found 668828 xserver-xorg-video-nouveau/1:1.0.1-5 thanksThis bug affects my iMac G5 running Debian 7.1 (wheezy). In my case, the main symptom was that emacs23 didn't display any text when moved into the bottom half of the screen. The xorg.conf file in message #45 works around the problem for me.
-- Ben Harris