When the window is signalled (by Wayland) to be closed, either by pressing "X" in the Gtk Titlebar or issued by the compositor and multiple tabs are open, only the focused tab is closed.
Created attachment 320282 [details] Patch
Comment on attachment 320282 [details] Patch Clearing flags on attachment: 320282 Committed r221819: <http://trac.webkit.org/changeset/221819>
All reviewed patches have been landed. Closing bug.