This is a known Plasma issue at the moment.
Note > Can’t login from external monitor
The way I overcame it, was to disable all but my primary screen at SDDM. I really only need the one screen to login anyway. Not a fix so much as a logical starting point.
If you run xrandr, it will show you our currently connected displays. This will filter the output to just the connected displays (note the space in front of ' connected').
xrandr | grep ' connected'
My output looks like this:
DP-0 connected 3840x2160+2560+0 (normal left inverted right x axis y axis) 600mm x 340mm
DP-2 connected primary 2560x1440+0+360 (normal left inverted right x axis y axis) 698mm x 393mm
So there I have DP-0 (secondary) and DP-2 (primary). If I want to disable all but my primary at the SDDM login, I edit /usr/share/sddm/scripts/Xsetup:
sudo nano /usr/share/sddm/scripts/Xsetup
And modify it to look like this:
#!/bin/sh
# Xsetup - run as root before the login dialog appears
xrandr --output DP-2 --auto --primary
xrandr --output DP-0 --off
You could use that config too, but you must change references to DP-2 and DP-0 to reflect your setup. Your displays will likely be called something different. Feel free to share your output of xrandr here if you’d like a hand with that.