Code review comment for lp:~vanvugt/compiz/fix-unredirect-flicker

"the window that compiz displays to is clipped so that it does not cover the area of the unredirected window"

That is relevant to regional sub-suffer redraws but not relevant to buffer swapping. Buffer swapping will swap everything, including unredirected/uninitialized regions. I don't believe that the X server has a hook into GLX to tell the driver to clip the swapped region around unredirected windows.

So why don't we see flickering all the time?...
With a single monitor, it /appears/ to mostly work either by luck or by design of the X server to ensure that unredirected windows are painted immediately after the buffer swap, but before vertical retrace. Either way, I can't find a documented reason why it behaves that way, or why it should work without flickering, ever.