[css-sizing-4] Address vmpstr's comments on #5668; always wait to record sizes until you're already doing the work normally.

Author: tabatkins

css-sizing-4/Overview.bs

@@ -574,13 +574,13 @@ Last Remembered Size</h4>
 	it can cause unsightly scrollbars or accidentally-hidden content.
 
 	The ''contain-intrinsic-size: auto'' value allows a middle-ground:
-	it lays out the element and records its sizes
-	<em>just before [=size containment=] begins to apply</em>,
-	then remembers this value
-	and uses it from then on,
-	offering a <em>probably accurate</em> size based on an actual layout of the contents
-	while still offering the performance benefits of [=size containment=]
-	from then onward.
+	if an element is ever <em>not</em> [=size containment|size-contained=],
+	this value causes the element to remember its size
+	(calculated as normal by layout);
+	then, if the element gains [=size containment=] later,
+	it will use the remembered size,
+	offering the performance benefits of [=size containment=]
+	while <em>probably</em> sizing accurately to its contents.
 
 	<div algorithm="last remembered size">
 		The [=last remembered size=] of an element is determined by:
@@ -591,21 +591,10 @@ Last Remembered Size</h4>
 			record its current inner dimensions
 			as its [=last remembered size=].
 
-		* If the element is being rendered for the first time,
-			or has just switched from not being rendered to being rendered
-			(for example, it was previously [=skipped contents|skipped=] due to being off-screen),
-			then, even if it has [=size containment=],
-			it must be laid out as if it does <em>not</em> have [=size containment=],
-			and the resulting inner dimensions
-			must be recorded as its [=last remembered size=].
-
-			(It is then rendered as usual,
-			applying [=size containment=],
-			but also using this [=last remembered size=]
-			due to ''contain-intrinsic-size: auto''.)
-
 		An element might not have a [=last remembered size=],
-		if it has not yet been rendered.
+		if it has never been rendered without [=size containment=].
+		(In this case, it will instead use the fallback value
+		provided along with ''contain-intrinsic-size/auto''.)
 	</div>
 
 <h4 id='cis-scrollbars'>