The KWin devs blame that on the driver. I agree as it does not happen with the nVidia closed driver.
Now watch a swarm of ATI fans here say
"Here we go again with bashing ATI drivers."
"The KWin devs only use nVidia."
"Why don't you send ATI cards to the devs?"
"Go back to Windows you noob troll"
I just saved you guys some typing. Or did I miss one?