This also could be helpful, depending on what exactly you're trying to accomplish:
window.moveTo(0,0);
window.resizeTo(screen.width,screen.height);
var navButtonsEtcHeight = screen.height - window.innerHeight;
var navButtonsEtcWidth = screen.width - window.innerWidth;
Note: For browser-independent solution to "window.innerHeight" support, see this link
Then you have the answer you're looking for -- mind you, with the downside of the non-fullscreen user seeing their window bounce around the screen first.
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