[scroll-animations-1] ScrollTimeline and ViewTimeline are inactive wh…

…en there is nothing to scroll. #7401

scroll-animations-1/Overview.bs

@@ -259,6 +259,7 @@ spec: cssom-view-1; type: dfn;
 	If the {{ScrollTimeline/source}} of a {{ScrollTimeline}}
 	is an element whose [=principal box=] does not exist
 	or is not a [=scroll container=],
+	or if there is no [=scrollable overflow=],
 	then the {{ScrollTimeline}} is [=timeline inactive state|inactive=].
 	It is otherwise in the [=timeline active state|active=] [=timeline state=].
 
@@ -427,6 +428,7 @@ spec: cssom-view-1; type: dfn;
 
 	If the {{ScrollTimeline/source}} or {{ViewTimeline/subject}} of a {{ViewTimeline}}
 	is an element whose [=principal box=] does not exist,
+	or if its nearest ancestor [=scroll container=] has no [=scrollable overflow=],
 	then the {{ViewTimeline}} is [=timeline inactive state|inactive=].
 	It is otherwise in the [=timeline active state|active=] [=timeline state=].