This is more probably about feature request. While using multiple monitors with different resolutions it would be handy if the UI auto scales while moving windows between screens. Some discussion was opened here: https://forum.kde.org/viewtopic.php?f=66&t=130848&p=372958#p372958
I've seen this somewhat working in 5.9.x releases. But after some fiddling with the resolutions and scaling of my monitors, it stopped happening.
It will under wayland, it can't under X
What if I want to use only one display at a time? For example, the laptop has resolution 2880x1620 but external monitors at work and home have 1920x1080 or 1920x1200. I ride the bus and work using my laptop screen, then I get to work and plug into a docking station; the laptop monitor goes dark and all my display gets routed to the monitor. I'd like the screen to be readable under both circumstances.