As https://github.com/upsuper noted, and as explicitly clarified in its scope section (https://drafts.csswg.org/css-scrollbars-1/#scope), CSS Scrollbars is purely for styling the controls themselves, not the scrollable area.
Scrollable area and scrollability in general is already defined by and in-scope for CSS Overflow. Am re-labeling this issue accordingly so it’ll get the proper consideration for that spec.
label: css-overflow-4