JMenu is not clean up its last look when restoring from minimized icon. After restored its last look is empty and remains there even I moved an application.
Here is the steps to generate:

1. Run SwingSet
2. Move your mouse to File menu in order to open File menu
3. While opening File menu, move the mouse below to exit its area and minimize
   SwingSet
4. Restore it
5. You will see empty gray color box on the screen