Tabs: override tablist's tabindex only when necessary

[ciampo]

What?

Extracted from #66097 (comment)

This PR tweaks the Tabs.Tablist component so that the element's tabbability is primarily controlled by ariakit

Why?

Currently the tablist element has a fixed tabIndex={-1}, which is in place to prevent browsers from focussing the tablist when it's a scroll container — we want to opt out from it since the component knows how to handle keyboard scrolling without interference from the browser.

But this override is a bit too invasive and prevents ariakit from setting tabindex={0} when the selected tab id is null — in that case, in fact, the tablist is meant to be tabbable.

How?

Tweak the tablist code, giving priority to ariakit's tabIndex value and only setting tabIndex={-1} if a value from ariakit is not provided.

Testing Instructions

• set up a controlled Tab component
• set selectedTabId to null
• make sure that no tabs are selected, no tab panels are shown
• press tab — make sure that the tablist receives focus, and that pressing arrow keys correctly selects tabs in the tablist

[github-actions]

Flaky tests detected in 7619679.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11386979837
📝 Reported issues:

• [Flaky Test] Should insert content using the global inserter #57525 in /test/e2e/specs/widgets/editing-widgets.spec.js

[github-actions]

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Co-authored-by: ciampo <[email protected]>
Co-authored-by: tyxla <[email protected]>

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[tyxla]

Makes sense and tests as expected 👍 🚀

Might need to tweak a bit if #66208 lands first.