Comment 10 for bug 1624407

Clicking on a window's close button sends the app mir_event_type_close_window. That's just an informational message that apps are meant to act on (e.g. pop up a dialog if not terminate themselves).

If you mean logout however, then I guess the right answer is that shells should send mir_event_type_{close_window,quit_app} to all apps on logout, and wait until the apps are no longer running (user responds to dialogs from apps with unsaved work or some timeout expires).