[cssom-view][css-contain-2] Per WG resolution, clarify that scrollIntoView() doesn't scroll to children of a 'c-v:hidden' element. Fixes #6529

Author: tabatkins

css-contain-2/Overview.bs

@@ -1620,6 +1620,13 @@ Restrictions and Clarifications {#cv-notes}
 	if this changes the element's size,
 	it might not align in the viewport exactly as requested.
 
+	If an element is not available to user-agent features
+	(for example, if it is [=skipped=]
+	due to a ''content-visiblity: hidden'' ancestor),
+	then scrolling operations
+	must not scroll to it at all,
+	as if it did not have a layout box.
+
 6. If an element with ''content-visibility: auto''
 	that is [=skipping its contents=]
 	is focused

cssom-view-1/Overview.bs

@@ -1105,7 +1105,9 @@ The <dfn method for=Element caniuse=scrollintoview>scrollIntoView(<var>arg</var>
     1. Set <var>block</var> to the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var>.
     1. Set <var>inline</var> to the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var>.
 1. Otherwise, if <var>arg</var> is false, then set <var>block</var> to "<code>end</code>".
-1. If the element does not have any associated <a>layout box</a>, then return.
+1. If the element does not have any associated <a>layout box</a>,
+    or is not available to user-agent features,
+    then return.
 1. <a lt='scroll an element into view'>Scroll the element into view</a>
     with <var>behavior</var>, <var>block</var>, and <var>inline</var>.
 1. Optionally perform some other action that brings the element to the user's attention.