Avoid blocking paint events from long-running mouse event handlers

If a mouse event handler calls Refresh(), increase the likelyhood that a
paint event can be issued before the next mouse event occurs, by requesting
more mouse events from the the end of the handler rather than the start.
See #18314

src/gtk/window.cpp

@@ -1815,19 +1815,6 @@ gtk_window_motion_notify_callback( GtkWidget * WXUNUSED(widget),
 
     wxCOMMON_CALLBACK_PROLOGUE(gdk_event, win);
 
-    if (gdk_event->is_hint)
-    {
-        int x = 0;
-        int y = 0;
-#ifdef __WXGTK3__
-        gdk_window_get_device_position(gdk_event->window, gdk_event->device, &x, &y, NULL);
-#else
-        gdk_window_get_pointer(gdk_event->window, &x, &y, NULL);
-#endif
-        gdk_event->x = x;
-        gdk_event->y = y;
-    }
-
     g_lastMouseEvent = (GdkEvent*) gdk_event;
 
     wxMouseEvent event( wxEVT_MOTION );
@@ -1875,6 +1862,20 @@ gtk_window_motion_notify_callback( GtkWidget * WXUNUSED(widget),
 
     g_lastMouseEvent = NULL;
 
+    // Request additional motion events. Done at the end to increase the
+    // chances that lower priority events requested by the handler above, such
+    // as painting, can be processed before the next motion event occurs.
+    // Otherwise a long-running handler can cause paint events to be entirely
+    // blocked while the mouse is moving.
+    if (gdk_event->is_hint)
+    {
+#ifdef __WXGTK3__
+        gdk_event_request_motions(gdk_event);
+#else
+        gdk_window_get_pointer(gdk_event->window, NULL, NULL, NULL);
+#endif
+    }
+
     return ret;
 }