After the 20171022 update, the display manager is always gdm, ignoring the
DISPLAYMANAGER setting in /etc/sysconfig/displaymanager.

First time I saw this, I checked using yast, and saw that the setting was still
sddm. I reset it to the default, then specified sddm again, rebooted, and still
got gdm. I then edited /etc/sysconfig/displaymanager by hand, trying various
values: sddm, lightdm (both are installed on my computer) and even
/usr/bin/sddm, rebooting each time; I still got gdm. I even tried making
/usr/bin/gdm a symbolic link to /usr/bin/sddm and still got gdm after a reboot.
The latter suggests that gdm is somehow hard-coded into the system.

Personal preferences aside, gdm leaves tons of gdm and gnome processes running,
and also seems to change the display DPI, which I had to force to 96 dpi in the
plasma settings, so it is not an appropriate display manager for those not
running gnome.