How to put Messsenger on top of full screen video?

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http://askubuntu.com – Just as the title says, how can I use messenger or any chat client while i'm watching video in browser with full screen? For example, i can use gtalk when watching youtube video in full screen in Windows, but can't figure out how to put chat window on full screen video using alt-tab. Thanks for any help. ;) (HowTos)