Comment 1 for bug 1638929

Your USC log shows USC is using the second display when you re-enable it:

[2016-11-03 14:51:24.574533] mirserver: 0.52: DisplayPort 24.9" 550x310mm
[2016-11-03 14:51:24.574558] mirserver: Disabled
...
[2016-11-03 14:51:29.648108] mirserver: 0.52: DisplayPort 24.9" 550x310mm
[2016-11-03 14:51:29.648126] mirserver: Current mode 2560x1440 59.95Hz
[2016-11-03 14:51:29.648137] mirserver: Preferred mode 2560x1440 59.95Hz
[2016-11-03 14:51:29.648145] mirserver: Logical position +1920+0

However Unity8 probably doesn't yet understand what that means. We have an agreement from last month that we would like Unity8 to just always clone to all displays. But your log shows we're still spanning (Logical position +1920+0). I would not expect Unity8 to understand what spanning means yet.

Since we have an agreement from last month that we'd like to just always clone in the absence of a design for spanning, we should enable that. USC already supports the option (--display-config=clone) so we just need to use it...

So long as your monitor's backlight is on and we're just not painting to it (hence black), this is not a Mir or USC bug. It just requires more work in Unity8 to paint across the span (once we have a design for that), or shorter term work in LightDM to add --display-config=clone for USC.