I have KDE 5 Plasma Installed on Arch on a ThinkPad P50 on a dock with an nVidia Quadro GPU running nouveau drivers. Laptop screen and external are both 4k.

When I installed Plasma connecting my external monitor worked fine, moving displays around worked fine, scaling worked fine, everything worked. I updated my whole system about a week ago and everything was working as configured still. I moved my desk around yesterday and decided to put my laptop on the other side of my monitor and when I went into display settings and switched the screens around it looked like it worked fine, until I clicked on something.

Basically what is happening is that to click buttons on a window on the left screen, I have to click on that coordinate on the right screen. When I switched to position of the displays, it moved the visual representation of the windows, but not their 'real' position, or something like that. If I put it back by blindly clicking around and using the tab index everything is fine. I tried to Unify the outputs and then reconfigure them from there but same thing. There are no errors, nothing is crashing, but occasionally when trying to fix it things crash now, which never happened a few weeks ago. Loading the Wayland WM usually locks up if my external monitor is connected and if I get into Wayland it's all kinds of broken, second screen displays nothing but can move my cursor into it, clicking anything hard locks system if I plugged things in according to a certain order, clicking log out pops up a crash error and doesn't log out.

For me, going into the monitor settings and swapping the monitor positions on the screen fixed this issue for me.

I did a unify/ununify displays and may have also swapped which monitor was marked as my primary prior to this as troubleshooting.