[css-scrollbars] Add thick value to scrollbar-width

[SebastianZ]

As noted in the specification, authors should generally refrain from overwriting the scrollbar width besides supporting specific UX goals. And @tabatkins pointed out that implementers object to add support for custom widths.

Though as outlined by several people in #6263, there's the use case of accessibility for having wider scrollbars.

The auto refers to the OS or UA settings, which is ok for most cases. Though if neither the OS nor the UA provide settings to change the width, users are stuck with the default width as the maximum.
Also, authors may want to emphasize the possibility to scroll and element.

Therefore, I suggest to add a new value thick to scrollbar-width that aligns with the definition of thin but makes the width thicker than auto.

The concrete spec. text might read like this:

thick
Implementations should use thicker scrollbars than auto. This may mean a thick variant of scrollbar provided by the platform, or a custom scrollbar thicker than the default platform scrollbar.

Sebastian

[nt1m]

I would object to this. Not all system frameworks support rendering scrollbars at thick sizes.

I would argue it's up to auto and the UA to consistently provide a decent UI for accessibility purposes, not the web author to randomly increase the size of the scrollbar (I doubt many people want this from an UX perspective).

[SebastianZ]

Yes, it's generally up to the UA/OS to provide a decent UI. The issue is that they sometimes don't.

And this is primarily a way for users to adjust this via a user style sheet even when the OS or UA doesn't provide a UI setting for this. It is less useful for web authors.

... not the web author to randomly increase the size of the scrollbar (I doubt many people want this from an UX perspective).

Bad UX may also be a counter-argument for the thin value, though it was specced and is already widely implemented.

I would object to this. Not all system frameworks support rendering scrollbars at thick sizes.

If they don't, UAs should render custom scrollbars if possible. If not, they might fall back to auto.

Sebastian