Sitching to fullscreen leads to a graphical glitch

Put the summary of the bug you're having here

What I expected to happen was...:
The game window should have gone fullscreen

What actually happened was...:
The window went fullscreen but the actual game remained a quarter of that size.

Steps to Reproduce:
1. Click on single player and choose a world
2. Click play selected world
3. while it is loading the world, maximize the game window before it finishes.