Per co-editor Rossen’s (https://github.com/atanassov) scope clarification as noted in issue #4693 (https://github.com/w3c/csswg-drafts/issues/4693), I agree it makes more sense to distinctly separate styling of scrollbar controls (specified by CSS Scrollbars), and any layout impacts or scrollability effects which have interactions with the rest of overflow and belong there (specified in CSS Overflow).
Thus I am retracting this issue and closing without any changes.