Greetings, CentOS refugee here. On my old Mate/CentOS 7 system (with two rotated screens), I configured and updated grub with "fbcon=rotate:3" to turn the greeter screen to conform to the rotated physical screen. So far, I haven't accomplished the same with Mate-Ubuntu Focal LTS 20-04.3, which I hope will replace CentOS on every machine in the house and office ... Real Soon Now.
After boot, I've successfully rotated the working Mate-Ubuntu desktop images with the gui and System/Hardware/Displays ... easy-peasy. I've found no obvious way to rotate the (lightdm? slick-greeter?) login screen.
While I can cope with "rotated mouse behavior" to maneuver the pointer into the login box on the greeter screen, my boss won't put up with such shenanigans ... she'll divorce me, or worse, bring Microsoft into the house. I can start the systems in properly-rotated command-line text boot, then do "startx", but my wife might hire Clippy the Assassin.
It's too late to save my sanity, but I'd like to save my wife's. Help?
Keith Lofstrom, Unix/Linux since BSD3.