Everything had been working in Debian 12 (Bookworm) . Then I installed
Debian 13 (Trixie).
Now I can't get past the login screen, using either gdm3 or sddm. It
displays the users but when I click on one it doesn't do anything. In
particular, it doesn't offer a password box. I reinstalled Debian 13
and it offered a password box, turned the screen blank, and offered the
login screen again.
try to get the binaries of gdm3 and sddm out of the way. Move it somewhere.
Then reboot, you will get the console. From the console as root use the
command "startx", which should start X and the windowmanager.
If it not does, you might see any errors in the console, which could give you
a clue what happens.
Moved gdm3 to gdm3-sv and sddm to sddm-sv, then ran startx.
I got a completely blank screen of the same olive-green color as the background for gdm3 or sddm, with no buttons at all.
There was nothing remarkable in the console output from startx. There were no (EE) lines in Xorg.o.log other than the complaint about nv.
Maybe I'll try going back to Debian 12 (Bookworm).