[css-view-transitions-2] Skip only a duplicate element instead of the whole transition (#13858)

* [css-view-transitions-2]  Skip only a duplicate element instead of the whole transition

Closes #13438

* Add console.warn note

Author: noamr

css-view-transitions-2/Overview.bs

@@ -553,8 +553,7 @@ spec:css2; type:dfn; text:viewport
 			and the transition is initiated.
 
 			Note that {{ViewTransitionUpdateCallback|updateCallback}}, if provided, is *always* called,
-			even if the transition cannot happen
-			(e.g. due to duplicate `view-transition-name` values).
+			even if the transition cannot happen.
 			The transition is an enhancement around the state change, so a failure to create a transition never prevents the state change.
 			See [[#transitions-as-enhancements]] for more details on this principle.
 
@@ -609,8 +608,7 @@ Note: An <code>element.startViewTransition()</code> call does not update
 			and the transition is initiated.
 
 			Note that {{ViewTransitionUpdateCallback|updateCallback}}, if provided, is *always* called,
-			even if the transition cannot happen
-			(e.g. due to duplicate `view-transition-name` values).
+			even if the transition cannot happen.
 			The transition is an enhancement around the state change, so a failure to create a transition never prevents the state change.
 			See [[#transitions-as-enhancements]] for more details on this principle.
 
@@ -716,8 +714,7 @@ the active view transition of an element is exposed to script via a property on
 			and the animation is about to start.
 
 			It rejects if the transition cannot begin.
-			This can be due to misconfiguration, such as duplicate 'view-transition-name's,
-			or if {{ViewTransition/updateCallbackDone}} returns a rejected promise.
+			This can be due to misconfiguration, e.g. if {{ViewTransition/updateCallbackDone}} returns a rejected promise.
 
 			The point that {{ViewTransition/ready}} fulfills
 			is the ideal opportunity to animate the [=view transition pseudo-elements=]
@@ -2348,6 +2345,9 @@ A 'view-transition-name' generated by <css>auto</css> is a [=tree-scoped name=].
 
 		: <dfn for="captured element">old border width</dfn>
 		:: 'border-width' [=used value=] (top, right, bottom and left pixel values) or null, initially null.
+
+		: <dfn for="captured element">duplicate</dfn>
+		:: A boolean, initiallly false.
 	</dl>
 
 	In addition, a [=captured element=] has the following <dfn for="captured element">style definitions</dfn>:
@@ -2483,6 +2483,10 @@ It has the following [=struct/items=]:
 			If failure is returned, then [=skip the view transition|skip=] |transition| with an "{{InvalidStateError}}" {{DOMException}} in |transition|'s [=relevant Realm=],
 			and return.
 
+		1. [=list/Remove=] from |transition|'s [=ViewTransition/named elements=] every entry whose value's [=captured element/duplicate=] is true.
+
+			Note: User agents are encouraged to alert the developer that the duplicate was removed, e.g. via a console warning.
+
 		1. [=list/For each=] |capturedElement| of |transition|'s [=ViewTransition/named elements=]' [=map/values=]:
 
 			1. If |capturedElement|'s [=captured element/new element=] is not null,
@@ -2512,8 +2516,6 @@ It has the following [=struct/items=]:
 
 		1. Let |namedElements| be |transition|'s [=ViewTransition/named elements=].
 
-		1. Let |usedTransitionNames| be a new [=/set=] of strings.
-
 		1. Let |captureElements| be a new [=/list=] of elements.
 
 		1. If the [=snapshot containing block size=] exceeds an [=implementation-defined=] maximum, then return failure.
@@ -2548,22 +2550,14 @@ It has the following [=struct/items=]:
 				or |element| is [=element-not-rendered|not rendered=],
 				then [=continue=].
 
-			1. If |usedTransitionNames| [=list/contains=] |transitionName|, then:
-
-				1. [=list/For each=] |element| in |captureElements|:
-
-					1. Set |element|'s [=captured in a view transition=] to false.
-
-				1. Return failure.
+			1. If |namedElements|[|transitionName|] exists, then set |namedElements|[|transitionName|]'s [=captured element/duplicate=] to true and [=continue=].
 
 			1. If <em>any other</em> [=active view transition=] contains |element|
 				in its [=captured elements=],
 				then for each such [=active view transition=],
 				in [=tree order=] of their corresponding [=ViewTransition/root elements=],
 				[=skip the view transition|skip=] that view transition with an "{{AbortError}}" {{DOMException}} in |document|’s [=relevant Realm=].
 
-			1. [=set/Append=] |transitionName| to |usedTransitionNames|.
-
 			1. Set |element|'s [=captured in a view transition=] to true.
 
 			1. [=list/Append=] |element| to |captureElements|.
@@ -2646,6 +2640,8 @@ It has the following [=struct/items=]:
 					This implies than names which only exist in the new DOM (entry animations) will be painted on top of names only in the old DOM (exit animations) and names in both DOMs (paired animations).
 					This might not be the right layering for all cases. See <a href="https://github.com/w3c/csswg-drafts/issues/8941">issue 8941</a>.
 
+			1. If |namedElements|[|transitionName|]'s [=new element=] is not null, then set |namedElements|[|transitionName|]'s [=captured element/duplicate=] to true.
+
 			1. Set |namedElements|[|transitionName|]'s [=new element=] to |element|.
 	</div>