I did a lot of searching on bing and google to try and figure this out on my own, but it appears that RimWorld used to have the opposite problem (meaning you couldn't force boarderless windowed mode without some trickery).
I am only really interested in forcing exclusive full screen to see if I can gain better performance. Does anyone know of a way to do this?
This thread (https://forum.unity3d.com/threads/exclusive-fullscreen-as-a-option-for-game-developers.390561/) from the Unity engine forums indicates there's a command-line switch
-window-mode exclusive
THANK YOU SO MUCH! This is working perfectly. I am going to do various test now to see if I can squeeze a bit more performance out of RimWorld late game. :)