+++ This bug was initially created as a clone of Bug #1030873 +++

If no default autologin session is defined, default.desktop (usually in
/usr/share/xsessions, but can eventually be in wayland-sessions as well) is
used by displaymanagers such as lightdm and sddm.

This link is now created by update-alternatives for IceWM, Xfce, LXQt (sr not
accepted yet), Plasma5 and enlightenment.

This means that if GDM is used if one of those is installed, it will list
default.desktop twice, as it is a symlink to a .desktop file in the same
directory.

lightdm and sddm are patched to recognize this properly.

GDM should respect the default.desktop selection if nothing else is defined in
/etc/sysconfig/displaymanager.

GNOME should also register itself with update-alternatives, with the same
priority as KDE (25), for reference:
https://build.opensuse.org/package/rdiff/KDE:Frameworks5/plasma5-session?linkrev=base&rev=108