Once the mouse leaves the window, the square doesn't follow the mouse anymore. This is so you can click on other windows in the background if you need to (like if an IM window pops up).

The reason the square doesn't go all the way off the screen when your mouse leaves is so you can't move the square off-screen to avoid red squares. If the square left the screen, then you could run up a high score by just accumulating the points you get from the timer.

Happy New Year,
Sam

An other idea : can't we restict the zone we can see? I mean that the square could go outside (and be invisible) but red squares too! So it would be much dangereous to stay in the dark... but we will never be stuck on the side?