When launching a GNOME session from a text-mode VT, the logind session
type is unlikely to be set to either "wayland" or "x11". We search for a
supported session type first with logind and then with
$XDG_SESSION_TYPE. As a fallback, we also test $DISPLAY in case of a
"tty" logind session to support starting through xinit. Ideally, such
setups should set XDG_SESSION_TYPE=x11.
If no supported session type is found, we throw an error.
https://bugzilla.gnome.org/show_bug.cgi?id=759388