Fix ghost shadows left by context menus

If the shadow is destroyed immediately before the window is destroyed,
we need to schedule a workspace repaint in order to prevent showing a
"ghost" shadow.

BUG: 425294

toplevel.cpp

@@ -583,6 +583,7 @@ void Toplevel::updateShadow()
 {
     QRect dirtyRect;  // old & new shadow region
     const QRect oldVisibleRect = visibleRect();
+    addWorkspaceRepaint(oldVisibleRect);
     if (shadow()) {
         dirtyRect = shadow()->shadowRegion().boundingRect();
         if (!effectWindow()->sceneWindow()->shadow()->updateShadow()) {