1. Launch the game
2. Enter fullscreen (F11)
3. Observe

Also see attached screenshot, which is the result of pressing F2 in fullscreen mode. The blue background seems to be the sky. Also, F2 doesn't seem to get the proportion right, the game displays about 3 times as small.

Workaround for still being able to play in fullscreen: Enter fullscreen on the main menu, then start/join a game.

This was fixed for a few hours after 1.8-pre1, but apparently it un-fixed itself, probably relating to LWJGL somehow.