I finally went ape and re-installed from the latest stretch build.
Initially, with the nouveau driver, XFCE$ synaptic (and others) drop
down menus worked. I installed nvidia-driver and it installed everything
related to nvidia. Rebooted and with the nvidia driver running, synaptic
still worked. THEN I created an /etc/X11/xorg.conf file, rebooted, and
it all went to h-e-double hockey sticks with the black screen and active
mouse pointer. Synaptic borken again.
And yes, I have a 4 monitor setup via two video cards. It has worked
well for several years. I rebooted into Xubuntu 16.04 on another drive,
it works just fine. I copied that xorg.conf file to my Debian drive
/etc/X11, rebooted, no go. I removed xorg.conf, rebooted and no problems
other than using one monitor out of four. Xubuntu works, Stretch not. I
would love to know where the difference is so I could make a sane bug
report.