OOKB - Olaf's Own Knowledge Base

In a multi-monitor configuration, the order of the monitors might be different on the logon screen and KDE Plasma (Wayland).

To correct this first find the configuration on the desktop:

xrandr |grep connected

DisplayPort-0 connected primary 3840x1600+3840+177 (normal left inverted right x axis y axis) 880mm x 367mm
DisplayPort-1 connected 3840x2160+0+0 (normal left inverted right x axis y axis) 607mm x 345mm
DisplayPort-2 disconnected (normal left inverted right x axis y axis)
HDMI-A-0 disconnected (normal left inverted right x axis y axis)

DisplayPort-0 is the right monitor
DisplayPort-1 is the left monitor
DisplayPort-2 and HDMI-A-0 are not connected
This information can be used to correct the SDDM configuration in /usr/share/sddm/scripts/Xsetup. At the end of the script add:

xrandr --output DisplayPort-2 --off --output HDMI-A-0 --off
xrandr --output DisplayPort-1 --mode 3840x2160 --pos 0x0 --rotate normal --output DisplayPort-0 --mode 3840x1600 --pos 3840x177 --rotate normal