[css-scroll-snap-1] Be more precise about layout viewport vs visual viewport #4393

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -511,13 +511,12 @@ Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
         : <dfn><<length-percentage>></dfn>
         ::
             Defines an inward offset from the corresponding edge of the [=scrollport=].
-            When the scrollport is the root scroller, the offset is relative to the layout viewport.
+            When applied to the root viewport,
+            the offset is calculated and applied relative to the layout viewport
+            (rather than the visual viewport);
+            the [=optimal viewing region=] is the remaining area
+            that intersects with the visual viewport.
 
-            <div class="note">
-                Though the [=scrollport=] is defined as the visual viewport of the scroll container,
-                scroll padding offsets are relative to the layout viewport when the scroll container is
-                the root scroller.
-            </div>
         : <dfn>auto</dfn>
         ::
             Indicates that the offset for the corresponding edge of the [=scrollport=] is UA-determined.
@@ -1262,6 +1261,35 @@ Changes Since 19 March 2019 CR {#changes-20190319}
 				in order to create the illusion of not scrolling.
 				</ins></p>
 			</blockquote>
+		<li id="change-2019-propagation">
+			Defined explicitly that 'scroll-snap-type' and 'scroll-padding' values
+			are propagated from the root element to the document viewport
+			as would be expected.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/3740">Issue 3740</a>)
+			<blockquote>
+				<p><ins>UAs must apply the 'scroll-snap-type' value set on the root element
+				to the document viewport.
+				Note that, unlike 'overflow',
+				'scroll-snap-type' values are <em>not</em> propagated from HTML <{body}>.</ins></p>
+			</blockquote>
+			<blockquote>
+				<p><ins>UAs must apply the 'scroll-padding' values set on the root element
+				to the document viewport.
+				(Note that, unlike 'overflow',
+				'scroll-padding' values are <em>not</em> propagated from HTML <{body}>.)</ins></p>
+			</blockquote>
+		<li id="change-2019-padding-viewport">
+			Clarified that while snap alignment is relative to the visual viewport,
+			'scroll-padding' is resolved against the layout viewport.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/4393">Issue 4393</a>)
+			<blockquote>
+				<p>Defines an inward offset from the corresponding edge of the [=scrollport=].
+				<ins>When applied to the root viewport,
+				the offset is calculated and applied relative to the layout viewport
+				(rather than the visual viewport);
+				the [=optimal viewing region=] is the remaining area
+				that intersects with the visual viewport.</ins></p>
+			</blockquote>
 	</ul>
 
 Changes Since 31 January 2019 CR {#changes-20190131}
@@ -1302,23 +1330,6 @@ Changes Since 31 January 2019 CR {#changes-20190131}
 				<em>even when snapping is off
 				or not applied on this element</em></ins>.</p>
 			</blockquote>
-		<li id="change-2019-propagation">
-			Defined explicitly that 'scroll-snap-type' and 'scroll-padding' values
-			are propagated from the root element to the document viewport
-			as would be expected.
-			(<a href="https://github.com/w3c/csswg-drafts/issues/3740">Issue 3740</a>)
-			<blockquote>
-				<p><ins>UAs must apply the 'scroll-snap-type' value set on the root element
-				to the document viewport.
-				Note that, unlike 'overflow',
-				'scroll-snap-type' values are <em>not</em> propagated from HTML <{body}>.</ins></p>
-			</blockquote>
-			<blockquote>
-				<p><ins>UAs must apply the 'scroll-padding' values set on the root element
-				to the document viewport.
-				(Note that, unlike 'overflow',
-				'scroll-padding' values are <em>not</em> propagated from HTML <{body}>.)</ins></p>
-			</blockquote>
 	</ul>
 
 Changes Since 14 August 2018 CR {#changes-20180814}