Running stretch, I confirm that the problem still exists today (after a dist-upgrade).
Installing dbus-user-session AND REBOOTING cured it. ii liblightdm-gobject-1-0 1.18.2-1 amd64 simple display manager (gobject library) ii lightdm 1.18.2-1 amd64 simple display manager ii dbus 1.10.8-1 amd64 simple interprocess messaging system (daemon and utilities) ii dbus-user-session 1.10.8-1 all simple interprocess messaging system (systemd --user integration) ii dbus-x11 1.10.8-1 amd64 simple interprocess messaging system (X11 deps)
