Yes i found a Catalyst bug the :

"The Catalyst+Tear-Free+Gpu-Acceleration-flash+Exit-Fullscreen Withe-Screen Bug."

If i use Kubuntu 11.4+Catalyst11.4 on my AGP-based HD4650 system i can not use Tear-Free Desktop because the GPU acceleration from Flash end in an Withe screen if i exit an Fullscreen flash video on youtube.

If i disable the GPU-Acceleration in flash it works and if i disable the tear free desktop the GPU-Acceleration works.

its a Tear-Free VS GPU acceleration bug.

My workaround is to disable the tear-free desktop.