Fix kde scaling issue with arch and nvidia on Macbook Pro with external monitor

Manually set the dpi. Edit...

sudo vim /etc/X11/mhwd.d/nvidia.conf

Add to "Screen"...

Option "UseEdidDpi" "False"
Option "DPI" "96x96"

Making something like...

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    SubSection     "Display"
        Depth       24
        Option "UseEdidDpi" "False"
        Option "DPI" "96x96"
    EndSubSection
EndSection

From: https://forum.manjaro.org/t/nvidia-proprietary-driver-ui-scaling-issue/38204/6