
If graphical applications want to start from systemd units, they need to start after we're properly ready to display them. This is particularly important under X where `_GTK_FRAME_EXTENTS` and other xprops are needed to have the right theming. We're doing this in an idle callback so that the dynamic starting of `gnome-session-x11-service.target` (which launches `gsd-xsettings`) as the result of a signal emission happens before us signalling we're ready for later things to start. https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/750