Code review comment for lp:~alan-griffiths/mir/fix-1535894

> Admittedly DefaultDisplayBufferCompositor is only used in USC and probably not
> in Unity8 any more. But we could do a similar fix in QtMir to address Unity8.
...
> It's not just other shells that don't exist yet which need this, but Unity8
> needs it right now. Consider Unity8 on desktop with two monitors -- It does
> external rendering like minimize animations, shadows, and the whole launcher,
> that surface_stack.cpp doesn't know about and will get wrong using your
> proposed implementation.

This highlights a serious failing in our customization points.

Unity8 uses QtCompositor from QtMir, not MultiThreadedCompositor from Mir - and [Default]DisplayBufferCompositor is only used by the latter.

So the difference between this approach and your suggested alternative makes no difference to Unity8.