Intent to Ship: Popover=hint

[Keith Cirkel]

Summary: Support for the additional ephemeral (hint) popover layer with `popover=hint` attribute

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1867743

Specification: https://html.spec.whatwg.org/#attr-popover-hint-state

Standards Body: WHATWG

Platform coverage: All

Preference: dom.element.popoverhint.enabled

Devtools bug: N/A

Other browsers:

  - Blink: Shipping since M133.

  - WebKit: Positive (https://github.com/WebKit/standards-positions/issues/305).

web-platform-tests: Extensive WPTs, for e.g. https://wpt.fyi/results/html/semantics/popovers/popover-light-dismiss-hint.html https://wpt.fyi/results/html/semantics/popovers/popover-top-layer-nesting-hints.html

popover=hint was proposed as a focus area for Interop 2026 (https://github.com/web-platform-tests/interop/issues/1129). I expressed my intent to prototype this feature in Jun 2024 (https://groups.google.com/a/mozilla.org/g/dev-platform/c/vaT1aQGETLM/m/GiQ4bzh-AAAJ). I've been slowly iterating on this feature when I can.

I have a set of patches which have implemented popover=hint per the specification, and relevant WPTs are passing. I have just queued the last of these patches to land - with the `dom.element.popoverhint.enabled` set to `true` by default. This will allow popover=hint to ride the 149 train and allow us to uncover any web compatibility issues - if any.

Provided everything goes well with 149 nightlies, then popover=hint will ship in Firefox 149 release on March 24. If we notice any web compatibility issues or any bug reports, I will disable the flag and evaluate our options, which may include deferring until 150 or later.

If anyone has any concerns please let me know!

Happy browsing.