Well, I made the mistake of tinkering with the Monitor & Display settings in the Control Center on my other machine, expecting that the 'Test Settings' button would work like the 'Test' button in the Configure Desktop => Screen Saver does, but no; 'Test Settings' seems to have stored its settings somewhere, there is no evident way to revert the display, and further tinkering has now the monitor is totally unreadable. (I'd provide a screenshot, but as usual, my old Samsung Galaxy 6 phone can't be read by Trinity.)
Fortunately, I can ssh into the broken machine from this one, but I have no idea where Monitor & Display stores its misconfiguration files.
Does someone know where they would be? I see /etc/trinity/tdedisplay/tdedisplayconfigrc, but it doesn't seem like it would have the effects I'm seeing:
| @14:04:11 leslie@─►chestnut◄─ | wd=/home/leslie | $ less /etc/trinity/tdedisplay/tdedisplayconfigrc | EnableAutoStartProfile=false | EnableDisplayControl=false | StartupProfileName= | rc=0
OTOH, on this machine, /etc/trinity/tdedisplay/ is empty,
| @14:02:05 leslie@pinto | wd=/home/leslie | $ ls -l /etc/trinity/tdedisplay/ | total 0 | rc=0
so maybe just removing that config file will fix it?
Leslie