Every so often the KDE lock screen is missing the password prompt. The mouse
cursor moves around, wakes up the display, with no way to get back to the
desktop. Normally I'd just force a log out or SHIFT-CTRL-BACKSPACE, but I had
a bunch of stuff running that I didn't want to unceremoniously kill. Of course,
searching for another answer these days provides all sorts of nonsensical
systemd stuff. One post mentioned loginctl, part of elogind. I then remembered
that Slackware switched to elogind. I remoted in from another desktop - running
Xfce which does not seem to suffer from this lock screen issue. 

$ loginctl
SESSION  UID USER    SEAT  TTY  
      1 1000 beowulf seat0 tty1 
      2 1000 beowulf       pts/4

2 sessions listed.

Now, we just pick the session to unlock.

$ loginctl unlock-session 1

ISSUE RESOLVED. 

Even after 25 years, I always enjoy learning new tricks for Linux, and for
Slackware in particular.