After switching from full screen mode, you can't use the upper-right button to make the game full screen. Nor can you resize the window. The only way to go full screen again or normal is to use either f11 or the full screen option in the options.

Reproduce:
1. Start Game
1a. You can resize the window as much as you want at this point
2. Make the game full screen through f11 or options menu
3. Return the game to windowed again through either method
4. No longer able to resize window or use full screen button next to the downsize button.