Debian 10 update results in graphics issue

Hello,

I updated Debian 10 ("apt-get update" then "apt-get upgrade") on a 2010
MacBook Pro. Everything updated as expected, but after rebooting, the
graphics in Xfce was garbled. The garbled appearance was over the entire
screen, and it changed slightly every time I hit a key (the keyboard
still worked, so I was able to reboot from a previously-opened terminal
window).

Interestingly, when I disabled wdm and simply ran "startx" from a text
login, I didn't see any problems with the graphics. So it's not clear
whether this is an Xorg, Xfce, xdm, Debian, or possibly a firmware issue
(the graphics hardware is NVIDIA GeForce 320M).

I was able to restore my system from a backup, so I should be able to
duplicate the problem.

thanks for any suggestions

-Stan Johnson