I’ve been scratching my head over this one. The game runs fine in fullscreen and it USED to run fine in fullscreen windowed as well. Now, it runs worse than when I tried playing Shaiya on my laptop in college. I’ve fiddled with the video/graphic settings for at least fifteen minutes trying to figure out. I’ve been bouncing between forums and reddit posts looking for relevant information, but nothing seems to help it out. All I can do for now is play fullscreen, which is kind of lame considering how much longer it takes to alt+tab out of. Two seconds? Repulsive.
If you doing this on a laptop. It has two video cards in it. I’m just saying it could be not switching over to the 3090 and staying on the onboard/CPU video card.
It’s a desktop. Also, why would Diablo know how to run Borderless Windowed before, but not now with the same settings? Even on a laptop, that wouldn’t make sense.
Have you tried this:
Using the Alt + Enter keyboard shortcut is the easiest way to enter full-screen mode in your games. This hotkey is compatible with nearly every game on Windows. Hence, it’s the first thing you should try. Pressing Alt + Enter again will switch your game back to windowed mode.
Looks like Alt + Enter just brings up the chat window.
I just tried this too. And any combination using the Enter or Return keys just opens Chat. I tried:
- Alt + Enter
- Alt + Return
- Ctrl + Enter
- Ctrl + Return
- Shift + Enter
- Shift + Return
- Ctrl + Alt + Enter
- Ctrl + Alt + Return
And, of course, the Enter key opens Chat by default.
I also checked the Game Options, in Video and Key Bindings sections, to see if there was a shortcut to switch screen modes. But, didn’t find anything.
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…you can give this a try: DxWnd / Home / Home…
Good luck (with the gazillion options)! Fingers crossed & thumbs pressed! 
Never did find an answer to this. Oh well.