Metacity's context menu for CSD's titlebars
Metacity should show its own context menu when user clicks on GTK's client-side titlebar. Currently Metacity does not support it, so GTK fallbacks to its own simple menu. It's inconsistent especially with GNOME applications.
Please look at this to see how it's implemented in Marco, Metacity's fork: https://github.com/mate-desktop/marco/pull/370/commits