Set a Permanent Custom Resolution on Ubuntu and KDE with xrandr and Xsetup
TL;DR Generate a modeline with cvt, add it with xrandr --newmode / --addmode, then enable it on your output. Session-only changes vanish on reboot and never fix the login screen. On KDE/Kubuntu with SDDM, put the same commands in Xsetup so they run before the greeter. After I switched from GNOME/Unity to KDE, SDDM would not pick up my ultrawide panel. It stuck at Full HD instead of 2560×1080. I had hit the same class of problem earlier on Ubuntu with an older monitor whose native mode never showed up in the display settings. ...