Returns true if the browser/device is in full screen mode.
Pure document-state probe — no Application context needed, since the
browser tracks exactly one fullscreen element per document regardless
of how many Applications are running.
Returns boolean
true when any of the four vendor variants of fullscreenElement is non-null
Returns true if the browser/device is in full screen mode.
Pure document-state probe — no Application context needed, since the browser tracks exactly one fullscreen element per document regardless of how many Applications are running.