Comment 27 for bug 1751593

The “black border” effects comes from the frame window resize.

The gdk_window_move_resize() in meta_ui_frame_move_resize() is what causes the border to become solid black, before it's repainted by the compositor.

With basic X11 clients, the delay between the gdk_window_move_resize() and the shadow being redrawn is so short that it cannot be seen, but when the X11 client is using a sync mechnaism, the update of the content is delayed until the client has finished updating its content, and in this case the black border is seen for a short time.

That explains why the black border shows most with clients with using NET_WM_SYNC protocol and server-side decorations.

As an experiment, commenting out the gdk_window_move_resize() in meta_ui_frame_move_resize() makes the problem disappear entirely:

  https://gitlab.gnome.org/GNOME/mutter/blob/master/src/ui/frames.c#L656

whereas commenting out the call to build_and_scan_frame_mask() in meta_window_actor_update_shape_region() makes the black border visible continuously:

  https://gitlab.gnome.org/GNOME/mutter/blob/master/src/compositor/meta-window-actor.c#L1699