Comment # 3 on bug 65810 from Michel Dänzer

(In reply to comment #2)
> I have the same after playing SpaceChem(game using Mono) on Steam.
> It goes away after stopping Steam.
> 
> xwininfo: Window id: 0x4a00073 (has no name)
> 
>   Absolute upper-left X:  0
>   Absolute upper-left Y:  0
>   Relative upper-left X:  0
>   Relative upper-left Y:  0
>   Width: 64
>   Height: 24

Yep, looks like a leftover window. Probably a steam bug then, but definitely
not a driver bug.