[css-contain-2] Keep non-initial viewport determination happen one frame later.

Author: emilio

css-contain-2/Overview.bs

@@ -1998,20 +1998,28 @@ Suppressing An Element's Contents Entirely: the 'content-visibility' property {#
 		All elements have a <dfn>currently relevant to the user</dfn> flag, initially false.
 	</div>
 
+	<div algorithm="update currently relevant to the user">
+		To <dfn>update currently relevant to the user</dfn> given an element <var>element</var>:
+
+		1. Let <var>currentlyRelevant</var> be true if <var>element</var> is [=relevant to the user=], otherwise false.
+		1. If <var>element</var>'s [=currently relevant to the user=] flag is equal to <var>currentlyRelevant</var>, then return false.
+		1. Set <var>element</var>'s [=currently relevant to the user=] flag to <var>currentlyRelevant</var>.
+		1. return true.
+	</div>
+
 	<div algorithm="update content relevancy for a document">
 		To <dfn export>update content relevancy for a document</dfn> given a document <var>doc</var>, and a boolean <var>updateNonInitial</var>:
 
 		1. Let <var>changed</var> be false.
-		1. For each <var>element</var> [=connected=] to doc with ''auto'' used value of ''content-visibility''
+		1. For each <var>element</var> [=connected=] to doc with ''auto'' used value of ''content-visibility'':
+			1. If <var>updateNonInitial</var> is true:
+				1. If [=update currently relevant to the user=] for <var>element</var> returns true.
+					1. Set <var>changed</var> to true.
 			1. Let <var>isInitialDetermination</var> be true if <var>element</var>'s [=proximity to the viewport=] is [=not determined=], otherwise false.
 			1. Determine [=proximity to the viewport=] for <var>element</var>.
-			1. If <var>updateNonInitial</var> is false and <var>isInitialDetermination</var> is false, then [=continue=].
-			1. If <var>element</var> is [=relevant to the user=], then:
-				1. If <var>element</var>'s [=currently relevant to the user=] flag is false, set <var>changed</var> to true.
-				1. Set <var>element</var>'s [=currently relevant to the user=] flag to true.
-			1. Otherwise:
-				1. If <var>element</var>'s [=currently relevant to the user=] flag is true, set <var>changed</var> to true.
-				1. Set <var>element</var>'s [=currently relevant to the user=] flag to false.
+			1. If <var>isInitialDetermination</var> is false, then [=continue=].
+			1. If [=update currently relevant to the user=] for <var>element</var> returns true.
+				1. Set <var>changed</var> to true.
 		1. return <var>changed</var>.
 
 		<div class=note>