When dragging a window with enabled OpenGL compositing to the top edge of the screen to maximize it or to the sides to spread it over half or a quarter of the screen a fully opaque box is shown which is either completely white or completely black.

Reproducible: Always

Steps to Reproduce:
Drag the window to the top, right or left edge of the screen to trigger maximization.
Actual Results:  
A black or white box is shown indicating which part of the screen the window would fill hiding everything that's below.

Expected Results:  
The box that appears should be white (it's sometimes black) and semi-transparent instead of opaque as it was in KDE 4.8.3.

Only happens when OpenGL compositing is enabled, works fine with Xrender.
Using Nouveau driver from Kernel 3.3.0.