Bug#478960: mayavi2: interpolation produces fake transparency effects
Do you know when the problem appeared? Could it be related to
xserver-xorg-video-intel 2.2.1 entering testing recently? This was the
first intel driver in testing with EXA 2d acceleration enabled by
default instead of XAA.
Could you try adding
Option "AccelMethod" "XAA"
to the Device section of your xorg.conf in case the problem is EXA related?
Also, could you check whether the problem still occurs with latest
xserver-xorg-video-intel (2.3.0 is in experimental, and 2.3.1 will enter