If the game is fullscreened when on the "Reading world data..." screen, the game becomes unresponsive until it gets past this screen. This makes it so you can't tab to other windows or un-fullscreen the game.

Steps to reproduce

1. Fullscreen Minecraft
2. Load a world, especially one with a large file size so it takes a long time on the "Reading world data..." screen
3. Press F11 to try to unfullscreen the game while it still says "Reading world data..."
4. Notice it doesn't work until it is done on that screen