I know the answer to this question, I've been looking there for years and its here too: Where is Xserver log file located?
Only it isn't - it's December 2020 and I'm logged into the desktop, but there are no logs since July (probably when I updated from 16.04 to 18.04)
$ ls -la /var/log/Xorg.*
-rw-r--r-- 1 root root 133637 Jul 23 19:38 /var/log/Xorg.0.log
-rw-r--r-- 1 root root 94763 Jul 23 16:31 /var/log/Xorg.0.log.old
-rw-r--r-- 1 root root 6382 Sep 2 2014 /var/log/Xorg.1.log
-rw-r--r-- 1 root root 65653 Jun 18 2014 /var/log/Xorg.1.log.old
-rw-r--r-- 1 root root 21314 May 27 2014 /var/log/Xorg.failsafe.log
-rw-r--r-- 1 root root 64 May 27 2014 /var/log/Xorg.failsafe.log.old
Any ideas?