I'm not sure I'm posting in the right forum, but I just tried to Demo a casual that doesn't fill the screen, no matter how I tweak its few Options. It uses about 3/4 of my screen and then imports all the shortcut icons and background image on the rest of my screen. Otherwise, it appears ready to play normally.

I have not encountered this before. I'm running Win10 on a 32" monitor, and every other game I've played has filled it. Is this a problem with the game itself, and there is no way to make it fit the screen?

32" is a big monitor.
Is this a 4k monitor?
If you Google "game not full screen on 4k monitor"
there are some results with suggestions like "disable display scaling."

This site has suggestions for adjusting Nvidia settings Solution #4.
https://windowsreport.com/indows-10-fullscreen-games/
If you have Radeon graphics, there may be a similar adjustment.

Thanks, Jenny. That is just the kind of lead I needed, to know what to do. I'm off to check out all you said.