Comment # 39 on bug 56405 from Alex Deucher

(In reply to comment #38)
> The distorted graphics problem disappears when setting the option
> Option "NoAccel" "on". 
> (This may be obvious to the experts.)
> 
> In this mode a new phenomenon appears: When moving th mouse to the
> Activities corner and the "summarizing display" is shown, the background
> consists of a corrupted image.

That option disables all hardware acceleration.  All rendering is done in
software.