Fix a sizing/layout problem with GTK3

Relying on "check-resize" to detect when a "size-allocate" is in progess is
insufficient, resulting in the possibility of a window ending up with the wrong
size or position after inital layout. Using our existing "size-allocate"
handlers should be enough to detect the cases we care about.
See #18865

src/gtk/toplevel.cpp

@@ -57,6 +57,7 @@ static wxTopLevelWindowGTK *g_activeFrame = NULL;
 
 extern wxCursor g_globalCursor;
 extern wxCursor g_busyCursor;
+extern bool g_inSizeAllocate;
 
 #ifdef GDK_WINDOWING_X11
 // Whether _NET_REQUEST_FRAME_EXTENTS support is working
@@ -240,6 +241,9 @@ size_allocate(GtkWidget*, GtkAllocation* alloc, wxTopLevelWindowGTK* win)
     if (win->m_clientWidth  != alloc->width ||
         win->m_clientHeight != alloc->height)
     {
+        const bool save_inSizeAllocate = g_inSizeAllocate;
+        g_inSizeAllocate = true;
+
         win->m_clientWidth  = alloc->width;
         win->m_clientHeight = alloc->height;
 
@@ -272,6 +276,8 @@ size_allocate(GtkWidget*, GtkAllocation* alloc, wxTopLevelWindowGTK* win)
             win->HandleWindowEvent(event);
         }
         // else the window is currently unmapped, don't generate size events
+
+        g_inSizeAllocate = save_inSizeAllocate;
     }
 }
 }