Comment 12 for bug 1532202

OK, I understand at least part of what is going on:

If, in the nested server, I hack DefaultDisplayBufferCompositor::composite() to check for an empty renderable_list and not call the renderer in that case we stop posting to the external display and performance is maintained so long as the client surface remains on the internal output.

As soon as the client surface overlaps (or is entirely on) the external output performance drops. This is cured by applying the same hack in the host compositor.

So the question is: does this "don't render nothing" optimization belongs in the renderer or the compositor?