dragging the titlebar against screenborder creates a solid white overlay instead of a semitrasparent.
this happens only with the opengl backend, with Xrender the overlay is trasparent as expected.

the gfx card is the intel sandybridge if it is relevant and this seems a regression from kde 4.8


Reproducible: Always

Steps to Reproduce:
1. drag the window to the screen borders
2. the overlay is flat white instead of trasparent
3.


Expected Results:  
i'd expect a trasparent overlay like it happens with the Xrender backend