On a fresh TW install using 2016-03-21 x64 iso, I went to install a minimal
Plasma desktop (as in, just the desktop without all the KDE apps).

I selected "Minimal X11 Install from the main selection screen, and the at the
final YaST2 software setup screen, I added patterns-openSUSE-kde_plasma back
in.

When the machine rebooted, it ended up at the XDG default login. Even though
the plasma pattern pulls in SDDM as a recommended, it doesn't configure it to
work correctly.

I checked the /etc/sysconfig setting via YaST2 for DISPLAY_MANAGER and it was
blank.

I would think that the default behavior of either the Plasma 5 pattern or maybe
the SDDM package would be it to set that variable if it wasn't configured
already to a non-XDG value, so that a fresh install would be working correctly.