Mir doesn't detect secondary monitor input switching

[Daniel van Vugt]

Mir doesn't detect secondary monitor input switching

This seems to be a weird edge case, but it's also my default desktop configuration...

Test case:
1. Set up two monitors, plugged into your machine, but only have the primary monitor set to the correct input. Leave the secondary monitor on some other input so you get no picture.
2. Start mir_proving_server --display-config sidebyside
3. Switch the secondary monitor's input to that coming from Mir.

Expected: Secondary monitor is detected so Mir now appears on both.
Observed (1): Secondary monitor is not detected and says it's about to turn off. Just the primary one still works.
Observed (2): I don't need to touch the cabling to get it to come on. Just tap Alt+P (which should turn the monitors off, but this time just turns them all on). Now Alt+P is confused and needs a few tries to become unconfused.

This problem seems to be limited to input switching within the monitor. If you switch by physically plugging the cable in then Mir uses it immediately.

So a cable that's plugged in already is counted as unplugged by Mir, but also never receives hot-plug events when you switch it to the correct input.

[Michał Sawicz (saviq)]

Syncing task from Mir.