[css-scroll-snap-1] Require scroll-into-view animations for re-snapping to a different box. #4609

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -411,6 +411,18 @@ Re-snapping After Layout Changes</h4>
     the scroll container must be re-snapped to that same snap position
     after the content change.
 
+    Scrolling required by a re-snap operation to a new or different box
+    must behave and animate the same way as
+    any other scroll-into-view operation,
+    including honoring controls such as 'scroll-behavior'.
+    Scrolling behavior for re-snapping to the same box as before
+    however, is UA-defined.
+    The UA may, for example,
+    when snapped to the start of a section,
+    choose not to animation the scroll to the section’s new position
+    as content is dynamically added earlier in the document
+    in order to create the illusion of not scrolling.
+
 	<div class="example">
 		In the following example,
 		the log console,
@@ -1204,6 +1216,24 @@ Changes Since 31 January 2019 CR {#changes-20190131}
 		<li id="change-2019-clarify-writing-mode">
 			Specified which writing mode is used to resolve 'scroll-snap-align'.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/3815">Issue 3815</a>)
+		<li id="change-2019-resnap-animation">
+			Require re-snapping to a new element to animate the same way
+			as any other scroll-into-view operation.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/4609">Issue 4609</a>)
+			<blockquote>
+				<p><ins>Scrolling required by a re-snap operation to a new or different box
+				must behave and animate the same way as
+				any other scroll-into-view operation,
+				including honoring controls such as 'scroll-behavior'.
+				Scrolling behavior for re-snapping to the same box as before
+				however, is UA-defined.
+				The UA may, for example,
+				when snapped to the start of a section,
+				choose not to animation the scroll to the section’s new position
+				as content is dynamically added earlier in the document
+				in order to create the illusion of not scrolling.
+				</ins></p>
+			</blockquote>
 	</ul>
 
 	Changes since the <a href="https://www.w3.org/TR/2019/CR-css-scroll-snap-1-20190131/">31 January 2019 Candidate Recommendation</a> include: