I know that there was an option to change overscan? Must have gone some time Plasma 5.27 ?
I will switch to Plasma 6 soon, but the many Kubuntu users will not. This is a dealbreaker.
I tried it now.
kscreen-doctor -o
Output: 1 eDP-1 enabled connected priority 1 Panel Modes ...
Output: 2 HDMI-A-1 enabled connected priority 2 HDMI Modes...
kscreen-doctor output.HDMI-A-1.overscan.20
It gave no errors but didnt change anything, also not after plugging out and back in