Comment 4 for bug 1506846

OK, this is what is happening here:

The host Mir doesn't apply the nested display config until after a frame is posted and the surface becomes active. So the two "fullscreen" surfaces created by the nested server are positioned on the only active screen.

While not allowing surfaces to become active (and have focus) until they've been painted is correct for most scenarios it is wrong for a system compositor. The creation of a (nested) session ought to be enough to make it active.