No, it's a condition that windows stipulates. It won't allow it unless it's full screen. That's what haasn was saying. I recently wrestled with it to see what it actually took to make HDR to pass through on windows. It was excruciating. If your PC is dedicated to only watching video, then sure. But that isn't my use case.
So all I've needed to do with mpv, is enable UHD 10 bit on my display, adjust the display settings for my environment....press play. All the other stuff is just because I'm a control freak
You didn't read correctly what Microsoft stipulated. It's perfectly possible to play HDR movies in a window. Did you tested it? I just did with VLC and with Microsoft movies app. If you think about it, that's logical: When you activate HDR in Windows, it force everything in HDR mode, desktop and all. The restriction is when streaming HDR movie on a built-in display. That's probably related to copy protection and/or how streaming provides (Netflix,...) detect if you can/are allowed to stream HDR content.