Created attachment 87888 [details] [review]
patch

IIRC this is related to tiling and not limited to steam windows, I saw this in
different apps with r600g when window height is in range from 120 to 128. Just
reducing the height of glxgears window to that range was enough to trigger the
bug (or setting the height in command line, e.g. "glxgears -geometry 100x121").

Attached patch for xf86-video-ati fixed it for me, but I'm not sure if it's a
correct solution.