On some of my (many) Ubuntu desktop installations, Hearts of Iron 4 has an annoying bug, in that right-clicking for partial border selection selects the entire border and borders can’t be edited. It seems to be caused by a weird combination of Gnome session, Wayland/Xorg and using a touchpad.
This command fixes the issue for me:
gsettings set org.gnome.desktop.peripherals.touchpad click-method 'default'
George's Techblog 