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

1. Needs info: I thought the goal here was to skip rendering on idle displays. That code doesn't seem to be present yet, so maybe it comes later? It would be: composite() returns "I had nothing new to render" so then post() never gets called. You either need to composite and post, or do neither. But never composite without post (wasted of GPU), and never post without composite (uninitialized pixels appear on screen).

Although that raises an important question: Is the bug safe to fix at all under the new display groups architecture? How do we safely tell Android to only post one of the displays in the group but not the other? I'm wondering if it's even possible to fix this under the display group approach... because you must either have one composite and one post, or zero of all.

2. Needs fixing: Please add the FIXME comment back in. It's an important one we shouldn't forget.

I'm also nervous that we're rewriting so much code that covers years of regression fixes. But that's not quite a blocking issue.