I opened emacs. According to Help -> About, it's using a Gtk 3 interface.
(I notice the
q key kills this About buffer).
Options -> Customize Emacs -> Top-level Customization Groups, opens a Customize buffer. I get a new menu Custom, which shows an Exit action, associated with the shortcut key
However pressing the
q key does not kill the buffer. It only switches back to a different buffer, but the Customize buffer remains open.
Using the Exit menu item, or the Exit button on the toolbar, does exactly the same.
Fedora 27 Workstation