I cannot be sure about how is guilty here ... however it happens only in e17 ( tested also in xfce4 ).
Actor involved here are:
- Youtube/Flash?
- Google Chrome
- Cairo-Dock
- E17
having all those nice piece of soft ( exclude flash from this definition :P ) running together cause the title issue: if i try to put the youtube video fullscreen I'm redirected back to the no fullscreen view.