1) Run SwingSet3 (any Swing application will do)
2) Switch to the Windows L&F (this step is option, it just makes the problem easier to see)
3) Click on SwingSet3's "File" menu. Notice it expands fine.
4) Hit WindowsKey+D (show desktop)
5) Hit WindowsKey+D a second time. SwingSet3 appears again.
6) Notice that the SwingSet3 menu bar is disabled and clicking on the "File" menu does not cause it to expand.
6) To fix it, click on the desktop and then back on SwingSet3's window. The menu bar will work again.

Can be reproduced 100% on Windows7 and build 1.6.0_18-ea-b03.