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?
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.
It probably breaks the right side of the page grid layout but if there are many columns to see, the ability to see them should grant an exception. I made that choice a few times with my customers. They wanted to see the data in those pages, as if they were in Excel.
I want to style scroll bars for this, not to make them prettier.