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[css-view-transitions-2] Define the view-transition-group-children pseudo element #12277

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[css-view-transitions-2] Define the view-transition-group-children pseudo element #12277

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77 changes: 63 additions & 14 deletions css-view-transitions-2/Overview.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -860,7 +860,7 @@ This example creates a transition where the [=view transition tree=] is nested i
}

.container,
::view-transition-group(container) {
::view-transition-group-children(container) {
clip-path: circle();
}

Expand All @@ -870,7 +870,22 @@ article {
}
```

By applying the ''clip-path'' to both the containing element and its generated pseudo-element, we preserve the clip during the transition,
The pseudo-element tree for this would be as follows:
```
::view-transition
├─ ::view-transition-group(container)
│ └─ ::view-transition-image-pair(container)
│ ├─ ::view-transition-old(container)
│ └─ ::view-transition-new(container)
│ └─ ::view-transition-group-children(container)
│ └─ ::view-transition-group(article)
│ ├─ ::view-transition-old(article)
│ └─ ::view-transition-new(article)
│ └─ ...other nested groups...
└─ …other groups…
```

By applying the ''clip-path'' to both the containing element and its generated ''::view-transition-group-children()'' pseudo-element, we preserve the clip during the transition,
and by applying ''view-transition-group'' to the internal element referencing the container, we make the tree "nested" in a way that would apply this clipping.
</main>

Expand All @@ -889,7 +904,21 @@ and by applying ''view-transition-group'' to the internal element referencing th
The 'view-transition-group' property can be used in conjuction with 'view-transition-name' to generate a hierarchy of [=named view transition pseudo-element=].

The [=used value=] for 'view-transition-group' resolves to a 'view-transition-name' in its ancestor chain, or to ''view-transition-name/none''. When generating the [=named view transition pseudo-element=], the ''::view-transition-group()'' with that name
would be the parent of the ''::view-transition-group()'' generated for this element's 'view-transition-name'.
would create a ''::view-transition-children()'' pseudo-element, which would be the parent of the ''::view-transition-group()'' generated for this element's 'view-transition-name'.

### View Transition Tree Root: the ''::view-transition-group-children'' pseudo-element ### {#view-transition-group-children-pseudo}

The <dfn>::view-transition-group-children()</dfn> [=pseudo-element=]
is a [=named view transition pseudo-element=]
that represents a matching named [=view transition=] capture.
A ''::view-transition-group-children()'' [=pseudo-element=]
is generated based on the 'view-transition-group' property,
as a [=tree/child=] of the corresponding ''::view-transition-group'' [=pseudo-element=],
and contains one or more ''::view-transition-group()'' elements, corresponding to the nested groups.
See the [=used group name=] and related algorithms.

The ''::view-transition-group-children()'' [=pseudo-element=] is positioned after its sibling ''view-transition-image-pair''.
It is used to control effects like clipping or masking of nested children.

# Algorithms # {#algorithms}

Expand Down Expand Up @@ -953,7 +982,10 @@ The [=captured element=] struct should contain these fields, in addition to the
:: Null or a string, initially null.

: <dfn for="captured element">transform from snapshot containing block</dfn>
:: A [=matrix=], initially the identity matrix.
:: A [=matrix=] or null, initially null.

: <dfn for="captured element">old border width</dfn>
:: 'border-width' [=used value=] (top, right, bottom and left pixel values) or null, initially null.
</dl>

## Resolving the ''@view-transition'' rule ## {#vt-rule-algo}
Expand Down Expand Up @@ -1219,6 +1251,8 @@ To resolve the <dfn>used group name</dfn> of an {{Element}} |element|, perform t
When [[css-view-transitions-1#capture-old-state-algorithm|capturing the old state for an element]], perform the following steps given a [=captured element=] |capturedElement|, a {{ViewTransition}} |transition|, and an [=element=] |element|:

1. Set |capturedElement|'s [=captured element/containing group name=] to |element|'s [=used group name=] given |transition|.

1. Set |capturedElement|'s [=captured element/old border width=] to |element|'s [=used value|used=] 'border-width'.
</div>

### Compute the new 'view-transition-group' ### {#vt-group-capture-new}
Expand All @@ -1233,19 +1267,23 @@ When [[css-view-transitions-1#capture-new-state-algorithm|capturing the new stat
When [[css-view-transitions-1#setup-transition-pseudo-elements-algorithm|setting up the transition pseudo-element]] for a [=captured element=] |capturedElement|, given a |transitionName| and a |transition|:

1. Let |containingGroupName| be |capturedElement|'s [=containing group name=].
1. If |containingGroupName| is not null, then:

1. Let |groupContainerElement| be |transition|'s [=ViewTransition/named elements=][|containingGroupName|].
1. If |containingGroupName| is null, return.

1. Let |group| be the ''::view-transition-group()'',
whose [=view transition name=] is set to |transitionName|.
1. Let |groupContainerElement| be |transition|'s [=ViewTransition/named elements=][|containingGroupName|].

1. Let |parentGroup| be the ''::view-transition-group()'',
whose [=view transition name=] is set to |containingGroupName|.
1. Let |group| be the ''::view-transition-group()'',
whose [=view transition name=] is set to |transitionName|.

1. Append |group| to |parentGroup|.
1. Let |childrenContainerGroup| be the ''::view-transition-group-children()'' whose [=view transition name=] is set to |containingGroupName|.

1. When setting the animation keyframes given |transform|, [=multiply=] |transform| by the inverse matrix of |groupContainerElement|'s [=old transform=].
1. If |childrenContainerGroup| does not exist:
Generate it and append to the ''::view-transition-group()'',
whose [=view transition name=] is set to |containingGroupName|.

1. Append |group| to |childrenContainerGroup|.

1. When setting the animation keyframes given |transform|, [=adjust nested view transition group transform|adjust=] |transform| given |groupContainerElement|.

Note: It is TBD how this is resolved when the old and new groups mismatch. See <a href="https://github.com/w3c/csswg-drafts/issues/10631">Issue 10631</a>.
</div>
Expand All @@ -1254,13 +1292,24 @@ When [[css-view-transitions-1#setup-transition-pseudo-elements-algorithm|setting
<div algorithm="additional pseudo-element style update steps (group)">
When [[css-view-transitions-1#style-transition-pseudo-elements-algorithm|updating the style of the transition pseudo-element]], perform the following steps before setting the [=captured element/group styles rule=], given a [=captured element=] |capturedElement|, given a |transform| and a {{ViewTransition}} |transition|:

1. Let |childrenTransform| be |transform|.

1. Set |capturedElement|'s [=transform from snapshot containing block=] to |transform|.

1. If |capturedElement|'s [=containing group name=] is not null, then:

1. Let |groupContainerElement| be |transition|'s [=ViewTransition/named elements=][|capturedElement|'s [=containing group name=]].
1. Let |groupContainerElement| be |transition|'s [=ViewTransition/named elements=][|capturedElement|'s [=containing group name=].

1. [=adjust nested view transition group transform|Adjust=] |transform| given |groupContainerElement|.

To <dfn>adjust nested view transition group transform</dfn> given a |transform| and a [=captured element=] |groupContainerElement|:
1. Let |childrenTransform| be |groupContainerElement|'s [=transform from snapshot containing block=].

1. If |childrenTransform| is null, set it to |groupContainerElement|'s [=captured element/old transform=].

1. Translate |childrenTransform| by |groupContainerElement|'s [=captured element/old border width=]'s left and top components.

1. [=Multiply=] |transform| by the inverse matrix of |groupContainerElement|'s [=transform from snapshot containing block=].
1. Multiply |transform| by the inverse matrix of |childrenTransform|.
</div>


Expand Down