Comment # 2 on bug 1008371 from Nolan Leasy

After further investigation, this appears to have been a problem with the
non-root user account.  The problem resolved after removing everything under
the user's home directory, including hidden files and sub-directories.

This user account was imported from a previous (non-SUSE) install, which is a
feature that I've never used in the past.

I'm not sure of the exact conditions that led to this problem, but I know that
after this initial install I was able to start GNOME and KDE correctly using
SDDM.  In any case, this does not appear to be an openSUSE Leap 42.2 issue. 
I'll flag this as RESOLVED.