Comment 3 for bug 1239955

Did some more digging.

It seems the culprit is actually some unpolishedinteraction between powerd, earlysuspend and mir.

When turning the screen off, the earlysuspend state is set through powerd. See libsuspend/ in powerd for the details, but effectively on nexus 4 this ends up writing "mem" to /sys/power/state. This seems to turn off the display itself!

You can observe:
echo "mem" > /sys/power/state
mir/integration-tests # Fails to post to FB device
cat /sys/power/wait_for_fb_wake # Will Block Indefinitely

On the other hand
echo "on" > /sys/power/state
mir/integration-tests: Suceeeds
cat /sys/power/wait_for_fb_wake # Returns immediately

Trying to work out how to best coordinate a fix. As a work around, we could of course just run "powerd-cli active" during the mir tests, to ensure we keep have this "on" state.