I’m using EndeavourOS with KDE.
The display is correctly oriented when logged in but it doesn’t rotate correctly when I’m logged out.
EDIT: corrected the post. This happens when logged out, locking the screen has it displayed correctly.
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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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
Unfortunately, I'm having to use X11 because of work (context). Thanks for the help!
I suppose xrandr can help you here: See the Arch wiki about xrandr