Mac: [NSApp keyWindow] is not valid in windowDidBecomeKey: This fixes weird behavior on Mac where our first responder reverts to the window itself, rather than the SDLTranslatorResponder, after the window has lost focus once. This causes Escape to call cancelOperation: on the NSWindow, which by default removes our fullscreen-ness. When someone has turned off SDL_TEXTINPUT we should probably set another initial responder that handles the Escape behavior, so that SDL_TEXTINPUT doesn't change fullscreen behavior (and possibly other behavior) like it does now.