Fixed assertion when quickly toggling from fullscreen back to fullscreen: "Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'backgroundWindows not nil in enterFullScreenTransitionWithOptions:animated:activatingIt:'" To reproduce this, run testsprite2, press Alt-Enter once, again while it's animating to fullscreen, and then again while it's animating out of fullscreen.