This sounds like the cumulative effect of a number of Mir bugs in the display configuration handling that resulted in the surfaces composited by a nested server appearing in the wrong place.
It is plausible that the fixes landed up to -r3056 have resolved this (they certainly address Mir issues in this area) - but I was waiting for the 0.18 release to test with the full stack.
This sounds like the cumulative effect of a number of Mir bugs in the display configuration handling that resulted in the surfaces composited by a nested server appearing in the wrong place.
It is plausible that the fixes landed up to -r3056 have resolved this (they certainly address Mir issues in this area) - but I was waiting for the 0.18 release to test with the full stack.