[css-overflow-4] drawing over the space reserved by `scrollbar-gutter`

The scrollbar-gutter spec.

There are cases when authors would like to reserve space for the scrollbar inside a container by using scrollbar-gutter, and they would also like to have certain elements that span the whole width of that container (and therefore would step over that gutter space).

The best example of this would be headers and dividers inside a list. Something like this:

However, it is not possible to implement that design with the current definition of scrollbar-gutter: there would be an empty gap on the side of the headers/dividers because they can not extend over the gutter:

This issue came up during the discussion about implementing scrollbar-gutter in Chromium (thread) and I think that it is a valid criticism of the spec. I am not sure about what changes or additions could be done to fix it.

Start with the linked CSS Overflow 4 scrollbar-gutter specification and review the Chromium implementation discussion linked in the issue. Use the two example images to understand the desired header and divider layout. Done would require an agreed specification change that permits those elements to span the reserved gutter, but the issue does not identify a specific solution.