Linux

If you're using Wayland, you can go to Settings -> Colors & Themes -> Login Screen (SDDM) and click "Apply Plasma Settings..."

If you're using X11, it looks like you'll have to resort to hacky scripts, unfortunately.

Source: https://discuss.kde.org/t/how-to-change-monitor-layout-and-orientation-in-sddm/3377

Put one hand on the top and one on bottom and rotate the screen by 90° or π/2 radians

Rotate the left display 90 degrees clockwise. Now they're both in landscape. Ta-da!

Sorry, I'll see myself out.

This is SDDM, the default login manager used by KDE.

The Arch Wiki has an article about it, look under section 2.6.

On the arch wiki for SSDM I found this:

Changes to your display configuration made in a Plasma Wayland session (e.g. monitor layout, resolution, etc) will not persist to SDDM. To make them persist open Plasma's System Settings and navigate to Startup and Shutdown> Login Screen (SDDM) and click "Apply Plasma Settings...". You will need to have permission to perform this action.

You should give that a try

From Archwiki > xrandr:

Tip: Both GDM and SDDM have startup scripts that are executed when X is initiated. For GDM, these are in /etc/gdm/, while for SDDM this is done at /usr/share/sddm/scripts/Xsetup. This method requires root access and mucking around in system configuration files, but will take effect earlier in the startup process than using xprofile.

That's not the point here but can you share your wallpaper? 😄

You just need to look at the problem from a different angle

Is this when the screen is locked or when you're logged out? Those are two different things and I suspect it's the latter. That's probably sddm and I suspect it can be fixed by using Wayland with it. Should be some option in /etc/sddm.conf or so.

Turn your head

Fix what? It looks perfect /s

Can I have that wallpaper?

usually monitors can be freely rotated. if yours can't, the back usually has a square vesa mount on the back and you can just take out the four screws and reattach it the way you like.