Seems to have been fixed in 1.8-pre3. Can anybody confirm this?
Confirmed for 1.8-pre1.
I have the exact same problem, and force quitting Minecraft didn't solve the problem. The fullscreen only starts correctly from the main menu (graphics take the entire screen as they should, as do controls). From anywhere else, this happens.
Seems to have been fixed in 1.8-pre3. Can anybody confirm this?