meta_window_handle_ungrabbed_event() triggers the show of the window menu in
gnome-shell via meta_window_show_menu() on hold of Meta + right mouse button
click.
Since meta_display_handle_event() was refactored lately and now forwards a
lot more events to Clutter (including the one triggering the window menu),
gnome-shell now sees this event after the menu has opened, figures that the
source-actor is outside of the menu, and immediately closes the menu again.
This is the correct behavior from the PopupMenuManager on the gnome-shell
side, it is the responsibility of the event handler that opens the menu (aka
meta_window_handle_ungrabbed_event()) to return CLUTTER_EVENT_STOP and stop
event propagation.
So fix this issue by adding a return value to
meta_window_handle_ungrabbed_event() and stopping event propagation in case
the event opened the window menu.
While at it, also return CLUTTER_EVENT_STOP for events triggering window
drags, so we can drop the extra check for that in
meta_display_handle_event().
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3623>
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