[css-scroll-snap-1] Add 'auto' value to scroll-padding, per WG resolution. Fixes #2929.

Author: tabatkins

css-scroll-snap-1/Overview.bs

@@ -400,26 +400,18 @@ Scroll Snap Strictness: the ''scroll-snap-type/none'', ''proximity'', and ''mand
 Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
 ----------------------------------------------------------------
 
-    <pre class="propdef">
+    <pre class="propdef shorthand">
     Name: scroll-padding
-    Value: <<length-percentage>>{1,4}
-    Initial: 0
+    Value: [ <<length-percentage>> | auto ]{1,4}
+    Initial: auto
     Applies to: <a>scroll containers</a>
     Inherited: no
     Percentages: relative to the corresponding dimension of the scroll container’s scrollport
     Computed value: as specified, with lengths made absolute
     Animatable: as length, percentage, or calc
     </pre>
 
-    This property is a <a>shorthand property</a> that sets
-    all of the <a href="#longhands"><css>scroll-padding-*</css> longhands</a>
-    in one declaration,
-    assigning values to the longhands representing each side
-    exactly as the 'padding' property does for its longhands.
-
-    Values must be non-negative
-    and represent inward offsets from the corresponding edge of the scrollport.
-    These offsets define the
+    This property specifies offsets that define the
     <dfn export>optimal viewing region</dfn> of the scrollport:
     the region used as the target region for placing things in view of the user.
     This allows the author to exclude regions of the scrollport
@@ -428,7 +420,35 @@ Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
     or simply to put more breathing room
     between a targetted element and the edges of the scrollport.
 
-    The 'scroll-padding' offsets effectively reduce the region of the viewport
+    The 'scroll-padding' property is a <a>shorthand property</a> that sets
+    all of the <a href="#longhands"><css>scroll-padding-*</css> longhands</a>
+    in one declaration,
+    assigning values to the longhands representing each side
+    exactly as the 'padding' property does for its longhands.
+    Values have the following meanings:
+
+    <dl dfn-for="scroll-padding, scroll-padding-inline, scroll-padding-inline-start, scroll-padding-inline-end, scroll-padding-block, scroll-padding-block-start, scroll-padding-block-end" dfn-type=value>
+        : <dfn><<length-percentage>></dfn>
+        ::
+            Defines an inward offset from the corresponding edge of the [=scrollport=].
+
+        : <dfn>auto</dfn>
+        ::
+            Indicates that the offset for the corresponding edge of the [=scrollport=] is UA-determined.
+            This will generally default to a used length of ''0px'',
+            but UAs may use heuristics to detect when a non-zero value is more appropriate.
+
+            <div class=example>
+                For example, a UA could detect when a ''position:fixed'' element
+                is being used as an opaque unscrollable “header”
+                that obscures the content below it,
+                and resolve the top offset to the height of that element
+                so that a “page down” operation (such as pressing <kbd>PgDn</kbd>)
+                automatically scrolls by one “visible page” of content.
+            </div>
+    </dl>
+
+    These offsets reduce the region of the viewport
     that is considered “viewable” <em>for scrolling operations</em>:
     they have no effect on layout,
     on the scroll origin or initial position,