I think that the pointer wrap could be used to cheat in this way. In osu!, you can still use the Pointer Wrap which is on the programming of certain mice and can be turned on and off at will. Pointer wrap is where when you move a mouse against the edge of the screen, it goes to the opposite edge. I use a *RocketFish Wireless 2.4G Laser Mouse. In the mouse settings, there is an option to enable pointer wrap. Perhaps you could make this disabled when osu! is fullscreen so that people don't use it to an unfair advantage.
Proof:
*This is a mouse which has its own driver and settings, which one of those is the Pointer Wrap.
Proof:
*This is a mouse which has its own driver and settings, which one of those is the Pointer Wrap.