The last UI element you want to malfunction when you’re playing with Display Settings is the Display Settings itself. 

Steps to reproduce:
1.On 1920x1080 or 1280x1024 display, Display -> Scaling -> 2x
2.Reboot
3.Try to change scaling back

I have a 1920x1080 13” laptop. I was not happy with some widgets at 1.5 so I made the mistake of trying 2x scaling factor. After a reboot there was no way to access the scaling setting or the any of the bottom buttons for the Display dialog. Meanwhile I had the huge waste of a laptop screen that was useless. Even if you hide the laptop screen, you still can’t access the buttons below. If you try maximizing / resizing, you get all kinds of rending artifacts. 

Of all the settings dialogs, this one should be the most resilient. Instead it seems to be one of the most fragile. Tested with KDE Neon (1280x1024) and KDE Arch (1920x1080)

After much time wasted deleting .local/share/kscreen and other KDE system files, I finally got out of this mess by using xrandr and faking a 2K display.

My suggested solution to this bug, is to allow the display settings to be any size, but if the buttons are off the screen put in a scroll bar. Also the display should shrink/be hidden before the buttons disappear.