[css-view-transitions-1] Fix tree-scoping from element-based to name-based.

[noamr]

The previous fix for using the tree context made it inconsistent with how shadow DOM styling works for things like anchor positioning, and made it so that e.g. ::part cannot set a view-transition-name.

Refactored to make the view-transition-name a tree-scoped name, rather than check the tree context of the element.

This aligns better with the existing resolution and should not require a new resolution.

Closes #10145

[noamr]

@nt1m can you check if this sits well with you, and if it requires changes to the WebKit implementation?
If you or @fantasai thinks this needs an additional CSSWG resolution I'm happy to add it to the agenda.

[khushalsagar]

Add a line to the appendix for changes below. LGTM otherwise. Thanks!

[noamr]

Add a line to the appendix for changes below. LGTM otherwise. Thanks!

Done

[noamr]

@nt1m can you check if this sits well with you, and if it requires changes to the WebKit implementation? If you or @fantasai thinks this needs an additional CSSWG resolution I'm happy to add it to the agenda.

WPT: web-platform-tests/wpt#47006

[nt1m]

Not sure the spec change is worded quite right, if you have:

<style>
::part(party) {
  view-transition-name: document-scoped;
}
</style>
#shadow-root
    <style>
      div {
        width: 100px;
        height: 100px;
        view-transition-name: shadow-scoped !important;
      }
    </style>
    <div part="party"></div>

I expect the effective view transition name of the <div> to be shadow-scoped, and document-scoped being ignored.

Since shadow-scoped is scoped to the shadow root and document-scoped ignored, the <div> wouldn't participate in the view transition.

The spec change suggests otherwise, although I think the Chromium change is correct.

Anyway it would be good to add coverage for that case + capturing ::before/::backdrop pseudo-elements (where we should use the originating element's scope) in those situations as well.

[nt1m]

Fwiw, the change is fairly easy to adopt on WebKit's side: WebKit/WebKit#30501

[noamr]

Not sure the spec change is worded quite right, if you have:

<style>
::part(party) {
  view-transition-name: document-scoped;
}
</style>
#shadow-root
    <style>
      div {
        width: 100px;
        height: 100px;
        view-transition-name: shadow-scoped !important;
      }
    </style>
    <div part="party"></div>

I expect the effective view transition name of the <div> to be shadow-scoped, and document-scoped being ignored.

Since shadow-scoped is scoped to the shadow root and document-scoped ignored, the <div> wouldn't participate in the view transition.

Hmm right, so the wording should be something along the line of the implementation, where we get the computed value for view-transition-name and check its associated context. Will revise and add a WPT for this (and for the other cases). Thanks!

[nt1m]

New version looks good, thanks!