[css-contain-2] Mention c-i-s:auto when discussion size skipping.

Author: tabatkins

css-contain-2/Overview.bs

@@ -1420,23 +1420,34 @@ Using ''content-visibility: auto''</h3>
 	can "jump around" as elements go off-screen and start [=skipping=].
 
 	<div class=example>
-		This can be fixed by making the element fixed-size,
-		or sometimes by carefully arranging a layout such as Grid
-		to size the element without depending on its contents.
-
-		If that can't be done, however,
-		'contain-intrinsic-size' can be set to an <em>estimate</em>
-		of the element's size,
-		ensuring it'll remain approximately the correct size when it's skipped.
+		This can be fixed in a number of ways:
+
+		* making the element fixed-size
+		* carefully arranging a layout such as Grid
+			to size the element without depending on its contents
+		* using 'contain-intrinsic-size' to set an <em>estimate</em> of the element's size
+		* using ''contain-intrinsic-size: auto'' to automatically "snapshot" the <em>exact</em> size of the element
+			from the last time it was rendered,
+			before it was [=skipped=]
+			(along with providing an estimate of the size
+			to be used before it's rendered and can have its size snapshotted)
 
 		For example,
 		on Twitter,
 		the average tweet is approximately 200px tall,
-		so ''contain-intrinsic-size: 500px 200px''
+		so ''contain-intrinsic-size: auto 500px 200px''
 		will ensure that the scrollbar thumb is in approximately the correct size and position
 		even when preceding or following tweets are [=skipped=],
 		while still allowing the tweets to size according to their contents
 		when they're on-screen.
+		As long as the tweets have all been viewed at least once
+		(and haven't changed size while they were [=skipped=]),
+		their sizes will be <em>exactly</em> correct while they're [=skipped=],
+		so the scrollbar thumb will be as well;
+		only freshly-loaded tweets
+		(such as those loaded at the top of the timeline
+		while you are scrolling further down)
+		will be forced to rely on the ''200px'' height estimate.
 	</div>
 
 Restrictions and Clarifications {#cv-notes}