Build the Design tab on first use instead of at startup

DesignPanel's constructor creates several hundred controls and its own GL
canvas, which every launch paid for whether or not the user ever opened the
tab. The notebook page is now an empty placeholder and the panel is built
into it the first time the tab is selected, so m_design_panel stays null
until then.

This also lifts the ordering constraint that had pushed the plater's
background colour and Hide() below the panel construction; they go back
where they were.

src/slic3r/GUI/MainFrame.cpp

@@ -1025,8 +1025,8 @@ void MainFrame::update_layout()
         size_t prepare_pos = (home_idx == wxNOT_FOUND) ? 0 : static_cast<size_t>(home_idx) + 1;
 #ifdef SLIC3R_CAD
         // Design sits between Home and Prepare, so it goes in first and pushes Prepare along.
-        m_design_panel->Reparent(m_tabpanel);
-        m_tabpanel->InsertPage(prepare_pos++, TAB_ID_DESIGN, m_design_panel, _L("Design"), "tab_design_active");
+        m_design_page->Reparent(m_tabpanel);
+        m_tabpanel->InsertPage(prepare_pos++, TAB_ID_DESIGN, m_design_page, _L("Design"), "tab_design_active");
 #endif
         m_tabpanel->InsertPage(prepare_pos, TAB_ID_PREPARE, m_plater, _L("Prepare"), "tab_3d_active");
         m_tabpanel->InsertPage(prepare_pos + 1, TAB_ID_PREVIEW, m_plater, _L("Preview"), "tab_preview_active");
@@ -1286,7 +1286,16 @@ void MainFrame::init_tabpanel() {
         //else if (panel == m_param_panel)
         //    m_param_panel->OnActivate();
 #ifdef SLIC3R_CAD
-        else if (panel == m_design_panel) {
+        else if (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.
+            if (m_design_panel == 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();
+            }
             // 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.
@@ -1299,7 +1308,7 @@ void MainFrame::init_tabpanel() {
 #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_panel) m_design_panel->on_tab_hidden();
+        if (m_design_panel != nullptr && panel != m_design_page) m_design_panel->on_tab_hidden();
 #endif
 #ifndef __APPLE__
         if (m_last_selected_tab == TAB_ID_PREPARE) {
@@ -1326,16 +1335,21 @@ void MainFrame::init_tabpanel() {
     }
 
     m_plater = new Plater(this, this);
-    // Register the plater with the app BEFORE constructing DesignPanel: its
-    // DesignCanvas reads wxGetApp().plater()->config() at construction time.
-    wxGetApp().plater_ = m_plater;
-#ifdef SLIC3R_CAD
-    m_design_panel = new DesignPanel(this);
-    start_mcp_control_if_enabled();   // opens the MCP socket iff SNAPORCA_MCP is set
-#endif
     m_plater->SetBackgroundColour(*wxWHITE);
     m_plater->Hide();
 
+    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.
+    m_design_page = new wxPanel(this);
+    m_design_page->SetSizer(new wxBoxSizer(wxVERTICAL));
+    m_design_page->Hide();
+    start_mcp_control_if_enabled();   // opens the MCP socket iff SNAPORCA_MCP is set
+#endif
+
     create_preset_tabs();
 
         //BBS add pages

src/slic3r/GUI/MainFrame.hpp

@@ -387,6 +387,9 @@ public:
     PrintHostQueueDialog* printhost_queue_dlg() { return m_printhost_queue_dlg; }
     Plater*               m_plater { nullptr };
 #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 };
 #endif
     //BBS: GUI refactor