Code review comment for lp:~alan-griffiths/mir/workaround-1525003

The main issue is waiting for the nested display surface to become exposed and focused; there's currently no good way to capture those events. It's important because the nested server will eventually end up doing mgn::MirClientHostConnection::hide_cursor which will race with the surface being exposed+focused

The mir_surface_set_state and mir_surface_configure_cursor will happen in order at the server (by default ipc threads is 1).