I read the release notes. I just opened a Finder window and it feels better, and it took me a second to figure out why.
It’s two things. There’s now a single vertical border between the sidebar and the content pane, so the two regions actually feel like two regions instead of one dark expanse where the divider was only implied by a shift in shade. And the inner corners got a slightly larger, more consistent radius that lines up with the outer window. That’s it. No new feature, nothing to learn.
The smallest possible line in the right place does more than any amount of gradient or shadow. One border to say “this is a boundary,” one radius to make the corners agree with each other, and the window stops feeling vague.
Good detail work is invisible until you go back to the old version and feel how much you weren’t seeing.