Comment 11 for bug 1580774

Looking at Mir's SurfaceInputDispatcher::deliver_pointer,

        auto target = find_target_surface(event_x_y);
        printf("input event target %p, pos: (%d,%d)\n", target.get(), event_x_y.x.as_int(), event_x_y.y.as_int());

prints this when hitting the barrier:
 input event target 0x562470, pos: (1177,323)
 input event target (nil), pos: (1213,323)
that is with mouse only about 300 pixels across the screen.

But I guess the real problem is that the Mir cursor coordinate is outside the physical display space, i.e. it should go beyond x=1200, and thus always be over the unity8 surface. Unity8 reads the cursor movement delta, not the position, to position its cursor