So, at least on Windows 8, when you go on Fullscreen, and back to normal, the Minecraft Screen will be locked to the small size. You can not press the middle button of the 3 in the upper right corner. It's just stuck there. To fix this, you need to restart Minecraft. This is rather annoying. But I have no idea what causes this to happen. I think I have tried it on Windows 7, and it seemed to work fine. So it must be something with Windows 8. But if it can be fixed, this should be fixed.