I'm seeing this as well --- under Xfce. I'm under the impression that applet.py is waiting for the DBus session bus to exit, but my system has dbus-user-session installed and it's possible that systemd is waiting for applet.py to exit before it will shut that down. Indeed if I send a SIGTERM to the session dbus-daemon that's enough to make applet.py exit.
I think that means applet.py needs some other (additional) way of being told when to exit. How do other applications do it? XSMP?
