After one of the latest Safari update (9.0.1 I believe) I can't use fullscreen option on Youtube properly. When I open youtube page and press F the player goes fullscreen, but the video area itself doesn't (and actually became smaller), so the rest of display is black and unused. If I press "space" (stop the video) or scroll using mouse the video area stretches to the full screen, but this is very inconvenient to press "space" twice each time I full screen.
I checked that the problem is still present when I sign out from my youtube account. But if I watch a youtube-video (embedded) on the other cites there is no problems with it. And there is no such a problem with Opera. 2b1af7f3a8