ENH: Add "Drop" button to move gizmo for objects, parts, and modifiers. And unify all Drop actions / checks (#16323)

* Add Drop to bed button for objects, parts and modifiers to the Position panel

Ported from BambuStudio PR #11511. The button appears under the position
inputs when the selection is not resting on the bed and moves it along
global Z. Works for a single full instance as well as a single part or
modifier, and is undoable.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7uDtamzhNGT6PFwmVdRJ8

* Reuse existing 'Drop to Bed' string so the button is already translated

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7uDtamzhNGT6PFwmVdRJ8

* Add tooltip to the Drop to Bed button

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7uDtamzhNGT6PFwmVdRJ8

* unify function usage across UI

* Update GizmoObjectManipulation.cpp

* show tooltip even its disabled

* update

* Update GizmoObjectManipulation.cpp

* update

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: yw4z <ywsyildiz@gmail.com>

src/slic3r/GUI/GUI_Factories.cpp

@@ -2400,7 +2400,7 @@ void MenuFactory::append_menu_item_drop(wxMenu* menu)
             if (plater()->canvas3D()->get_canvas_type() != GLCanvas3D::ECanvasType::CanvasView3D)
                 return false;
             else {
-                return (std::abs(plater()->get_view3D_canvas3D()->get_selection().get_bounding_box().min.z()) > -SINKING_Z_THRESHOLD);
+                return plater()->get_view3D_canvas3D()->get_selection().can_drop();
             } //disable if model is on the bed / not in View3D
         }, m_parent);
 }

src/slic3r/GUI/Gizmos/GizmoObjectManipulation.cpp

@@ -57,24 +57,6 @@ const double GizmoObjectManipulation::mm_to_in = 0.0393700787;
 const double GizmoObjectManipulation::oz_to_g = 28.34952;
 const double GizmoObjectManipulation::g_to_oz = 0.035274;
 
-// Helper function to be used by drop to bed button. Returns lowest point of this
-// volume in world coordinate system.
-static double get_volume_min_z(const GLVolume* volume)
-{
-    const Transform3f& world_matrix = volume->world_matrix().cast<float>();
-
-    // need to get the ModelVolume pointer
-    const ModelObject* mo = wxGetApp().model().objects[volume->composite_id.object_id];
-    const ModelVolume* mv = mo->volumes[volume->composite_id.volume_id];
-    const TriangleMesh& hull = mv->get_convex_hull();
-
-    float min_z = std::numeric_limits<float>::max();
-    for (const stl_vertex& vert : hull.its.vertices) {
-        min_z = std::min(min_z, Vec3f::UnitZ().dot(world_matrix * vert));
-    }
-    return min_z;
-}
-
 GizmoObjectManipulation::GizmoObjectManipulation(GLCanvas3D& glcanvas)
     : m_glcanvas(glcanvas), m_alignment_helper(glcanvas)
 {
@@ -295,12 +277,14 @@ void GizmoObjectManipulation::update_reset_buttons_visibility()
 {
     const Selection& selection = m_glcanvas.get_selection();
 
+    // Assembly view has its own selection and transformations, which drop_selection() doesn't update
+    m_enable_drop_to_bed = m_glcanvas.get_canvas_type() == GLCanvas3D::ECanvasType::CanvasView3D && selection.can_drop();
+
     if (selection.is_single_full_instance() || selection.is_single_volume_or_modifier()) {
         const GLVolume *               volume = selection.get_first_volume();
 
         Vec3d rotation;
         Vec3d scale;
-        double min_z = 0.;
 
         if (selection.is_single_full_instance()) {
             rotation = volume->get_instance_rotation();
@@ -309,12 +293,10 @@ void GizmoObjectManipulation::update_reset_buttons_visibility()
         else {
             rotation = volume->get_volume_rotation();
             scale = volume->get_volume_scaling_factor();
-            min_z = get_volume_min_z(volume);
         }
         m_show_clear_rotation = !rotation.isApprox(m_init_rotation);
         m_show_reset_0_rotation = !rotation.isApprox(Vec3d::Zero());
         m_show_clear_scale = (m_cache.scale / 100.0f - Vec3d::Ones()).norm() > 0.001;
-        m_show_drop_to_bed = (std::abs(min_z) > EPSILON);
     }
 }
 
@@ -1072,6 +1054,17 @@ void GizmoObjectManipulation::do_render_move_window(ImGuiWrapper *imgui_wrapper,
 
     GLGizmoUtils::render_tooltip_button(imgui_wrapper, m_glcanvas, m_shortcuts_move, x, y);
 
+    // ORCA use existing function for drop and show button on bottom bar to keep gizmo compact
+    imgui_wrapper->disabled_begin(!m_enable_drop_to_bed);
+    ImGui::SameLine(0, space_size * 1.5f);
+    if (imgui_wrapper->button(_L("Drop"))){
+        wxGetApp().plater()->drop_selection();
+        UpdateAndShow(true);
+    }
+    imgui_wrapper->disabled_end();
+    if(ImGui::IsItemHovered(ImGuiHoveredFlags_AllowWhenDisabled))
+        imgui_wrapper->tooltip(_L("Move the selection along Z so that its lowest point touches the bed."), ImGui::GetFontSize() * 20.0f);
+
     ImGui::SameLine();
     GLGizmoUtils::begin_right_aligned_buttons({ _L("Done") });
     if (imgui_wrapper->button(_L("Done"))) {

src/slic3r/GUI/Gizmos/GizmoObjectManipulation.hpp

@@ -103,7 +103,7 @@ public:
     bool            m_show_reset_0_rotation{false};
     bool            m_show_clear_rotation { false };
     bool            m_show_clear_scale { false };
-    bool            m_show_drop_to_bed { false };
+    bool            m_enable_drop_to_bed { false };
     enum class RotateType { None, Relative, Absolute
     };
     RotateType m_last_rotate_type{RotateType::None}; // 0:no input 1:relative 2:absolute

src/slic3r/GUI/NativeCommands.cpp

@@ -365,7 +365,7 @@ std::vector<NativeCommand> build_command_catalog()
         return object_op(wxGetApp().plater(), [](Plater* p) { return !p->is_selection_empty(); }, [](Plater* p) { p->center_selection(); });
     });
     add_with_icon("obj_drop", L("Drop to Bed"), L("Object"), "toolbar_flatten", [](const std::string&) {
-        return object_op(wxGetApp().plater(), [](Plater* p) { return !p->is_selection_empty(); }, [](Plater* p) { p->drop_selection(); });
+        return object_op(wxGetApp().plater(), [](Plater* p) { return p->get_selection().can_drop(); }, [](Plater* p) { p->drop_selection(); });
     });
     add("obj_fit_volume", L("Scale to Fit Print Volume"), L("Object"), [](const std::string&) {
         return object_op(

src/slic3r/GUI/Selection.cpp

@@ -558,15 +558,37 @@ void Selection::center()
     return;
 }
 
+double Selection::get_drop_min_z() const
+{
+    if (!m_valid)
+        return 0.;
+    if (m_mode == Volume)
+        return get_bounding_box().min.z();
+
+    double min_z = DBL_MAX;
+    for (const auto& [obj_idx, inst_idxs] : m_cache.content) {
+        if (obj_idx >= (int)m_model->objects.size())
+            continue; // wipe tower
+        for (int inst_idx : inst_idxs)
+            min_z = std::min(min_z, m_model->objects[obj_idx]->get_instance_min_z(inst_idx));
+    }
+    return min_z == DBL_MAX ? 0. : min_z;
+}
+
+bool Selection::can_drop() const
+{
+    return std::abs(get_drop_min_z()) > -SINKING_Z_THRESHOLD;
+}
+
 void Selection::drop()
 {
-    if (std::abs(this->get_bounding_box().min.z()) < -SINKING_Z_THRESHOLD) {
-        return; // shouldnt happen, but better check anyways, already checked in append_menu_item_drop()
-    }
+    const double min_z = get_drop_min_z();
+    if (std::abs(min_z) < -SINKING_Z_THRESHOLD)
+        return; // already checked in can_drop()
 
     wxGetApp().plater()->take_snapshot(L("Move object"));
 
-    this->move_to_center(Vec3d(0, 0, -this->get_bounding_box().min.z()));
+    this->move_to_center(Vec3d(0, 0, -min_z));
 
     for (unsigned int i : m_list) {
         GLVolume&    volume = *(*m_volumes)[i];

src/slic3r/GUI/Selection.hpp

@@ -248,6 +248,9 @@ public:
     void clone(int numbers = 1);
     void center();
     void drop();
+    bool can_drop() const;
+    // Lowest world Z that drop() moves to the bed; whole objects count model parts only, as auto drop does.
+    double get_drop_min_z() const;
     void center_plate(const int plate_idx);
     void set_printable(bool printable);
     bool get_auto_drop() const;