I've actually gotten a report about that before, but I haven't been able to recreate it. It doesn't do that for me, when I test it the box is always centred in the middle of the screen. So I haven't quite figured out what causes that yet. I've been thinking it might be an issue in how it interacts with fullscreen, or perhaps it might be browser dependant. The last user who reported this issue was using Firefox, could you tell me what browser you are using, and if you're running the game in fullscreen or not?