MediaViewer has some fullscreen-handling code which would have a better place in core. Something very simple, just to abstract away browser prefixes - something like $.fullscreen.isSupported() / $.fullscreen.isActive() / $.fullscreen.request() / $.fullscreen.cancel() / $.fullscreen.toggle(), plus triggering the fullscreenchange event if the browser does not support it, plus maybe adding a top-level class.
Docs: https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Using_full_screen_mode