[css-scroll-snap-1] Explicitly tie together the behavior of fixedpos elements and scroll-padding, since that was the goal, and UAs do fiddly things with fixedpos on mobile. w3c#4393

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -513,7 +513,9 @@ Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
             Defines an inward offset from the corresponding edge of the [=scrollport=].
             When applied to the root viewport,
             the offset is calculated and applied relative to the layout viewport
-            (rather than the visual viewport);
+            (rather than the visual viewport)
+            the same way as the corresponding [=inset properties=]
+            on [=fixed-positioned boxes=];
             the [=optimal viewing region=] is the remaining area
             that intersects with the visual viewport.
 
@@ -1280,13 +1282,16 @@ Changes Since 19 March 2019 CR {#changes-20190319}
 			</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.
+			'scroll-padding' is resolved against the layout viewport,
+			so that 'scroll-padding' and 'inset' are consistent on the root 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);
+				(rather than the visual viewport)
+				the same way as the corresponding [=inset properties=]
+				on [=fixed-positioned boxes=];
 				the [=optimal viewing region=] is the remaining area
 				that intersects with the visual viewport.</ins></p>
 			</blockquote>