This is just a detail that bit me while trying to get maximum res from my desktop system, thought I'd pass it along.
I could NOT get the game to run at 4K resolution - which was not important to me for in-game visual reasons and has its downside in the teeny text problems, but meant my 3-monitor system did not shift in and out of changed resolutions, messing up other apps and icon positions and the like. Every time I tried, I got a giant game window that overlapped the center and right monitors and pushed critical buttons (like Back and Exit) offscreen.
I finally traced the problem to my use of app scaling for the center monitor. For day to day use, the icons and app interfaces are way too small on a 28" 4K display, so I had scaling on that display dialed up to 150%. Turning that down to 100% lets CoX run in full native resolution - a little slow on framerate, but it doesn't muck up my desktop and other apps every time I cycle in and out. I do have to open Display Settings and bump the scaling down, then back up, but it's a small price to pay for overall harmony.
TL;DR - if you can't get max res out of the game without oversized game windows, check your display scaling.
Word of caution with Win10 and App Scaling. Anything above 125% tends to cause some very bad things with some apps. Even causing them not to load (presumably because the app doesn't know how to handle it).
I've had no problem with my side monitors set to 100 and center display set to 150; I run a variety of apps and tools including the complete Adobe set. I can see how some older apps and those not fully optimized for scaling past original design limits could choke.