I have upgrade my OS to Linux Mint 12 (Code name Lisa) that use GNOME 3 as her desktop environment, there is a little problem when you run netbeans in GNOME 3, when you click menu in netbeans, it will be dissapear as soon as you release mouse button, the menu does not dissapear if you drag it. You can see complete report about this bugs in here and you can see the discussion about it here.
So how do I solved the problem? After little “googling”, I found the best way and of course it is very tricky :
ALT + SHIFT + ENTER twice
and voila, you get your menu back! this is JDK Issue, but I think there is no update yet for this issue, so this tricky way can help you for a moment until there is an update for it.