The /etc/X11/prefdm script lacks this line:
exec xdm "$@" >/dev/null 2>&1 </dev/null
After the fallback attempts on gdm and kdm. Would be nice to have xdm as the third fallback attempt. Without this line, the user is required to create /etc/sysconfig/desktop with the following contents:
In order for the system to boot up to a login screen. Without the DISPLAYMANAGER setting, the system will appear to hang on boot up. Adding the 'exec xdm' fallback attempt line would be nice to have.
This is intentional, per the request of the xdm package maintainer.