I think I've solved it. I removed gdm3 and lightdm
apt purge gdm3
apt purge lightdm
then reinstalled gdm3 and ubuntu desktop.
apt install gdm3 ubuntu-desktop
It now seems to be working as expected.
It appears to me screen blanking may not work with lightdm, but I'm not sure
perhaps it was
I am running the default 20.04 gnome desktop, except for the display
manager.
I'm not sure how to trigger the issue since the problem is that the
screensaver doesn't start when it's supposed to.
Ran
journalctl -b 0
Checked the journal nothing there related to gnome. There are a lot of
Public bug reported:
After a do-release-upgrade from 18.04 to 20.04 screen blanking stopped working.
I had my desktop set to blank but no lock the screen.
After the upgrtade it does not ever blank.
preferences are set in Settings-> Power -> Blank Screen 10 minutes
Automatic Suspend is OFF
