Comment 7 for bug 714646

The raster backend is the one we are going to use for the foreseeable future, especially for Natty.
The X11 backend is not as fast and not recommended by the Qt developers
The OpenGL backend will not work on all hardware/software configurations. I would like in the future to enable it when possible and for the detection use the code written by Loïc Molinari for Unity 3D.

I believe the cause of the issue has changed from:

"[..] appears only when the radius of the background Rectangle is not 0."

to:

We do not use any background Rectangle anymore but Images instead. I am afraid the fix will not be simple.