Steps to recreate:
1) Have another application running in windows aside from NB
2) Open an editor and place cursor anywhere in the editor (just to set focus)
3) Alt-tab from NB to another application.
4) Alt-tab back to NB
The menu is now active, as if you had pressed the alt button with in NB. In order to get back to the editor you have to
cancel out of the menu (pressing alt or esc, or clicking on editor, etc ).
Unfortunately this is Swing bug, not directly related to Netbeans, we can't workaround it...
See older report for more details.
*** This issue has been marked as a duplicate of 73960 ***