Comment 15 for bug 1216472

Further notes... The issue seems to occur on frames where some but not all of the outputs are dirty. So one output makes it through to callback (sna_xmir_copy_to_mir), but the other does not:

_X_EXPORT void
xmir_screen_for_each_damaged_window(xmir_screen *xmir, xmir_window_proc callback)
{
    xmir_window *xmir_win, *tmp_win;
    xorg_list_for_each_entry_safe(xmir_win, tmp_win, &xmir->damage_list, link_damage) {
        if (xmir_window_has_free_buffer(xmir_win) &&
            xmir_window_is_dirty(xmir_win))
            (*callback)(xmir_win, xmir_window_get_dirty(xmir_win));
    }
}

XMir is intentionally designed to only redraw dirty outputs. So not all get swapped every frame. I wonder however if that is somehow incompatible with how the intel DDX works?