That's not so much "styling the scroll bar"; that's "trying to tell the OS widget library's metrics-policy logic its business, because you know better than it does, because it's insufficiently smart."
If you think about it, a "smarter" OS widget library would always "float" scrollbars at the edge of the visible part of the viewport that the overflowing surface occupies. No configurability needed — it's just always better.
So why expose the ability to do that for certain scrollbars in CSS — rather than just making all scrollbars everywhere always do that?
That's cool and all, but until I have enough influence personally to dictate Microsoft's design, I don't really have a choice but to depend on web APIs to achieve the desired effect.
Also, I think for Chrome at least, it draws scrollbars using its own widget library — built on top of Skia, I think? Not sure what the widget-library part itself is called. You could definitely contribute to that.
If you think about it, a "smarter" OS widget library would always "float" scrollbars at the edge of the visible part of the viewport that the overflowing surface occupies. No configurability needed — it's just always better.
So why expose the ability to do that for certain scrollbars in CSS — rather than just making all scrollbars everywhere always do that?