Reproduction steps:
1) Run attached test application on Win OS (or at least somewhere where Swing Windows LF is available)
2) Open menu and try to go to first submenu
When mouse crosses the border of submenu, you'll see painting bug, selection rectangle overpaints submenu, which looks bad and is certainly painting bug.
Painting isn't perfect in second submenu either, but as submenu items are enabled, it is less visible.
added image to clarify what is the problem