp"
 #include "Widgets/SideTools.hpp"
 #include "SelectMachinePop.hpp"
+#include "Lazy.hpp"
 
 namespace Slic3r {
 namespace GUI {
@@ -72,16 +73,16 @@ public:
 	void msw_rescale();
 };
 
-class MonitorPanel : public wxPanel
+class MonitorPanel : public wxPanel, public StagedBuild, public LazyInstance<MonitorPanel>
 {
 private:
     Tabbook*		m_tabpanel{ nullptr };
     wxSizer*        m_main_sizer{ nullptr };
 
-    StatusPanel*        m_status_info_panel;
-    MediaFilePanel*     m_media_file_panel;
-    UpgradePanel*       m_upgrade_panel;
-    HMSPanel*           m_hms_panel;
+    StatusPanel*        m_status_info_panel{ nullptr };
+    MediaFilePanel*     m_media_file_panel{ nullptr };
+    UpgradePanel*       m_upgrade_panel{ nullptr };
+    HMSPanel*           m_hms_panel{ nullptr };
 
 	/* side tools */
     SideTools*      m_side_tools{nullptr};
@@ -96,7 +97,6 @@ private:
     wxBitmap m_arrow_img;
 
     int last_status;
-    bool m_initialized { false };
     bool update_flag{false};
     wxTimer* m_refresh_timer = nullptr;
 
@@ -125,6 +125,8 @@ public:
 
     StatusPanel* get_status_panel() {return m_status_info_panel;};
 	void select_machine(std::string machine_sn);
+    // Resets the selected printer's calibration results and syncs the sidebar, without the Device tab.
+    static void on_machine_selected(MachineObject* obj);
     void on_timer(wxTimerEvent& event);
     void on_select_printer(wxCommandEvent& event);
     void on_printer_clicked(wxMouseEvent &event);

src/slic3r/GUI/MultiMachineManagerPage.cpp

@@ -21,8 +21,9 @@ MultiMachineItem::MultiMachineItem(wxWindow* parent, MachineObject* obj)
     Bind(wxEVT_MOTION, &MultiMachineItem::OnMove, this);
     Bind(EVT_MULTI_DEVICE_VIEW, [obj](auto& e) {
         wxGetApp().mainframe->jump_to_monitor(obj->get_dev_id());
-        if (wxGetApp().mainframe->m_monitor->get_status_panel()->get_media_play_ctrl()) {
-            wxGetApp().mainframe->m_monitor->get_status_panel()->get_media_play_ctrl()->jump_to_play();
+        MonitorPanel* monitor = MonitorPanel::if_built();
+        if (monitor && monitor->get_status_panel()->get_media_play_ctrl()) {
+            monitor->get_status_panel()->get_media_play_ctrl()->jump_to_play();
         }
     });
     wxGetApp().UpdateDarkUIWin(this);

src/slic3r/GUI/MultiMachinePage.cpp

@@ -11,6 +11,7 @@ namespace GUI {
 MultiMachinePage::MultiMachinePage(wxWindow* parent, wxWindowID id, const wxPoint& pos, const wxSize& size, long style)
     : wxPanel(parent, id, pos, size, style)
 {
+    SetBackgroundColour(*wxWHITE);
     init_tabpanel();
     m_main_sizer = new wxBoxSizer(wxHORIZONTAL);
     m_main_sizer->Add(m_tabpanel, 1, wxEXPAND | wxLEFT, 0);

src/slic3r/GUI/MultiMachinePage.hpp

@@ -7,6 +7,7 @@
 #include "MultiTaskManagerPage.hpp"
 #include "MultiMachineManagerPage.hpp"
 #include "Tabbook.hpp"
+#include "Lazy.hpp"
 
 #include "wx/button.h"
 
@@ -19,7 +20,7 @@ namespace GUI {
 #define PICK_LEFT_DEV_STATUS 250
 #define PICK_DEVICE_MAX 6
     
-class MultiMachinePage : public wxPanel
+class MultiMachinePage : public wxPanel, public LazyInstance<MultiMachinePage>
 {
 private:
     wxTimer*                    m_refresh_timer      = nullptr;

src/slic3r/GUI/ParamsPanel.cpp

@@ -290,7 +290,7 @@ ParamsPanel::ParamsPanel( wxWindow* parent, wxWindowID id, const wxPoint& pos, c
 
         m_compare_btn = new ScalableButton(m_top_panel, wxID_ANY, "compare", wxEmptyString, wxDefaultSize, wxDefaultPosition, wxBU_EXACTFIT | wxNO_BORDER, true);
         m_compare_btn->SetToolTip(_L("Compare presets"));
-        m_compare_btn->Bind(wxEVT_BUTTON, ([](wxCommandEvent e) { wxGetApp().mainframe->diff_dialog.show(); }));
+        m_compare_btn->Bind(wxEVT_BUTTON, ([](wxCommandEvent e) { DiffPresetDialog::ensure()->show(); }));
 
         m_setting_btn = new ScalableButton(m_top_panel, wxID_ANY, "table", wxEmptyString, wxDefaultSize, wxDefaultPosition, wxBU_EXACTFIT | wxNO_BORDER, true);
         m_setting_btn->SetToolTip(_L("View all object's settings"));
@@ -550,18 +550,36 @@ void ParamsPanel::clear_page()
 void ParamsPanel::OnActivate()
 {
     if (m_current_tab == NULL)
-    {
-        //the first time
-        BOOST_LOG_TRIVIAL(debug) << __FUNCTION__ << boost::format(": first time opened, set current tab to print");
-        // BBS: open/close tab
-        //m_current_tab = m_tab_print;
-        set_active_tab(m_tab_print ? m_tab_print : m_tab_filament);
-    }
+        select_default_tab();
     Tab* cur_tab = dynamic_cast<Tab *> (m_current_tab);
     if (cur_tab)
         cur_tab->OnActivate();
 }
 
+void ParamsPanel::select_default_tab()
+{
+    BOOST_LOG_TRIVIAL(debug) << __FUNCTION__ << boost::format(": first time opened, set current tab to print");
+    // BBS: open/close tab
+    //m_current_tab = m_tab_print;
+    set_active_tab(m_tab_print ? m_tab_print : m_tab_filament);
+}
+
+bool ParamsPanel::SettingsPagePrebuild::built() const
+{
+    Tab* tab = dynamic_cast<Tab*>(m_panel.m_current_tab);
+    return tab != nullptr && !tab->page_build_pending();
+}
+
+bool ParamsPanel::SettingsPagePrebuild::build_step()
+{
+    if (m_panel.m_current_tab == nullptr) {
+        m_panel.select_default_tab();
+        return !built();
+    }
+    Tab* tab = dynamic_cast<Tab*>(m_panel.m_current_tab);
+    return tab != nullptr && tab->page_build_step();
+}
+
 void ParamsPanel::OnToggled(wxCommandEvent& event)
 {
     if (m_mode_region && m_mode_region->GetId() == event.GetId()) {

src/slic3r/GUI/ParamsPanel.hpp

@@ -30,6 +30,7 @@
 #include "wxExtensions.hpp"
 #include "GUI_Utils.hpp"
 #include "Widgets/Button.hpp"
+#include "Lazy.hpp"
 
 class ModeSwitchButton;
 class SwitchButton;
@@ -121,6 +122,22 @@ class ParamsPanel : public wxPanel
 
         wxPanel* m_current_tab { nullptr };
 
+        // Builds the selected page's option groups at idle; while no tab is selected yet,
+        // its first unit selects the default one.
+        class SettingsPagePrebuild : public LazyBase
+        {
+        public:
+            explicit SettingsPagePrebuild(ParamsPanel& panel) : m_panel(panel) {}
+            const std::string& name() const override { return m_name; }
+            bool               built() const override;
+            bool               build_step() override;
+            int                prebuild_order() const override { return 0; }
+
+        private:
+            ParamsPanel& m_panel;
+            std::string  m_name{ "settings_page" };
+        } m_settings_page_prebuild{ *this };
+
         bool m_has_object_config { false };
 
         struct Highlighter
@@ -149,6 +166,10 @@ class ParamsPanel : public wxPanel
         //clear the right page
         void clear_page();
         void OnActivate();
+        // The print tab, or the filament tab without one.
+        void select_default_tab();
+        // The settings page's prebuild task.
+        LazyBase& settings_page_prebuild() { return m_settings_page_prebuild; }
         void set_active_tab(wxPanel*tab);
         bool is_active_and_shown_tab(wxPanel*tab);
         void update_mode();

src/slic3r/GUI/Plater.cpp

@@ -10412,8 +10412,8 @@ void Plater::priv::reset(bool apply_presets_change, bool reload_presets)
     // Same reason, one level up: the Design tab keeps the editable document, not the Model, so
     // clearing the recipe alone leaves the tab showing the previous project's feature tree —
     // and its next edit syncs that tree straight back into the new project.
-    if (wxGetApp().mainframe != nullptr && wxGetApp().mainframe->m_design_panel != nullptr)
-        wxGetApp().mainframe->m_design_panel->clear_document();
+    if (DesignPanel* design = DesignPanel::if_built())
+        design->clear_document();
 #endif
     assemble_view->get_canvas3d()->reset_explosion_ratio();
     update();
@@ -13184,17 +13184,17 @@ void Plater::priv::on_tab_selection_changing(wxBookCtrlEvent& e)
         // Pointer test, not a name lookup: in printer-agents mode this page is TAB_ID_MONITOR_WEB
         // while the native Device tab holds TAB_ID_MONITOR, and in legacy-web mode it holds
         // TAB_ID_MONITOR itself.
-        const bool selecting_web_device_tab = main_frame->m_printer_view &&
-            main_frame->m_tabpanel->GetPage(new_sel) == main_frame->m_printer_view;
+        const bool selecting_web_device_tab = main_frame->m_printer_view_page &&
+            main_frame->m_tabpanel->GetPage(new_sel) == main_frame->m_printer_view_page;
         if (selecting_web_device_tab) {
             // Use the selected discovered machine when the preset has no host.
             main_frame->load_printer_url();
         } else if (new_name == TAB_ID_MONITOR && wxGetApp().preset_bundle != nullptr) {
             auto     cfg = wxGetApp().preset_bundle->printers.get_edited_preset().config;
             wxString url = from_u8(PrintHost::get_print_host_webui(&cfg));
-            if (main_frame->m_printer_view && url.empty()) {
+            if (PrinterWebView* view = PrinterWebView::if_built(); view != nullptr && url.empty()) {
                 // It's missing_connection page, reload so that we can replay the gif image
-                main_frame->m_printer_view->reload();
+                view->reload();
             }
         }
     }
@@ -13925,8 +13925,8 @@ void Plater::priv::unbind_canvas_event_handlers()
     // The Design tab's viewport is a fourth GLCanvas3D on the same shared GL context, owned by
     // MainFrame rather than by us — same reach as reset() uses for clear_document(). Null until
     // the tab has been opened once, so most sessions skip it.
-    if (wxGetApp().mainframe != nullptr && wxGetApp().mainframe->m_design_panel != nullptr)
-        wxGetApp().mainframe->m_design_panel->unbind_canvas_event_handlers();
+    if (DesignPanel* design = DesignPanel::if_built())
+        design->unbind_canvas_event_handlers();
 #endif
 }
 
@@ -13939,8 +13939,8 @@ void Plater::priv::reset_canvas_volumes()
         preview->get_canvas3d()->reset_volumes();
 
 #ifdef SLIC3R_CAD
-    if (wxGetApp().mainframe != nullptr && wxGetApp().mainframe->m_design_panel != nullptr)
-        wxGetApp().mainframe->m_design_panel->reset_canvas_volumes();
+    if (DesignPanel* design = DesignPanel::if_built())
+        design->reset_canvas_volumes();
 #endif
 }
 
@@ -20002,7 +20002,7 @@ int Plater::export_config_3mf(int plate_idx, Export3mfProgressFn proFn)
 void Plater::send_calibration_job_finished(wxCommandEvent & evt)
 {
     p->main_frame->request_select_tab(TAB_ID_CALIBRATION);
-    auto calibration_panel = p->main_frame->m_calibration;
+    CalibrationPanel* calibration_panel = CalibrationPanel::ensure();
     if (calibration_panel) {
         auto curr_wizard = static_cast<CalibrationWizard*>(calibration_panel->get_tabpanel()->GetPage(evt.GetInt()));
         wxCommandEvent event(EVT_CALIBRATION_JOB_FINISHED);
@@ -20034,8 +20034,8 @@ void Plater::print_job_finished(wxCommandEvent &evt)
 
     dev->set_selected_machine(evt.GetString().ToStdString());
     p->main_frame->request_select_tab(TAB_ID_MONITOR);
-    //jump to monitor and select device status panel
-    MonitorPanel* curr_monitor = p->main_frame->m_monitor;
+    // Selects the status page on a built Device tab; one built by the switch starts there.
+    MonitorPanel* curr_monitor = MonitorPanel::if_built();
     if(curr_monitor)
        curr_monitor->get_tabpanel()->ChangeSelection(MonitorPanel::PrinterTab::PT_STATUS);
 }
@@ -20775,8 +20775,8 @@ void Plater::update_print_error_info(int code, std::string msg, std::string extr
     if (p->m_send_to_sdcard_dlg) {
         p->m_send_to_sdcard_dlg->update_print_error_info(code, msg, extra);
     }
-    if (p->main_frame->m_calibration)
-        p->main_frame->m_calibration->update_print_error_info(code, msg, extra);
+    if (CalibrationPanel* calibration = CalibrationPanel::if_built())
+        calibration->update_print_error_info(code, msg, extra);
 }
 
 wxString Plater::get_project_filename(const wxString& extension) const
@@ -21069,7 +21069,8 @@ void Plater::pop_warning_and_go_to_device_page(wxString printer_name, PrinterWar
 {
     printer_name.Replace("Bambu Lab", "", false);
     wxString content;
-    bool device_page = (wxGetApp().mainframe == nullptr) && (wxGetApp().mainframe->m_monitor->IsShown());
+    MainFrame* frame       = wxGetApp().mainframe;
+    bool       device_page = frame != nullptr && frame->m_monitor_page->in_book();
     if (type == PrinterWarningType::NOT_CONNECTED) {
         if (device_page) {
             content = wxString::Format(_L("Printer not connected. Please go to the device page to connect %s before syncing."),

src/slic3r/GUI/PrebuildQueue.hpp

@@ -0,0 +1,89 @@
+#pragma once
+
+#include <algorithm>
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "Lazy.hpp"
+
+namespace Slic3r { namespace GUI {
+
+// Holders to build at idle, ordered by prebuild_order(), run one slice at a time. The
+// holders outlive the queue. Main thread only.
+class PrebuildQueue
+{
+public:
+    struct Slice
+    {
+        std::string name;                // the task that ran; empty when nothing was pending
+        int         units{ 0 };          // units run
+        long long   ms{ 0 };             // time spent
+        bool        completed{ false };  // the task has no more units
+        bool        remaining{ false };  // some task is still pending
+    };
+
+    // Lower order runs first, equal order in the order added.
+    void add(LazyBase& task)
+    {
+        auto after = std::find_if(m_tasks.begin(), m_tasks.end(), [&](const LazyBase* t) { return t->prebuild_order() > task.prebuild_order(); });
+        m_tasks.insert(after, &task);
+    }
+
+    void clear() { m_tasks.clear(); }
+
+    bool pending() const
+    {
+        return std::any_of(m_tasks.begin(), m_tasks.end(), [](const LazyBase* t) { return t->pending(); });
+    }
+
+    // The task names in queue order, comma separated.
+    std::string names() const
+    {
+        std::string out;
+        for (const LazyBase* t : m_tasks)
+            out += (out.empty() ? "" : ", ") + t->name();
+        return out;
+    }
+
+    // Runs units of the first pending task until it completes, budget_ms of now_ms() have
+    // passed, or interrupt() is true after a unit; on_unit gets the task name and each
+    // unit's duration. A task with no work is passed over and stays in the queue, so one
+    // whose work returns is pending again.
+    Slice run_slice(int                                                       budget_ms,
+                    const std::function<long long()>&                         now_ms,
+                    const std::function<bool()>&                              interrupt,
+                    const std::function<void(const std::string&, long long)>& on_unit = {})
+    {
+        Slice slice;
+        auto  next = std::find_if(m_tasks.begin(), m_tasks.end(), [](const LazyBase* t) { return t->pending(); });
+        if (next == m_tasks.end())
+            return slice;
+        // The pointer is copied out, since a unit may add tasks and reallocate m_tasks.
+        LazyBase* const task    = *next;
+        slice.name              = task->name();
+        const long long started = now_ms();
+        for (;;) {
+            const long long unit_started = now_ms();
+            const bool      more         = task->build_step();
+            const long long now          = now_ms();
+            ++slice.units;
+            if (on_unit)
+                on_unit(slice.name, now - unit_started);
+            slice.ms = now - started;
+            if (!more) {
+                slice.completed = true;
+                break;
+            }
+            if (slice.ms >= budget_ms || interrupt())
+                break;
+        }
+        slice.remaining = pending();
+        return slice;
+    }
+
+private:
+    std::vector<LazyBase*> m_tasks;
+};
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/Preferences.cpp

@@ -1044,8 +1044,8 @@ wxBoxSizer *PreferencesDialog::create_item_checkbox(wxString title, wxString too
             if (m_bambu_cloud_checkbox)      m_bambu_cloud_checkbox->Enable(!enabled);
         }
         else if (param == "hide_login_side_panel") {
-            if (wxGetApp().mainframe && wxGetApp().mainframe->m_webview) {
-                wxGetApp().mainframe->m_webview->SendCloudProvidersInfo();
+            if (WebViewPanel* home = WebViewPanel::if_built()) {
+                home->SendCloudProvidersInfo();
             }
         }
         // ORCA: apply the preview dimming change immediately to the currently loaded preview
@@ -1254,9 +1254,8 @@ wxBoxSizer *PreferencesDialog::create_item_bambu_cloud(wxString title, wxString
         app_config->save();
 
         // Update homepage visibility immediately
-        auto *mainframe = wxGetApp().mainframe;
-        if (mainframe && mainframe->m_webview)
-            mainframe->m_webview->SendCloudProvidersInfo();
+        if (WebViewPanel* home = WebViewPanel::if_built())
+            home->SendCloudProvidersInfo();
     });
 
     m_sizer->Add(cb, 0, wxALIGN_CENTER);

src/slic3r/GUI/PrinterWebView.hpp

@@ -26,6 +26,7 @@
 #include "wx/textctrl.h"
 #include <wx/timer.h>
 #include <memory>
+#include "Lazy.hpp"
 
 
 namespace Slic3r {
@@ -34,7 +35,7 @@ namespace GUI {
 class PrinterWebViewHandler;
 
 
-class PrinterWebView : public wxPanel {
+class PrinterWebView : public wxPanel, public LazyInstance<PrinterWebView> {
 public:
     PrinterWebView(wxWindow *parent);
     virtual ~PrinterWebView();

src/slic3r/GUI/Project.cpp

@@ -44,6 +44,7 @@ const std::vector<std::string> license_list = {
 
 ProjectPanel::ProjectPanel(wxWindow *parent, wxWindowID id, const wxPoint &pos, const wxSize &size, long style) : wxPanel(parent, id, pos, size, style)
 {
+    SetBackgroundColour(*wxWHITE);
     m_project_home_url = wxString::Format("file://%s/web/model/index.html", from_u8(resources_dir()));
     wxString strlang = wxGetApp().current_language_code_safe();
     if (strlang != "")
@@ -67,6 +68,7 @@ ProjectPanel::ProjectPanel(wxWindow *parent, wxWindowID id, const wxPoint &pos,
     m_auxiliary = new AuxiliaryPanel(this);
     m_auxiliary->Hide();
     main_sizer->Add(m_auxiliary, wxSizerFlags().Expand().Proportion(1));
+    add_build_steps_of(*m_auxiliary);
     Bind(EVT_AUXILIARY_DONE, [this](wxCommandEvent& e) { update_model_data();});
 
     SetSizer(main_sizer);

src/slic3r/GUI/Project.hpp

@@ -35,6 +35,7 @@
 #include "libslic3r/ProjectTask.hpp"
 #include "wxExtensions.hpp"
 #include "Auxiliary.hpp"
+#include "Lazy.hpp"
 
 #define AUFILE_GREY700 wxColour(107, 107, 107)
 #define AUFILE_GREY500 wxColour(158, 158, 158)
@@ -59,7 +60,7 @@ struct project_file{
     std::string size;
 };
 
-class ProjectPanel : public wxPanel
+class ProjectPanel : public wxPanel, public StagedBuild, public LazyInstance<ProjectPanel>
 {
 private:
     std::atomic<bool> m_web_init_completed{false};
@@ -90,7 +91,6 @@ public:
     void msw_rescale();
     void update_model_data();
     void clear_model_info();
-    void init_auxiliary() { m_auxiliary->init_auxiliary(); }
 
     bool Show(bool show);
     void OnScriptMessage(wxWebViewEvent& evt);

src/slic3r/GUI/SelectMachine.cpp

@@ -3470,7 +3470,7 @@ void SelectMachineDialog::navigate_to_timelapse_page()
         main_frame->jump_to_monitor();
 
         // then switch to Storage (Media) tab inside Monitor
-        auto* monitor = dynamic_cast<MonitorPanel*>(main_frame->m_monitor);
+        MonitorPanel* monitor = MonitorPanel::if_built();
         if (monitor) {
             auto* tabpanel = monitor->get_tabpanel();
             if (tabpanel) {

src/slic3r/GUI/StagedBuild.hpp

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <algorithm>
+#include <functional>
+#include <vector>
+
+namespace Slic3r { namespace GUI {
+
+// A panel whose constructor builds the skeleton and queues the rest as steps, run in order
+// on the main thread, one at a time.
+class StagedBuild
+{
+public:
+    virtual ~StagedBuild() = default;
+
+    bool built() const
+    {
+        return m_next_step == m_steps.size() &&
+               std::all_of(m_children.begin(), m_children.end(), [](const StagedBuild* child) { return child->built(); });
+    }
+
+    // Runs one step and returns true while more remain. Own steps first, then whatever a
+    // child queued after its steps were forwarded.
+    bool build_step()
+    {
+        if (m_next_step < m_steps.size()) {
+            // Copied out, since a step may queue more steps and reallocate m_steps.
+            auto step = std::move(m_steps[m_next_step++]);
+            step();
+        } else if (StagedBuild* child = unfinished_child()) {
+            child->build_step();
+        }
+        return !built();
+    }
+
+protected:
+    void add_build_step(std::function<void()> step) { m_steps.push_back(std::move(step)); }
+
+    // Queues one step per step the child has now, and keeps the child so that built() waits
+    // for any it queues later.
+    void add_build_steps_of(StagedBuild& child)
+    {
+        m_children.push_back(&child);
+        for (size_t i = child.m_next_step; i < child.m_steps.size(); ++i)
+            add_build_step([&child] { child.build_step(); });
+    }
+
+private:
+    StagedBuild* unfinished_child() const
+    {
+        auto it = std::find_if(m_children.begin(), m_children.end(), [](const StagedBuild* child) { return !child->built(); });
+        return it == m_children.end() ? nullptr : *it;
+    }
+
+    std::vector<std::function<void()>> m_steps;
+    std::vector<StagedBuild*>          m_children;
+    size_t                             m_next_step{ 0 };
+};
+
+}} // namespace Slic3r::GUI

src/slic3r/GUI/StatusPanel.cpp

@@ -1328,18 +1328,21 @@ StatusBasePanel::StatusBasePanel(wxWindow *parent, wxWindowID id, const wxPoint
 
     wxBoxSizer *bSizer_left = new wxBoxSizer(wxVERTICAL);
 
-    auto m_monitoring_sizer = create_monitoring_page();
-    bSizer_left->Add(m_monitoring_sizer, 1, wxEXPAND | wxALL, 0);
+    // The sizers are nested here so each step only appends to its own.
+    add_build_step([this, bSizer_left] {
+        auto m_monitoring_sizer = create_monitoring_page();
+        bSizer_left->Add(m_monitoring_sizer, 1, wxEXPAND | wxALL, 0);
 
-    auto m_panel_separotor1 = new wxPanel(this, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxTAB_TRAVERSAL);
-    m_panel_separotor1->SetBackgroundColour(STATUS_PANEL_BG);
-    m_panel_separotor1->SetMinSize(wxSize(-1, PAGE_SPACING));
-    m_panel_separotor1->SetMaxSize(wxSize(-1, PAGE_SPACING));
-    m_monitoring_sizer->Add(m_panel_separotor1, 0, wxEXPAND, 0);
+        auto m_panel_separotor1 = new wxPanel(this, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxTAB_TRAVERSAL);
+        m_panel_separotor1->SetBackgroundColour(STATUS_PANEL_BG);
+        m_panel_separotor1->SetMinSize(wxSize(-1, PAGE_SPACING));
+        m_panel_separotor1->SetMaxSize(wxSize(-1, PAGE_SPACING));
+        m_monitoring_sizer->Add(m_panel_separotor1, 0, wxEXPAND, 0);
 
-    m_project_task_panel = new PrintingTaskPanel(this, PrintingTaskType::PRINGINT);
-    m_project_task_panel->init_bitmaps();
-    m_monitoring_sizer->Add(m_project_task_panel, 0, wxALL | wxEXPAND , 0);
+        m_project_task_panel = new PrintingTaskPanel(this, PrintingTaskType::PRINGINT);
+        m_project_task_panel->init_bitmaps();
+        m_monitoring_sizer->Add(m_project_task_panel, 0, wxALL | wxEXPAND , 0);
+    });
 
 //    auto m_panel_separotor2 = new wxPanel(this, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxTAB_TRAVERSAL);
 //    m_panel_separotor2->SetBackgroundColour(STATUS_PANEL_BG);
@@ -1359,8 +1362,10 @@ StatusBasePanel::StatusBasePanel(wxWindow *parent, wxWindowID id, const wxPoint
     m_machine_ctrl_panel->SetDoubleBuffered(true);
     auto m_machine_control = create_machine_control_page(m_machine_ctrl_panel);
     m_machine_ctrl_panel->SetSizer(m_machine_control);
-    m_machine_ctrl_panel->Layout();
-    m_machine_control->Fit(m_machine_ctrl_panel);
+    add_build_step([this, m_machine_control] {
+        m_machine_ctrl_panel->Layout();
+        m_machine_control->Fit(m_machine_ctrl_panel);
+    });
 
     bSizer_status_below->Add(m_machine_ctrl_panel, 0, wxALL, 0);
 
@@ -1375,8 +1380,11 @@ StatusBasePanel::StatusBasePanel(wxWindow *parent, wxWindowID id, const wxPoint
     m_panel_separotor_bottom->SetBackgroundColour(STATUS_PANEL_BG);
 
     bSizer_status->Add(m_panel_separotor_bottom, 0, wxEXPAND | wxALL, 0);
-    this->SetSizerAndFit(bSizer_status);
-    this->Layout();
+    this->SetSizer(bSizer_status);
+    add_build_step([this, bSizer_status] {
+        bSizer_status->SetSizeHints(this);
+        this->Layout();
+    });
 }
 
 StatusBasePanel::~StatusBasePanel()
@@ -1614,16 +1622,24 @@ wxBoxSizer *StatusBasePanel::create_machine_control_page(wxWindow *parent)
 
     wxBoxSizer *bSizer_control = new wxBoxSizer(wxVERTICAL);
 
-    auto temp_axis_ctrl_sizer = create_temp_axis_group(parent);
-    auto m_filament_load_sizer = create_filament_group(parent);
-
+    // The slot sizers keep each group's place in bSizer_control.
+    wxBoxSizer *temp_axis_slot = new wxBoxSizer(wxVERTICAL);
+    wxBoxSizer *filament_slot  = new wxBoxSizer(wxVERTICAL);
     /* ams control box or live nozzle-rack panel (rack printers switch between the two) */
     wxSizer *ams_rack_sizer = new wxBoxSizer(wxHORIZONTAL);
-    ams_rack_sizer->Add(create_ams_group(parent), 0, wxEXPAND | wxLEFT);
 
-    m_panel_nozzle_rack = new wgtDeviceNozzleRack(parent);
-    m_panel_nozzle_rack->Show(false);
-    ams_rack_sizer->Add(m_panel_nozzle_rack, 0, wxEXPAND | wxLEFT);
+    add_build_step([this, parent, temp_axis_slot, filament_slot] {
+        temp_axis_slot->Add(create_temp_axis_group(parent), 0, wxEXPAND);
+        filament_slot->Add(create_filament_group(parent), 0, wxEXPAND);
+    });
+    add_build_step([this, parent, ams_rack_sizer] {
+        ams_rack_sizer->Add(create_ams_group(parent), 0, wxEXPAND | wxLEFT);
+    });
+    add_build_step([this, parent, ams_rack_sizer] {
+        m_panel_nozzle_rack = new wgtDeviceNozzleRack(parent);
+        m_panel_nozzle_rack->Show(false);
+        ams_rack_sizer->Add(m_panel_nozzle_rack, 0, wxEXPAND | wxLEFT);
+    });
 
     m_ams_rack_switch = new SwitchBoard(parent, _L("Filament"), _L("Hotends"), wxSize(FromDIP(126), FromDIP(26)));
     m_ams_rack_switch->updateState("left");
@@ -1631,12 +1647,12 @@ wxBoxSizer *StatusBasePanel::create_machine_control_page(wxWindow *parent)
     m_ams_rack_switch->Bind(wxCUSTOMEVT_SWITCH_POS, &StatusBasePanel::on_ams_rack_switch, this);
 
     bSizer_control->Add(0, 0, 0, wxTOP, FromDIP(8));
-    bSizer_control->Add(temp_axis_ctrl_sizer,   0, wxALIGN_CENTER|wxLEFT|wxRIGHT, FromDIP(8));
+    bSizer_control->Add(temp_axis_slot,         0, wxALIGN_CENTER|wxLEFT|wxRIGHT, FromDIP(8));
     bSizer_control->Add(m_ams_rack_switch,      0, wxALIGN_CENTRE|wxTOP, FromDIP(6));
     bSizer_control->Add(0, 0, 0, wxTOP, FromDIP(6));
     bSizer_control->Add(ams_rack_sizer,         0, wxALIGN_CENTER|wxLEFT|wxRIGHT, FromDIP(8));
     bSizer_control->Add(0, 0, 0, wxTOP, FromDIP(6));
-    bSizer_control->Add(m_filament_load_sizer,  0, wxALIGN_CENTER|wxLEFT|wxRIGHT, FromDIP(8));
+    bSizer_control->Add(filament_slot,          0, wxALIGN_CENTER|wxLEFT|wxRIGHT, FromDIP(8));
     bSizer_control->Add(0, 0, 0, wxTOP, FromDIP(4));
 
     bSizer_right->Add(bSizer_control, 1, wxEXPAND | wxALL, 0);
@@ -2228,8 +2244,10 @@ void StatusBasePanel::expand_filament_loading(wxMouseEvent& e)
     m_filament_step->Show(tag_show);
     Layout();
     Fit();
-    wxGetApp().mainframe->m_monitor->get_status_panel()->Layout();
-    wxGetApp().mainframe->m_monitor->Layout();
+    if (MonitorPanel* monitor = MonitorPanel::if_built()) {
+        monitor->get_status_panel()->Layout();
+        monitor->Layout();
+    }
 }
 
 void StatusBasePanel::show_ams_group(bool show)
@@ -2240,7 +2258,8 @@ void StatusBasePanel::show_ams_group(bool show)
         m_ams_control->Fit();
         Layout();
         Fit();
-        wxGetApp().mainframe->m_monitor->Layout();
+        if (MonitorPanel* monitor = MonitorPanel::if_built())
+            monitor->Layout();
     }
 
     // On rack printers, don't clobber the rack view when the user has the switch on "Hotends".
@@ -2253,7 +2272,8 @@ void StatusBasePanel::show_ams_group(bool show)
         m_ams_control->Fit();
         Layout();
         Fit();
-        wxGetApp().mainframe->m_monitor->Layout();
+        if (MonitorPanel* monitor = MonitorPanel::if_built())
+            monitor->Layout();
     }
 }
 
@@ -2277,8 +2297,10 @@ void StatusBasePanel::show_filament_load_group(bool show)
         Layout();
         Fit();
 
-        wxGetApp().mainframe->m_monitor->get_status_panel()->Layout();
-        wxGetApp().mainframe->m_monitor->Layout();
+        if (MonitorPanel* monitor = MonitorPanel::if_built()) {
+            monitor->get_status_panel()->Layout();
+            monitor->Layout();
+        }
     }
 }
 
@@ -2403,6 +2425,12 @@ void StatusPanel::update_camera_state(MachineObject* obj)
 
 StatusPanel::StatusPanel(wxWindow *parent, wxWindowID id, const wxPoint &pos, const wxSize &size, long style, const wxString &name)
     : StatusBasePanel(parent, id, pos, size, style)
+{
+    // Wires the controls the base class builds in steps, so it is the last step.
+    add_build_step([this] { wire_controls(); });
+}
+
+void StatusPanel::wire_controls()
 {
     init_scaled_buttons();
     m_buttons.push_back(m_bpButton_z_10);
@@ -2514,47 +2542,50 @@ StatusPanel::StatusPanel(wxWindow *parent, wxWindowID id, const wxPoint &pos, co
 
 StatusPanel::~StatusPanel()
 {
-    // Disconnect Events
-    m_project_task_panel->get_bitmap_thumbnail()->Disconnect(wxEVT_LEFT_DOWN, wxMouseEventHandler(StatusPanel::refresh_thumbnail_webrequest), NULL, this);
-    m_project_task_panel->get_partskip_button()->Disconnect(wxEVT_LEFT_DOWN, wxCommandEventHandler(StatusPanel::on_subtask_partskip), NULL, this);
-    m_project_task_panel->get_pause_resume_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_subtask_pause_resume), NULL, this);
-    m_project_task_panel->get_abort_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_subtask_abort), NULL, this);
-    m_project_task_panel->get_market_scoring_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_market_scoring), NULL, this);
-    m_project_task_panel->get_market_retry_buttom()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_market_retry), NULL, this);
-    m_project_task_panel->get_clean_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_print_error_clean), NULL, this);
+    // The controls only exist once the last build step has run.
+    if (built()) {
+        // Disconnect Events
+        m_project_task_panel->get_bitmap_thumbnail()->Disconnect(wxEVT_LEFT_DOWN, wxMouseEventHandler(StatusPanel::refresh_thumbnail_webrequest), NULL, this);
+        m_project_task_panel->get_partskip_button()->Disconnect(wxEVT_LEFT_DOWN, wxCommandEventHandler(StatusPanel::on_subtask_partskip), NULL, this);
+        m_project_task_panel->get_pause_resume_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_subtask_pause_resume), NULL, this);
+        m_project_task_panel->get_abort_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_subtask_abort), NULL, this);
+        m_project_task_panel->get_market_scoring_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_market_scoring), NULL, this);
+        m_project_task_panel->get_market_retry_buttom()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_market_retry), NULL, this);
+        m_project_task_panel->get_clean_button()->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_print_error_clean), NULL, this);
 
-    m_setting_button->Disconnect(wxEVT_LEFT_DOWN, wxMouseEventHandler(StatusPanel::on_camera_enter), NULL, this);
-    m_setting_button->Disconnect(wxEVT_LEFT_DCLICK, wxMouseEventHandler(StatusPanel::on_camera_enter), NULL, this);
-    m_tempCtrl_bed->Disconnect(wxEVT_KILL_FOCUS, wxFocusEventHandler(StatusPanel::on_bed_temp_kill_focus), NULL, this);
-    m_tempCtrl_bed->Disconnect(wxEVT_SET_FOCUS, wxFocusEventHandler(StatusPanel::on_bed_temp_set_focus), NULL, this);
-    m_tempCtrl_nozzle->Disconnect(wxEVT_KILL_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_kill_focus), NULL, this);
-    m_tempCtrl_nozzle->Disconnect(wxEVT_SET_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_set_focus), NULL, this);
+        m_setting_button->Disconnect(wxEVT_LEFT_DOWN, wxMouseEventHandler(StatusPanel::on_camera_enter), NULL, this);
+        m_setting_button->Disconnect(wxEVT_LEFT_DCLICK, wxMouseEventHandler(StatusPanel::on_camera_enter), NULL, this);
+        m_tempCtrl_bed->Disconnect(wxEVT_KILL_FOCUS, wxFocusEventHandler(StatusPanel::on_bed_temp_kill_focus), NULL, this);
+        m_tempCtrl_bed->Disconnect(wxEVT_SET_FOCUS, wxFocusEventHandler(StatusPanel::on_bed_temp_set_focus), NULL, this);
+        m_tempCtrl_nozzle->Disconnect(wxEVT_KILL_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_kill_focus), NULL, this);
+        m_tempCtrl_nozzle->Disconnect(wxEVT_SET_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_set_focus), NULL, this);
 
-    m_tempCtrl_nozzle_deputy->Disconnect(wxEVT_KILL_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_kill_focus), NULL, this);
-    m_tempCtrl_nozzle_deputy->Disconnect(wxEVT_SET_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_set_focus), NULL, this);
+        m_tempCtrl_nozzle_deputy->Disconnect(wxEVT_KILL_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_kill_focus), NULL, this);
+        m_tempCtrl_nozzle_deputy->Disconnect(wxEVT_SET_FOCUS, wxFocusEventHandler(StatusPanel::on_nozzle_temp_set_focus), NULL, this);
 
-    m_switch_lamp->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_lamp_switch), NULL, this);
-    /*m_switch_nozzle_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
-    m_switch_printing_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
-    m_switch_cham_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);*/
+        m_switch_lamp->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_lamp_switch), NULL, this);
+        /*m_switch_nozzle_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
+        m_switch_printing_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
+        m_switch_cham_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);*/
 
-    //m_switch_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
-    //m_switch_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
-    m_switch_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
+        //m_switch_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
+        //m_switch_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
+        m_switch_fan->Disconnect(wxEVT_COMMAND_TOGGLEBUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_nozzle_fan_switch), NULL, this);
 
-    m_bpButton_xy->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_xy), NULL, this);
-    m_bpButton_z_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_up_10), NULL, this);
-    m_bpButton_z_1->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_up_1), NULL, this);
-    m_bpButton_z_down_1->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_down_1), NULL, this);
-    m_bpButton_z_down_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_down_10), NULL, this);
-    m_bpButton_e_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_e_up_10), NULL, this);
-    m_bpButton_e_down_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_e_down_10), NULL, this);
-    m_nozzle_btn_panel->Disconnect(wxCUSTOMEVT_SWITCH_POS, wxCommandEventHandler(StatusPanel::on_nozzle_selected), NULL, this);
-    m_switch_speed->Disconnect(wxEVT_LEFT_DOWN, wxCommandEventHandler(StatusPanel::on_switch_speed), NULL, this);
-    m_calibration_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_start_calibration), NULL, this);
-    m_options_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_show_print_options), NULL, this);
-    m_safety_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_show_safety_options), NULL, this);
-    m_parts_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_show_parts_options), NULL, this);
+        m_bpButton_xy->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_xy), NULL, this);
+        m_bpButton_z_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_up_10), NULL, this);
+        m_bpButton_z_1->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_up_1), NULL, this);
+        m_bpButton_z_down_1->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_down_1), NULL, this);
+        m_bpButton_z_down_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_z_down_10), NULL, this);
+        m_bpButton_e_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_e_up_10), NULL, this);
+        m_bpButton_e_down_10->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_axis_ctrl_e_down_10), NULL, this);
+        m_nozzle_btn_panel->Disconnect(wxCUSTOMEVT_SWITCH_POS, wxCommandEventHandler(StatusPanel::on_nozzle_selected), NULL, this);
+        m_switch_speed->Disconnect(wxEVT_LEFT_DOWN, wxCommandEventHandler(StatusPanel::on_switch_speed), NULL, this);
+        m_calibration_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_start_calibration), NULL, this);
+        m_options_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_show_print_options), NULL, this);
+        m_safety_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_show_safety_options), NULL, this);
+        m_parts_btn->Disconnect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(StatusPanel::on_show_parts_options), NULL, this);
+    }
 
     // remove warning dialogs
     if (abort_dlg != nullptr)
@@ -5216,7 +5247,9 @@ void StatusPanel::set_default()
     m_filament_step->Hide();
     error_info_reset();
 #ifndef __WXGTK__
-    SetFocus();
+    // Also reached while the panel is built off screen.
+    if (IsShownOnScreen())
+        SetFocus();
 #endif
 }
 

src/slic3r/GUI/StatusPanel.hpp

@@ -36,6 +36,7 @@
 #include "HMS.hpp"
 #include "PartSkipDialog.hpp"
 #include "DeviceErrorDialog.hpp"
+#include "StagedBuild.hpp"
 
 class StepIndicator;
 
@@ -375,7 +376,7 @@ public:
     void paint(wxPaintEvent&);
 };
 
-class StatusBasePanel : public wxScrolledWindow
+class StatusBasePanel : public wxScrolledWindow, public StagedBuild
 {
 protected:
     wxBitmap m_item_placeholder;
@@ -443,7 +444,7 @@ protected:
 
 
     wxMediaCtrl3 *  m_media_ctrl;
-    MediaPlayCtrl * m_media_play_ctrl;
+    MediaPlayCtrl * m_media_play_ctrl{nullptr};
 
     Label *         m_staticText_printing;
     wxStaticBitmap *m_bitmap_thumbnail;
@@ -629,6 +630,7 @@ class StatusPanel : public StatusBasePanel
 {
 private:
     friend class MonitorPanel;
+    void wire_controls();
 
 protected:
     std::shared_ptr<SliceInfoPopup> m_slice_info_popup;
@@ -679,7 +681,7 @@ protected:
     std::map<std::string, std::string> m_print_connect_types;
     std::vector<Button *>       m_buttons;
     int last_status;
-    ScoreData *m_score_data;
+    ScoreData *m_score_data = nullptr;
     wxBitmap* calib_bitmap = nullptr;
     CalibMode m_calib_mode;
     CalibrationMethod m_calib_method;

src/slic3r/GUI/Tab.cpp

@@ -804,6 +804,10 @@ void Tab::OnActivate()
     }
 #endif
 
+    // The OnActivate() that shows the tab builds the page.
+    if (wxGetApp().mainframe != nullptr && !wxGetApp().mainframe->is_active_and_shown_tab(m_parent))
+        return;
+
     // BBS: select on first active
     if (!m_active_page)
         restore_last_select_item();
@@ -7159,6 +7163,16 @@ void Tab::restore_last_select_item()
     m_tabctrl->SelectItem(item);
 }
 
+bool Tab::page_build_pending() const
+{
+    return m_active_page != nullptr && m_active_page->build_pending();
+}
+
+bool Tab::page_build_step()
+{
+    return m_active_page != nullptr && m_active_page->build_step(m_mode);
+}
+
 void Tab::update_description_lines()
 {
     if (m_active_page && m_active_page->title() == "Dependencies" && m_parent_preset_description_line)
@@ -7375,7 +7389,7 @@ void Tab::OnKeyDown(wxKeyEvent& event)
 
 void Tab::compare_preset()
 {
-    wxGetApp().mainframe->diff_dialog.show(m_type);
+    DiffPresetDialog::ensure()->show(m_type);
 }
 
 void Tab::transfer_options(const std::string &name_from, const std::string &name_to, std::vector<std::string> options)
@@ -7543,8 +7557,10 @@ void Tab::save_preset(std::string name /*= ""*/, bool detach, bool save_to_proje
             wxGetApp().get_tab(preset_type)->update_tab_ui();
     }
 
-    // update preset comboboxes in DiffPresetDlg
-    wxGetApp().mainframe->diff_dialog.update_presets(m_type);
+    // show() reloads the presets, so only a visible Compare dialog needs updating.
+    DiffPresetDialog* diff_dialog = DiffPresetDialog::if_built();
+    if (diff_dialog != nullptr && diff_dialog->IsShown())
+        diff_dialog->update_presets(m_type);
 }
 
 // Called for a currently selected preset.
@@ -8604,20 +8620,8 @@ void Page::activate(ConfigOptionMode mode, std::function<void()> throw_if_cancel
 #else
     //m_vsizer->AddSpacer(10);
 #endif
-#if HIDE_FIRST_SPLIT_LINE
-    // BBS: no line spliter for first group
-    bool first = true;
-#endif
-    for (auto group : m_optgroups) {
-        if (!group->activate(throw_if_canceled))
-            continue;
-        m_vsizer->Add(group->sizer, 0, wxEXPAND | (group->is_legend_line() ? (wxLEFT|wxTOP) : wxALL), m_parent->FromDIP(5)); // ORCA use less margin on parameters section
-        group->update_visibility(mode);
-#if HIDE_FIRST_SPLIT_LINE
-        if (first) group->stb->Hide();
-        first = false;
-#endif
-        group->reload_config();
+    for (size_t i = 0; i < m_optgroups.size(); ++i) {
+        activate_group(i, mode, throw_if_canceled);
         throw_if_canceled();
     }
 
@@ -8636,6 +8640,41 @@ void Page::activate(ConfigOptionMode mode, std::function<void()> throw_if_cancel
 #endif
 }
 
+// Builds one option group; false when it already has its controls.
+bool Page::activate_group(size_t i, ConfigOptionMode mode, std::function<void()> throw_if_canceled)
+{
+    auto& group = m_optgroups[i];
+    if (!group->activate(throw_if_canceled))
+        return false;
+    m_vsizer->Add(group->sizer, 0, wxEXPAND | (group->is_legend_line() ? (wxLEFT|wxTOP) : wxALL), m_parent->FromDIP(5)); // ORCA use less margin on parameters section
+    group->update_visibility(mode);
+#if HIDE_FIRST_SPLIT_LINE
+    // BBS: no line spliter for first group
+    if (i == 0) group->stb->Hide();
+#endif
+    group->reload_config();
+    return true;
+}
+
+// The first group without controls.
+size_t Page::next_group_to_build() const
+{
+    return std::find_if(m_optgroups.begin(), m_optgroups.end(), [](const auto& group) { return !group->is_activated(); }) - m_optgroups.begin();
+}
+
+bool Page::build_pending() const
+{
+    return next_group_to_build() < m_optgroups.size();
+}
+
+bool Page::build_step(ConfigOptionMode mode)
+{
+    const size_t i = next_group_to_build();
+    if (i < m_optgroups.size())
+        activate_group(i, mode, [] {});
+    return build_pending();
+}
+
 void Page::clear()
 {
     for (auto group : m_optgroups)

src/slic3r/GUI/Tab.hpp

@@ -94,6 +94,10 @@ public:
 	void		reload_config();
     void        update_visibility(ConfigOptionMode mode, bool update_contolls_visibility);
     void        activate(ConfigOptionMode mode, std::function<void()> throw_if_canceled);
+    // Whether an option group has no controls yet.
+    bool        build_pending() const;
+    // Builds the next option group that has no controls yet; true while some remain.
+    bool        build_step(ConfigOptionMode mode);
     void        clear();
     void        msw_rescale();
     void        sys_color_changed();
@@ -121,6 +125,8 @@ public:
     std::map<std::string, std::string> m_opt_id_map;
 
 protected:
+    size_t      next_group_to_build() const;
+    bool        activate_group(size_t i, ConfigOptionMode mode, std::function<void()> throw_if_canceled);
 	// Color of TreeCtrlItem. The wxColour will be updated only if the new wxColour pointer differs from the currently rendered one.
 	const wxColour*		m_item_color;
 };
@@ -437,6 +443,10 @@ public:
 	// BBS: new layout
 	void set_expanded(bool value);
 	void restore_last_select_item();
+	// page_build_pending() says whether the selected page has groups without controls, and
+	// page_build_step() builds one.
+	bool page_build_pending() const;
+	bool page_build_step();
 
 	static bool validate_custom_gcode(const wxString& title, const std::string& gcode);
 	bool        validate_custom_gcodes();

src/slic3r/GUI/UnsavedChangesDialog.cpp

@@ -1962,8 +1962,9 @@ DiffPresetDialog::DiffPresetDialog(MainFrame* mainframe)
 
     assert(wxGetApp().preset_bundle);
 
-    m_preset_bundle_left  = std::make_unique<PresetBundle>(*wxGetApp().preset_bundle);
-    m_preset_bundle_right = std::make_unique<PresetBundle>(*wxGetApp().preset_bundle);
+    // show() copies the app's bundle into both before anything is displayed.
+    m_preset_bundle_left  = std::make_unique<PresetBundle>();
+    m_preset_bundle_right = std::make_unique<PresetBundle>();
 
     // Create UI items
 

src/slic3r/GUI/UnsavedChangesDialog.hpp

@@ -10,6 +10,7 @@
 #include "libslic3r/PresetBundle.hpp"
 #include "Widgets/Button.hpp"
 #include "Widgets/ScrolledWindow.hpp"
+#include "Lazy.hpp"
 
 class ScalableButton;
 class wxStaticText;
@@ -416,7 +417,7 @@ public:
 //------------------------------------------
 //          DiffPresetDialog
 //------------------------------------------
-class DiffPresetDialog : public DPIDialog
+class DiffPresetDialog : public DPIDialog, public LazyInstance<DiffPresetDialog>
 {
     DiffViewCtrl*           m_tree              { nullptr };
     wxBoxSizer*             m_presets_sizer     { nullptr };

src/slic3r/GUI/WebViewDialog.cpp

@@ -219,6 +219,7 @@ WebViewPanel::WebViewPanel(wxWindow *parent)
     Bind(wxEVT_CLOSE_WINDOW, &WebViewPanel::OnClose, this);
 
     m_LoginUpdateTimer = nullptr;
+    update_mode();
  }
 
 WebViewPanel::~WebViewPanel()
@@ -416,8 +417,7 @@ void WebViewPanel::OnClose(wxCloseEvent& evt)
 
 void WebViewPanel::OnFreshLoginStatus(wxTimerEvent &event)
 {
-    auto mainframe = Slic3r::GUI::wxGetApp().mainframe;
-    if (mainframe && mainframe->m_webview == this) {
+    if (WebViewPanel::if_built() == this) {
         auto* app_config = Slic3r::GUI::wxGetApp().app_config;
         if (app_config && app_config->get_stealth_mode()) return;
         Slic3r::GUI::wxGetApp().get_login_info(ORCA_CLOUD_PROVIDER);

src/slic3r/GUI/WebViewDialog.hpp

@@ -24,6 +24,7 @@
 #include <wx/tbarbase.h>
 #include "wx/textctrl.h"
 #include <wx/timer.h>
+#include "Lazy.hpp"
 
 
 namespace Slic3r {
@@ -33,7 +34,7 @@ class NetworkAgent;
 namespace GUI {
 
 
-class WebViewPanel : public wxPanel
+class WebViewPanel : public wxPanel, public LazyInstance<WebViewPanel>
 {
 public:
     WebViewPanel(wxWindow *parent);

tests/slic3rutils/CMakeLists.txt

@@ -5,6 +5,9 @@ add_executable(${_TEST_NAME}_tests
     test_creality_cfs_match.cpp
     test_dev_mapping.cpp
     test_filament_bitmap_utils.cpp
+    test_lazy.cpp
+    test_prebuild_queue.cpp
+    test_staged_build.cpp
     test_network_versions.cpp
     test_action_source.cpp
     test_plugin_host_api.cpp

tests/slic3rutils/test_lazy.cpp

@@ -0,0 +1,240 @@
+#include <catch2/catch_all.hpp>
+
+#include <functional>
+#include <memory>
+#include <stdexcept>
+#include <vector>
+
+#include "slic3r/GUI/Lazy.hpp"
+
+using Slic3r::GUI::Lazy;
+using Slic3r::GUI::LazyBase;
+using Slic3r::GUI::LazyInstance;
+using Slic3r::GUI::StagedBuild;
+
+namespace {
+
+struct Plain
+{
+    int value{ 1 };
+};
+
+struct One : LazyInstance<One>
+{
+    int value{ 2 };
+};
+
+// Two steps after the constructor.
+struct Staged : StagedBuild, LazyInstance<Staged>
+{
+    std::vector<int> ran;
+    Staged()
+    {
+        add_build_step([this] { ran.push_back(1); });
+        add_build_step([this] { ran.push_back(2); });
+    }
+    void add_step(std::function<void()> step) { add_build_step(std::move(step)); }
+};
+
+// Owns what the factories make, since a Lazy does not.
+template <class T>
+struct Made
+{
+    std::vector<std::unique_ptr<T>> objects;
+    T*                              make()
+    {
+        objects.push_back(std::make_unique<T>());
+        return objects.back().get();
+    }
+    typename Lazy<T>::Factory factory()
+    {
+        return [this] { return make(); };
+    }
+};
+
+} // namespace
+
+TEST_CASE("The factory runs on the first unit, not at construction", "[Lazy]")
+{
+    Made<Plain> made;
+    Lazy<Plain> lazy("plain", 0, made.factory());
+    REQUIRE(made.objects.empty());
+    REQUIRE_FALSE(lazy.built());
+    REQUIRE(lazy.pending());
+    REQUIRE(lazy.get() == nullptr);
+
+    REQUIRE_FALSE(lazy.build_step()); // the only unit
+    REQUIRE(made.objects.size() == 1);
+    REQUIRE(lazy.built());
+    REQUIRE_FALSE(lazy.pending());
+    REQUIRE(lazy.get() == made.objects[0].get());
+    REQUIRE_FALSE(lazy.build_step());
+    REQUIRE(made.objects.size() == 1);
+}
+
+TEST_CASE("A staged type takes one unit for the constructor and one per step", "[Lazy]")
+{
+    Made<Staged> made;
+    Lazy<Staged> lazy("staged", 0, made.factory());
+    REQUIRE(lazy.build_step());
+    REQUIRE(made.objects.size() == 1);
+    REQUIRE(lazy.get() == nullptr); // exists but incomplete
+    REQUIRE(lazy.build_step());
+    REQUIRE(made.objects[0]->ran == std::vector<int>{1});
+    REQUIRE_FALSE(lazy.build_step());
+    REQUIRE(made.objects[0]->ran == std::vector<int>{1, 2});
+    REQUIRE(lazy.get() == made.objects[0].get());
+}
+
+TEST_CASE("ensure builds whatever is left and is a no-op afterwards", "[Lazy]")
+{
+    Made<Staged> made;
+    Lazy<Staged> lazy("staged", 0, made.factory());
+    lazy.build_step();
+    Staged* s = lazy.ensure();
+    REQUIRE(s == made.objects[0].get());
+    REQUIRE(s->ran == std::vector<int>{1, 2});
+    REQUIRE(lazy.ensure() == s);
+    REQUIRE(made.objects.size() == 1);
+}
+
+TEST_CASE("when_built waits for completion, then runs at once", "[Lazy]")
+{
+    Made<Staged> made;
+    Lazy<Staged> lazy("staged", 0, made.factory());
+    std::vector<int> seen;
+    lazy.when_built([&](Staged& s) { seen.push_back(int(s.ran.size())); });
+    lazy.build_step();
+    lazy.build_step();
+    REQUIRE(seen.empty());
+    lazy.build_step();
+    REQUIRE(seen == std::vector<int>{2});
+    lazy.when_built([&](Staged&) { seen.push_back(9); });
+    REQUIRE(seen == std::vector<int>{2, 9});
+}
+
+TEST_CASE("A LazyInstance type reaches its holder through the statics", "[Lazy]")
+{
+    REQUIRE(One::if_built() == nullptr);
+    REQUIRE(One::ensure() == nullptr);
+    Made<One> made;
+    {
+        Lazy<One> lazy("one", 0, made.factory());
+        REQUIRE(One::if_built() == nullptr);
+        One* one = One::ensure();
+        REQUIRE(one == made.objects[0].get());
+        REQUIRE(One::if_built() == one);
+        int seen = 0;
+        One::when_built([&](One& o) { seen = o.value; });
+        REQUIRE(seen == 2);
+    }
+    REQUIRE(One::if_built() == nullptr);
+}
+
+TEST_CASE("A newer holder replaces the registration; the older one leaves it alone", "[Lazy]")
+{
+    Made<One> made;
+    auto      first = std::make_unique<Lazy<One>>("first", 0, made.factory());
+    first->ensure();
+    Lazy<One> second("second", 0, made.factory());
+    REQUIRE(One::if_built() == nullptr); // the new holder has not built yet
+    second.ensure();
+    REQUIRE(One::if_built() == made.objects[1].get());
+    first.reset();
+    REQUIRE(One::if_built() == made.objects[1].get());
+}
+
+TEST_CASE("The holder reports the name and order it was given", "[Lazy]")
+{
+    Made<Plain> made;
+    Lazy<Plain> lazy("plain", 7, made.factory());
+    LazyBase&   base = lazy;
+    REQUIRE(base.name() == "plain");
+    REQUIRE(base.prebuild_order() == 7);
+}
+
+TEST_CASE("A unit that re-enters the holder builds nothing twice", "[Lazy]")
+{
+    Made<Plain>  made;
+    Lazy<Plain>* self         = nullptr;
+    int          nested_units = 0;
+    Lazy<Plain>  lazy("plain", 0, [&] {
+        if (self->build_step()) // as if the constructor pumped the event loop into a slice
+            ++nested_units;
+        return made.make();
+    });
+    self = &lazy;
+    REQUIRE_FALSE(lazy.build_step());
+    REQUIRE(nested_units == 0);
+    REQUIRE(made.objects.size() == 1);
+    REQUIRE(lazy.built());
+}
+
+TEST_CASE("A factory that returns null leaves the holder unbuilt and not pending", "[Lazy]")
+{
+    int         calls = 0;
+    Lazy<Plain> lazy("plain", 0, [&] { ++calls; return static_cast<Plain*>(nullptr); });
+    REQUIRE_FALSE(lazy.build_step());
+    REQUIRE_FALSE(lazy.built());
+    REQUIRE_FALSE(lazy.pending());
+    REQUIRE(lazy.get() == nullptr);
+    REQUIRE_FALSE(lazy.build_step()); // not retried
+    REQUIRE(calls == 1);
+}
+
+TEST_CASE("ensure returns null for a factory that returned null", "[Lazy]")
+{
+    Lazy<Plain> lazy("plain", 0, [] { return static_cast<Plain*>(nullptr); });
+    REQUIRE(lazy.ensure() == nullptr);
+    REQUIRE_FALSE(lazy.built());
+}
+
+TEST_CASE("A nested ensure inside the factory returns null", "[Lazy]")
+{
+    Made<Plain>  made;
+    Lazy<Plain>* self   = nullptr;
+    Plain*       nested = reinterpret_cast<Plain*>(1);
+    Lazy<Plain>  lazy("plain", 0, [&] {
+        nested = self->ensure(); // as if the constructor pumped the event loop into a caller
+        return made.make();
+    });
+    self = &lazy;
+    Plain* built = lazy.ensure();
+    REQUIRE(built == made.objects[0].get());
+    REQUIRE(nested == nullptr);
+}
+
+TEST_CASE("A nested ensure during a staged step returns null", "[Lazy]")
+{
+    Made<Staged>  made;
+    Lazy<Staged>* self   = nullptr;
+    Staged*       nested = reinterpret_cast<Staged*>(1);
+    Lazy<Staged>  lazy("staged", 0, [&] {
+        Staged* s = made.make();
+        s->add_step([&] { nested = self->ensure(); }); // as if a step pumped the event loop into a caller
+        return s;
+    });
+    self = &lazy;
+    Staged* built = lazy.ensure();
+    REQUIRE(built == made.objects[0].get());
+    REQUIRE(nested == nullptr);
+}
+
+TEST_CASE("A unit that throws leaves the holder free to build the rest", "[Lazy]")
+{
+    Made<Staged> made;
+    bool         thrown = false;
+    Lazy<Staged> lazy("staged", 0, [&] {
+        Staged* s = made.make();
+        s->add_step([&] { thrown = true; throw std::runtime_error("step"); });
+        return s;
+    });
+    lazy.build_step();
+    lazy.build_step();
+    lazy.build_step();
+    REQUIRE_THROWS(lazy.build_step());
+    REQUIRE(thrown);
+    REQUIRE(lazy.pending());
+    REQUIRE_FALSE(lazy.build_step()); // the next unit runs
+    REQUIRE(lazy.built());
+}

tests/slic3rutils/test_prebuild_queue.cpp

@@ -0,0 +1,183 @@
+#include <catch2/catch_all.hpp>
+
+#include <string>
+#include <vector>
+
+#include "slic3r/GUI/PrebuildQueue.hpp"
+
+using Slic3r::GUI::LazyBase;
+using Slic3r::GUI::PrebuildQueue;
+
+namespace {
+
+// A task with `left` units, each taking `unit_ms` of the shared fake clock and logging its id.
+struct Counter : LazyBase
+{
+    std::string      id;
+    int              left;
+    int              order;
+    long long        unit_ms{ 1 };
+    inline static std::vector<int> log;
+    inline static long long        now = 0;
+
+    Counter(int id, int left, int order, long long unit_ms = 1) : id(std::to_string(id)), left(left), order(order), unit_ms(unit_ms) {}
+
+    const std::string& name() const override { return id; }
+    bool               built() const override { return left == 0; }
+    bool               build_step() override
+    {
+        now += unit_ms;
+        log.push_back(std::stoi(id));
+        return --left > 0;
+    }
+    int prebuild_order() const override { return order; }
+};
+
+// Resets the shared log and clock at the start of a case.
+struct Reset
+{
+    Reset() { Counter::log.clear(); Counter::now = 0; }
+};
+
+const auto fake_clock    = [] { return Counter::now; };
+const auto no_input = [] { return false; };
+
+// Runs slices with an unlimited budget until nothing is pending; one task per slice.
+void drain(PrebuildQueue& q)
+{
+    while (q.pending())
+        q.run_slice(1000000, fake_clock, no_input);
+}
+
+} // namespace
+
+TEST_CASE("Tasks run lowest order first, equal order in the order added", "[PrebuildQueue]")
+{
+    Reset   reset;
+    Counter a{ 1, 1, 10 }, b{ 2, 1, 10 }, c{ 3, 1, 50 }, d{ 4, 1, 100 };
+    PrebuildQueue q;
+    q.add(c);
+    q.add(a);
+    q.add(b);
+    q.add(d);
+    REQUIRE(q.names() == "1, 2, 3, 4");
+    drain(q);
+    REQUIRE(Counter::log == std::vector<int>{1, 2, 3, 4});
+}
+
+TEST_CASE("A task with nothing pending is skipped, not removed", "[PrebuildQueue]")
+{
+    Reset   reset;
+    Counter a{ 1, 0, 0 }, b{ 2, 2, 1 };
+    PrebuildQueue q;
+    q.add(a);
+    q.add(b);
+    REQUIRE(q.pending());
+    auto slice = q.run_slice(1, fake_clock, no_input); // one unit of b
+    REQUIRE(slice.units == 1);
+    REQUIRE(Counter::log == std::vector<int>{2});
+
+    a.left = 1; // a's work returned; it comes first again
+    q.run_slice(1, fake_clock, no_input);
+    REQUIRE(Counter::log == std::vector<int>{2, 1});
+}
+
+TEST_CASE("A slice with nothing pending runs no unit", "[PrebuildQueue]")
+{
+    Reset         reset;
+    PrebuildQueue q;
+    REQUIRE_FALSE(q.pending());
+    auto slice = q.run_slice(40, fake_clock, no_input);
+    REQUIRE(slice.units == 0);
+    REQUIRE_FALSE(slice.completed);
+    REQUIRE_FALSE(slice.remaining);
+}
+
+TEST_CASE("A slice stops once its budget is spent, after the unit that crossed it", "[PrebuildQueue]")
+{
+    Reset   reset;
+    Counter a{ 1, 10, 0, 15 };
+    PrebuildQueue q;
+    q.add(a);
+    auto slice = q.run_slice(40, fake_clock, no_input);
+    REQUIRE(slice.units == 3); // units end at 15, 30 and 45 ms; the one crossing 40 is the last
+    REQUIRE(slice.ms == 45);
+    REQUIRE_FALSE(slice.completed);
+    REQUIRE(slice.remaining);
+    REQUIRE(a.left == 7);
+}
+
+TEST_CASE("A slice stops after the unit during which input arrived", "[PrebuildQueue]")
+{
+    Reset   reset;
+    Counter a{ 1, 10, 0 };
+    bool    input = false;
+    PrebuildQueue q;
+    q.add(a);
+    auto slice = q.run_slice(40, fake_clock, [&] { input = a.left == 8; return input; });
+    REQUIRE(slice.units == 2);
+    REQUIRE_FALSE(slice.completed);
+    REQUIRE(slice.remaining);
+}
+
+TEST_CASE("A slice reports completion, whether work remains, and each unit's time", "[PrebuildQueue]")
+{
+    Reset                  reset;
+    Counter                a{ 1, 2, 0, 5 }, b{ 2, 1, 1 };
+    std::vector<long long> unit_ms;
+    PrebuildQueue          q;
+    q.add(a);
+    q.add(b);
+    auto slice = q.run_slice(40, fake_clock, no_input, [&](const std::string& name, long long ms) {
+        REQUIRE(name == "1");
+        unit_ms.push_back(ms);
+    });
+    REQUIRE(slice.units == 2);
+    REQUIRE(slice.completed);
+    REQUIRE(slice.name == "1");
+    REQUIRE(slice.remaining); // b
+    REQUIRE(unit_ms == std::vector<long long>{5, 5});
+    slice = q.run_slice(40, fake_clock, no_input);
+    REQUIRE(slice.completed);
+    REQUIRE_FALSE(slice.remaining);
+    REQUIRE_FALSE(q.pending());
+}
+
+TEST_CASE("A unit may add a task to the queue it runs from", "[PrebuildQueue]")
+{
+    Reset         reset;
+    PrebuildQueue q;
+    Counter       later{ 2, 1, 5 };
+    struct Adder : LazyBase
+    {
+        PrebuildQueue& q;
+        Counter&       later;
+        std::string    id{ "1" };
+        bool           done{ false };
+        Adder(PrebuildQueue& q, Counter& later) : q(q), later(later) {}
+        const std::string& name() const override { return id; }
+        bool               built() const override { return done; }
+        bool               build_step() override
+        {
+            Counter::log.push_back(1);
+            done = true;
+            q.add(later);
+            return false;
+        }
+        int prebuild_order() const override { return 0; }
+    } first{ q, later };
+    q.add(first);
+    drain(q);
+    REQUIRE(Counter::log == std::vector<int>{1, 2});
+}
+
+TEST_CASE("clear drops every task", "[PrebuildQueue]")
+{
+    Reset   reset;
+    Counter a{ 1, 1, 0 };
+    PrebuildQueue q;
+    q.add(a);
+    q.clear();
+    REQUIRE_FALSE(q.pending());
+    REQUIRE(q.run_slice(40, fake_clock, no_input).units == 0);
+}

tests/slic3rutils/test_staged_build.cpp

@@ -0,0 +1,105 @@
+#include <catch2/catch_all.hpp>
+
+#include <vector>
+
+#include "slic3r/GUI/StagedBuild.hpp"
+
+using Slic3r::GUI::StagedBuild;
+
+namespace {
+
+// Exposes the protected queueing calls and records the order steps ran in.
+struct Staged : StagedBuild
+{
+    std::vector<int> ran;
+    void queue(int id) { add_build_step([this, id] { ran.push_back(id); }); }
+    void queue_child(Staged& child) { add_build_steps_of(child); }
+    void queue_nested(int id, int nested)
+    {
+        add_build_step([this, id, nested] {
+            ran.push_back(id);
+            queue(nested);
+        });
+    }
+};
+
+} // namespace
+
+TEST_CASE("Steps run in the order they were queued, one per build_step", "[StagedBuild]")
+{
+    Staged s;
+    s.queue(1);
+    s.queue(2);
+    s.queue(3);
+    REQUIRE_FALSE(s.built());
+
+    REQUIRE(s.build_step());
+    REQUIRE(s.ran == std::vector<int>{1});
+    REQUIRE(s.build_step());
+    REQUIRE(s.ran == std::vector<int>{1, 2});
+    REQUIRE_FALSE(s.build_step());
+    REQUIRE(s.ran == std::vector<int>{1, 2, 3});
+    REQUIRE(s.built());
+    REQUIRE_FALSE(s.build_step());
+    REQUIRE(s.ran.size() == 3);
+}
+
+TEST_CASE("A panel with no steps is built from the start", "[StagedBuild]")
+{
+    Staged s;
+    REQUIRE(s.built());
+    REQUIRE_FALSE(s.build_step());
+}
+
+TEST_CASE("A step may queue another step, which runs after the ones already queued", "[StagedBuild]")
+{
+    Staged s;
+    s.queue_nested(1, 3);
+    s.queue(2);
+
+    REQUIRE(s.build_step());
+    REQUIRE_FALSE(s.built());
+    REQUIRE(s.build_step());
+    REQUIRE_FALSE(s.build_step());
+    REQUIRE(s.ran == std::vector<int>{1, 2, 3});
+    REQUIRE(s.built());
+}
+
+TEST_CASE("A parent waits for steps a child queues after being adopted", "[StagedBuild]")
+{
+    Staged child;
+    child.queue_nested(1, 2); // step 1 queues step 2 while it runs
+    Staged parent;
+    parent.queue_child(child); // one forwarder, for step 1
+    parent.queue(10);
+
+    REQUIRE(parent.build_step()); // child step 1, which queues step 2
+    REQUIRE(parent.build_step()); // 10; own steps exhausted, the child still has 2
+    REQUIRE_FALSE(parent.built());
+    REQUIRE_FALSE(parent.build_step()); // child step 2
+    REQUIRE(child.ran == std::vector<int>{1, 2});
+    REQUIRE(parent.ran == std::vector<int>{10});
+    REQUIRE(parent.built());
+}
+
+TEST_CASE("A child's remaining steps are forwarded one per parent step", "[StagedBuild]")
+{
+    Staged child;
+    child.queue(1);
+    child.queue(2);
+    child.queue(3);
+    REQUIRE(child.build_step()); // the parent adopts only what is left
+
+    Staged parent;
+    parent.queue_child(child);
+    parent.queue(10);
+
+    REQUIRE(parent.build_step());
+    REQUIRE(child.ran == std::vector<int>{1, 2});
+    REQUIRE(parent.build_step());
+    REQUIRE(child.ran == std::vector<int>{1, 2, 3});
+    REQUIRE(child.built());
+    REQUIRE_FALSE(parent.build_step());
+    REQUIRE(parent.ran == std::vector<int>{10});
+    REQUIRE(parent.built());
+}