After a new install of openSUSE 13.2 - KDE, my desktop was completely broken
(windows were not rendered properly, menus would only partially display on
mouse over).
My graphics card is a GeForce 7600 GT.

I somehow managed to turn off desktop effects (like groping in the dark) and
the problem was gone. Conclusion: desktop effects should be turned off by
default together with the nouveau driver.

Addition (not critical):
Later I installed the nvidia driver via zypper and the problem also didn't
appear. However, with desktop effects turned on in "Configure Desktop", these
don't work. When e.g. changing the animation speed, an error message appears
"24 desktop effects could not be loaded"; details: "desktop effect system is
not running".