Multiple monitor setup correct login screen order

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

The configuration in Debian 13 Trixie is even a little more tricky:
while SDDM is running in a X11 session, you might want to run the desktop session in Wayland.
xrandr might use different output device names in X11 and Wayland.

X11
xrandr | grep -w connected
 
DP-2 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
DP-4 connected primary 3440x1440+2560+0 (normal left inverted right x axis y axis) 797mm x 333mm
Wayland
xrandr | grep -w connected
 
DP-2 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
DP-3 connected primary 3440x1440+2560+0 (normal left inverted right x axis y axis) 797mm x 333mm

To correct the monitor configuration first find the configuration on the desktop, but ensure you started the desktop session with Plasma (X11) not Plasma (Wayland). Open a bash as user and run: 

xrandr | grep -w connected
DP-2 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
DP-4 connected primary 3440x1440+2560+0 (normal left inverted right x axis y axis) 797mm x 333mm

DP-2 is the left monitor
DP-4 is the right monitor

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 DP-2 --mode 2560x1440 --pos 0x0 --rotate normal --output DP-4 --mode 3440x1440 --pos 2560x0 --rotate normal

Ensure /etc/sddm.conf.d/kde_settings.conf and /etc/sddm.conf contain:

[X11]
Xsetup=/usr/share/sddm/scripts/Xsetup