I have my desktop arranged with 3×27 monitors, two of which are in portrait mode. This works fine once I'm logged in to the normal display manager, but makes my lockscreen/login screen terrible (everything is rotated and unusable.)
In addition I have a few secondary user accounts for my friends etc who occasionally use the desktop; the monitor arrangment for them is broken horribly.
For lockscreens I found a few old answers involving hand-editing lightdm files (which don't even exist anymore :/).
What's the proper modern way to fix this?
(I'm going to ask a second question just about this, but I'm particular interested in a solution that'll also fix the question for xmonad, which I prefer to use.)
This applies on Ubuntu 20.04 (and also earlier versions.)