Unity elements sometimes appear while full screen in Virtualbox - how to stop it?

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http://askubuntu.com – Launcher is great, and it helps me finds stuff. I have the latest release of 12. But when I'm full screen in VirtualBox running another operating system and I hit some key combination I haven't yet figured out, Unity suddenly grabs keyboard focus. It might be alt-tab, it might be left-control x. I have physically pulled both the windows key and FN key off my laptop because they get in the way, so it's probably not the windows key. You don't know the focus has been stolen because the full screen app (VirtualBox) is still full screen and the Unity element is in the background. But you're busy (HowTos)