[css-view-transitions-2] Resolve finished promise at own task when animations are done

Closes w3c#12442

Author: noamr

css-view-transitions-2/Overview.bs

@@ -3133,13 +3133,17 @@ 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 transition|'s [=ViewTransition/phase=] to "`animating`",
 
-			1. Set |transition|'s [=ViewTransition/phase=] to "`done`".
+			1. Set |transition|'s [=ViewTransition/phase=] to "`pending-done`".
 
-			1. [=Clear view transition=] |transition|.
+			1. [=Queue a global task=] on the [=DOM manipulation task source=],
+				given |transition|'s [=relevant global object=],
+				to perform the following step:
+					If |transition|'s [=ViewTransition/phase=] is "`pending-done`", then:
+						1. [=Resolve=] |transition|'s [=ViewTransition/finished promise=].
 
-			1. [=Resolve=] |transition|'s [=ViewTransition/finished promise=].
+						1. [=Clear view transition|clear=] |transition|.
 
 			1. Return.