GIMP 2.10.22 on Slackware64-current is unable to do Print Preview. There is no visible error message and nothing happens. In a terminal window or ~/.xsession-errors you will see a usage message from xpdf.

I determined that GIMP is doing Print Preview by trying to run 'xpdf -q /tmp/FILE.pdf'. However, the -q option was removed from xpdf between version 3.04 in Slackware-14.2 and version 4.02 in Slackware64-current. So it doesn't work.

But that's as far as I got. I read in the Gtk+3 documentation that the default Gtk+3 print preview command uses evince, not xpdf. So either this was changed somehow in Slackware, or GIMP isn't using Gtk+3 to do print preview.

There really should be a wrapper script between GIMP and xpdf anyway, so that the temporary file can be deleted after viewing. That's what I did in GIMP-2.8.x on Slackware-14.2, using a setting in ~/.gtkrc-2.0. But I can't figure out how to do this in Gtk+3 - or if it would work with GIMP.

Any ideas?

Just tried gimps print preview in a vm and it worked fine. The vm is fully up to date Fri Jan 15.
So can't really say where your problem lies. I can say gimp 2.10.22 is definitely gtk2 based if that's any help.

Yes: That was the missing piece. I just assumed GIMP-2.10 was using Gtk+3, but no, that isn't planned until GIMP-3.

The problem was in my Gtk+2 setup. I had ~/gtkrc-2.0 setting gtk-print-preview-command to point to a script, which used xpdf -q, and cleaned up the temporary files after. I removed -q from the script and it works now.

Also it turns out if you don't set gtk-print-preview-command, GIMP defaults to first trying evince (which will fail with a message), then trying xpdf, which will work. Except it won't clean up the temporary files.

Thanks for the missing clue.