[css-scrollbars-1] Loosen requirement for scrollbar-width:thin

[upsuper]

I think the main point of having thin scrollbars is to give content more room when the space is already tight, so it is probably better using the concept of "scrollbar gutter" for this value, which is also clearer when overlay scrollbar / auto-expand scrollbar involves.

Also when the scrollbar is already fully overlay (e.g. on macOS when trackpad is used, or on Android), I believe there is no point to create an even thinner scrollbar, so the value probably shouldn't use a "must".

[tantek]

This looks good except for the "In that case, this value should have no effect." I disagree. Even in that case the author intent is to visually provide something thinner and less noisy even when overlaid. The presence/absence of a gutter also doesn't clarify whether it is clickable or not. E.g. some overlay scrollbars are clickable, and thus making them thin absolutely should have the effect of narrowing the clickable area (typically because you don't want it obscure/prevent clicking the thing being scrolled!). This is especially important in narrow fields and lists.

[upsuper]

Updated, and remove the gutter part. It now just mentions thinner scrollbar.

[tantek]

Yes this is an improvement. Approving this and we can iterate further with minor tweaks as needed, e.g. "implementations may treat"