Comment 12 for bug 1611804

David, re: "An output ID must be specified" USC requires that clients (assumed to be nested servers) allocate fullscreen surfaces on specified outputs. (Actually, I'm surprised it doesn't reject the connection before trying to create the surface as DM probably didn't tell it to expect that client.)

You can't specify an output when connecting mir_demo_client_basic. Try one of the egl clients and set the output ID from the commandline.