Reading the
https://docs.blender.org/manual/en/dev/getting_started/installing/linux.html
you'll find the line
KDE: System Settings ‣ Window Behavior ‣ Window Behavior ‣ Window Actions, Switch ‘Alt’ for ‘Meta’ key.
I switched to an installation of Mint 19, setup KDE and found the setting is as shown in the image:
This shows the KDE
System Settings > Windows Management > Window Behaviour > Window Actions
