Earlier quoted context omitted.
Can you post screenshots of what you mean? I see grossly rounded corners in some apps, but I don't see the other stuff like gaps in window corners for full screen apps. I may have some config bit flipped that has disabled those. Yeah, the new corner radius is ugly but by and large, it's not much different than before, from what I see so far.
https://imgur.com/a/jLPM9oV this is what I'm seeing with Safari, WhatsApp and Chrome all maximized but with various radius on each corner.
That is not surprising since Chrome does not use native UIKit.
If you draw your own window frame, you're gonna have to update for every OS release.
I believe though that apps which have not been recompiled for macOS 26 operate in a legacy mode, and that includes the older window corner radius.