Comment # 8 on bug 104598 from Michel Dänzer

On further thinking, this should be addressed in the xserver present code. Even
if the kernel didn't return an error from the page flip ioctl, it still would
from the vblank ioctls, which can currently result in some of the client's
PresentPixmap requests being executed out of order (which might explain at
least some of the intermittent artifacts).