[css-view-transitions-2] Specify ViewTransition.waitUntil() (#12925)

Specify ViewTransition.waitUntil()

Author: vmpstr

css-view-transitions-2/Overview.bs

@@ -1717,6 +1717,7 @@ Issue: Is this updated by a <code>rootEl.startViewTransition()</code> call, too?
 			undefined skipTransition();
 			attribute ViewTransitionTypeSet types;
 			readonly attribute Element transitionRoot;
+			undefined waitUntil(Promise<any> promise);
 		};
 	</xmp>
 
@@ -1772,6 +1773,11 @@ Issue: Is this updated by a <code>rootEl.startViewTransition()</code> call, too?
 
 			For [=view transitions=] started on the {{Document}},
 			this is the [=document element=].
+
+		: <code>{{ViewTransition|viewTransition}}.{{ViewTransition/waitUntil}}()</code>
+		:: Delays view transition finish until the given promise is settled
+
+			When invoked with Promise |p|, run [=delay finish for promise=] steps with |p|.
 	</dl>
 
 	A {{ViewTransition}} has the following:
@@ -2090,6 +2096,12 @@ and by applying ''view-transition-group'' to the internal element referencing th
 			Initially false.
 
 			Note: this value can be read [=in parallel=] while navigating.
+
+		: <dfn>wait until promise count</dfn>
+		:: an integer, initially 0.
+
+			Represents the count of promises that have been added via {{ViewTransition/waitUntil}} function
+			and have not settled.
 	</dl>
 
 ### Additions to Elements ### {#elements-concept}
@@ -3115,6 +3127,19 @@ To <dfn>adjust nested view transition group transform</dfn> given a |transform|
 			Note: This is necessary since the descendant will generate its own snapshot which will be displayed and animated independently.
 	</div>
 
+	<div algorithm="delay finish for promise">
+		To <dfn>delay finish for promise</dfn>, given promise |p|:
+
+		1. If |p| is settled, abort the remainder of the steps and return.
+
+		1. Increment [=this=]'s [=wait until promise count=].
+
+		1. [=Upon fulfillment=] or [=upon rejection|rejection=] of |p|:
+
+				1. Decrement [=this=]'s [=wait until promise count=].
+
+				Note: If [=this=]'s [=wait until promise count=] is 0, the transition will finish at the next [=rendering opportunity=].
+
 ## [=Handle transition frame=] ## {#handle-transition-frame-algorithm}
 
 	<div algorithm>
@@ -3133,7 +3158,7 @@ To <dfn>adjust nested view transition group transform</dfn> given a |transform|
 
 				- |document|'s [=pending animation event queue=] has any events associated with |animation|.
 
-		1. If |hasActiveAnimations| is false:
+		1. If |hasActiveAnimations| is false and [=this=]'s [=wait until promise count=] is 0:
 
 			1. Set |transition|'s [=ViewTransition/phase=] to "`done`".
 
@@ -3327,6 +3352,7 @@ This appendix is <em>informative</em>.
 Changes from <a href="https://www.w3.org/TR/2024/WD-css-view-transitions-2-20240516/">2024-05-16 Working Draft</a>
 </h3>
 
+* Add waitUntil() (<a href="https://github.com/w3c/csswg-drafts/issues/9901">#9901</a>)
 * First pass at layered capture (<a href="https://github.com/w3c/csswg-drafts/issues/10585">#10585</a>)
 * Allow transitions when traversing into a document that was created using cross-origin redirects (<a href="https://github.com/w3c/csswg-drafts/issues/11063">#11063</a>#11063)
 * Clarify a few nesting details (<a href="https://github.com/w3c/csswg-drafts/issues/10780">#10780</a> and <a href="https://github.com/w3c/csswg-drafts/issues/10633">#10633</a>)