[css-scrollbars] Interaction between scrollbar-color and scrollbar-width with ::-webkit-scrollbar

[frivoal]

The css scrollbar spec has a section explicitly declaring ::-webkit-* scrollbar pseudos, and any other similar approach as out of scope: https://drafts.csswg.org/css-scrollbars/#out-of-scope

As documented in multiple previous issues, this is based on the consensus (including implementers) that this approach is a bad idea, and that shipping ::-webkit-* scrollbar pseudos in the first place was a bad idea / accident which we wish we had avoided.

However, the wpt test suite contains a significant number of tests about the interaction between scrollbar-color and scrollbar-width with ::-webkit-scrollbar.

It has 17 tests, for a total of 21 subtests

• scrollbar-color-001.html
• scrollbar-color-002.html
• scrollbar-color-003.html (2 tests)
• scrollbar-color-004.html
• scrollbar-color-005.html
• scrollbar-color-006.html
• scrollbar-color-007.html
• scrollbar-color-008.html
• scrollbar-color-009.html
• scrollbar-color-010.html
• scrollbar-width-010.html
• scrollbar-width-011.html
• scrollbar-width-012.html
• scrollbar-width-013.html
• scrollbar-width-014.html
• scrollbar-width-015.html (2 tests)
• scrollbar-width-016.html (3 tests)

These tests are designed to pass if the user agent does not support ::-webkit-scrollbar at all (which is the case for Firefox), but they do expect a specific behavior if ::-webkit-scrollbar is supported, and that behavior is not specified anywhere as far as I can tell.

What should we do?

• while keeping ::-webkit-scrollbar optional / deprecated, specify the expected interaction, maybe in css-scrollbars, maybe in https://compat.spec.whatwg.org/
• delete the tests, to avoid constraining the behavior of a feature we're trying to get rid of
• keep the tests, but mark them as .tentative
• something else?

[lukewarlow]

I would prefer to keep them in some form they're useful to make sure that Webkit and chromium match in their behaviour.

[frivoal]

Marking them as .tentative for now, which lets us keep them while indicating that they're not part of the spec. We can always decide to do something else later.

[bfgeek]

+1 to keeping them - can also use .optional instead of .tentative if needed.

[frivoal]

Don't think .optional is appropriate, those are for MAY statements, which presumes that there's a spec, which we don't have.

[frivoal]

I think we could specify the interaction without specifying the legacy mechanism, by saying that any non-auto value of scrollbar-width or scrollbar-height takes precedence over any non-standard alternative mechanism offered to authors to style the scrollbars.