1871584 - Consider rewriting ::cue to use the same machinery as CSS shadow parts.

Status: NEW

(Core :: CSS Parsing and Computation, enhancement)

Description

[Emilio Cobos Álvarez (:emilio)]

Right now, ::cue is the only JS-implemented psuedo-element. But that's a weird set-up and causes us to have custom property hacks like the ones causing trouble in bug 1871540, and in general is a bit of rather weird code.

It seems like what the video controls code wants is effectively just ::part(cue) or so. It should be possible to reuse all the part machinery and remove the special JS pseudo-element stuff.

The idea would be something like:

• Make the selector parser output Part { is_pseudo: bool, parts: Box<[...]> }
• Change the "js implemented" flag to be "this pseudo is a part" flag
• So ::cue would effectively output Part { true, [cue] }
• Then check is_pseudo == element.is_in_ua_shadow_tree() in the selector-matching code to make ::cue match but ::part(cue) not and vice versa.
• Maybe we need to tweak GeckoElement::inheritance_parent too.

But then effectively all the weird stuff goes away and videocontrols.js just needs to add part=cue to the relevant element.

Comment 1

[Emilio Cobos Álvarez (:emilio)]

Attachment: WIP: Bug 1871584 - Add the selectors infrastructure for ::part - like pseudo-elements.

Comment 2

[Emilio Cobos Álvarez (:emilio)]

Attachment: WIP: Bug 1871584 - Make ::cue a part-like pseudo-element.

Things to do here:

• ::part() doesn't work on UA sheets, so default styles don't quite work as
  intended.

• We need to keep the pseudo association for property restrictions, but we
  could do that better probably.

Depends on D197478