UV editor: keep the mouse capture balanced

The canvas captured the mouse on every button press without checking whether
it already held one, released it in a single place, and handled no capture
loss at all. Two sequences leaked a capture: pressing a second button during a
drag nested a second one that the single release on button-up could not undo,
and a modal R/S skipped that release entirely while waiting for a confirming
click that may never come.

A leaked capture is not a local problem on macOS, where wxEVT_MOUSE_CAPTURE_LOST
is never sent and nothing recovers it. While any wx window holds a capture,
wxOSX routes every mouse event to that window and never calls through to
NSWindow, so the application stops seeing motion and enter/leave, and native
tooltips stop appearing anywhere in it.

Capture through grab_mouse()/drop_mouse() so there is at most one, give it back
on any button-up including a modal gesture (which tracks the pointer and needs
no capture), and cancel on wxEVT_MOUSE_CAPTURE_LOST: commit nothing, put back
what a modal rotate or scale already applied, and do not release a capture that
is already gone.

src/slic3r/GUI/UVEditorCanvas.cpp

@@ -176,6 +176,7 @@ UVEditorCanvas::UVEditorCanvas(wxWindow *parent)
     Bind(wxEVT_MIDDLE_DOWN, &UVEditorCanvas::on_mouse, this);
     Bind(wxEVT_MIDDLE_UP, &UVEditorCanvas::on_mouse, this);
     Bind(wxEVT_MOTION, &UVEditorCanvas::on_mouse, this);
+    Bind(wxEVT_MOUSE_CAPTURE_LOST, &UVEditorCanvas::on_capture_lost, this);
     Bind(wxEVT_MOUSEWHEEL, &UVEditorCanvas::on_mouse, this);
     Bind(wxEVT_LEAVE_WINDOW, &UVEditorCanvas::on_leave, this);
     Bind(wxEVT_KEY_DOWN, &UVEditorCanvas::on_key, this);
@@ -727,10 +728,47 @@ void UVEditorCanvas::end_gesture()
     m_rot_raw_deg       = 0.f;
     m_rot_applied_deg   = 0.f;
     m_modal_scale_accum = 1.f;
+    drop_mouse();
+}
+
+void UVEditorCanvas::cancel_gesture()
+{
+    // Undo what the gesture already applied live, as the Esc path does, and commit nothing.
+    if (m_on_island_edit) {
+        if (m_gesture == Gesture::RotateIslandModal && m_rot_applied_deg != 0.f)
+            m_on_island_edit(m_selected_island, Vec2f::Zero(), -m_rot_applied_deg, 1.f, false);
+        if (m_gesture == Gesture::ScaleIslandModal && m_modal_scale_accum != 1.f)
+            m_on_island_edit(m_selected_island, Vec2f::Zero(), 0.f, 1.f / m_modal_scale_accum, false);
+    }
+
+    m_gesture           = Gesture::None;
+    m_rot_raw_deg       = 0.f;
+    m_rot_applied_deg   = 0.f;
+    m_modal_scale_accum = 1.f;
+    m_vertex_edit_moved = false;
+}
+
+void UVEditorCanvas::grab_mouse()
+{
+    if (!HasCapture())
+        CaptureMouse();
+}
+
+void UVEditorCanvas::drop_mouse()
+{
     if (HasCapture())
         ReleaseMouse();
 }
 
+// The capture was taken from us (a dialog opened, another application grabbed the pointer). wx
+// requires this to cancel the gesture: no commit, no Skip(), and no ReleaseMouse() - the capture is
+// already gone, and releasing it again would unbalance the stack.
+void UVEditorCanvas::on_capture_lost(wxMouseCaptureLostEvent &)
+{
+    cancel_gesture();
+    Refresh();
+}
+
 void UVEditorCanvas::on_key(wxKeyEvent &evt)
 {
     const int key = evt.GetKeyCode();
@@ -909,7 +947,7 @@ void UVEditorCanvas::on_mouse(wxMouseEvent &evt)
             }
             m_gesture = (m_selected_island >= 0) ? Gesture::MoveIsland : Gesture::Pan;
         }
-        CaptureMouse();
+        grab_mouse();
         Refresh();
     } else if (type == wxEVT_RIGHT_DOWN && m_selected_island >= 0 && m_select_mode == SelectMode::Island) {
         const Vec2f rel      = screen_to_uv(pos) - island_centroid(m_selected_island);
@@ -919,12 +957,16 @@ void UVEditorCanvas::on_mouse(wxMouseEvent &evt)
         m_rot_base_deg       = island_rotation_deg(m_selected_island);
         m_rot_display_deg    = m_rot_base_deg;
         m_gesture_last_angle = std::atan2(rel.y(), rel.x());
-        CaptureMouse();
+        grab_mouse();
     } else if (type == wxEVT_MIDDLE_DOWN) {
         m_gesture      = Gesture::Pan;
         m_drag_last_px = pos;
-        CaptureMouse();
+        grab_mouse();
     } else if (type == wxEVT_LEFT_UP || type == wxEVT_RIGHT_UP || type == wxEVT_MIDDLE_UP) {
+        // The drag is over either way. A modal R/S keeps running until a click confirms it, but it
+        // tracks the pointer over this canvas and needs no capture to do so, so the capture goes back
+        // here rather than waiting for that click - which may never come.
+        drop_mouse();
         if (m_gesture != Gesture::RotateIslandModal && m_gesture != Gesture::ScaleIslandModal) {
             end_gesture();
             Refresh();

src/slic3r/GUI/UVEditorCanvas.hpp

@@ -10,6 +10,7 @@
 // pulls in the platform's real GL/gl.h, and glad/gl.h errors out if that happens first (it wants
 // to be the one to define the standard include guards GL/gl.h itself defines).
 #include <glad/gl.h>
+#include <wx/event.h>
 #include <wx/glcanvas.h>
 #include <wx/panel.h>
 #include <wx/button.h>
@@ -194,6 +195,7 @@ private:
     void on_paint(wxPaintEvent &evt);
     void on_size(wxSizeEvent &evt);
     void on_mouse(wxMouseEvent &evt);
+    void on_capture_lost(wxMouseCaptureLostEvent &evt);
     void on_key(wxKeyEvent &evt);
     void on_leave(wxMouseEvent &evt); // drops the +/- cursor hint when the pointer leaves the canvas
     void on_erase_background(wxEraseEvent &evt) {} // required to avoid flicker on MSW, deliberately a no-op
@@ -238,6 +240,16 @@ private:
     // vertex of some *other* island, in texture-UV space. Zero if nothing is within reach (#2).
     Vec2f snap_correction(int island) const;
     void  end_gesture();
+    // Cancels the gesture instead of finishing it: nothing is committed, and a modal rotate/scale is
+    // put back the way Esc puts it back. Used when the capture is taken away from us.
+    void  cancel_gesture();
+    // One capture at a time, released exactly once. Pressing a second button mid-drag would otherwise
+    // nest a second capture that the single release on button-up cannot undo, and macOS never sends
+    // wxEVT_MOUSE_CAPTURE_LOST to recover from that. A leaked capture is not a local problem there:
+    // while any wx window holds one, wxOSX routes every mouse event to it and the application stops
+    // seeing enter/leave and motion entirely, which also takes its tooltips down.
+    void  grab_mouse();
+    void  drop_mouse();
     // Rebuilds the status line from the current gesture/selection and pushes it to m_on_status.
     void  update_status();
     // Re-picks what a click at `pos` would grab in the current select mode, and repaints when that changed.