Add back ScrollDirection::auto

Author: theres-waldo

Overview.bs

@@ -427,6 +427,7 @@ partial interface DocumentTimeline {
 
 <pre class="idl">
 enum ScrollDirection {
+  "auto",
   "block",
   "inline",
   "horizontal",
@@ -437,27 +438,28 @@ enum ScrollDirection {
 The {{ScrollDirection}} enumeration specifies a direction of scroll of a
 scrollable element.
 
+:   <code>auto</code>
+::  If only one direction is scrollable, selects that direction.
+    Otherwise selects the direction along the [=block axis=].
+
 :   <code>block</code>
-::  Selects the direction along the [=block axis=]
+::  Selects the direction along the [=block axis=].
 
 :   <code>inline</code>
-::  Selects the direction along the [=inline axis=]
+::  Selects the direction along the [=inline axis=].
 
 :   <code>horizontal</code>
-::  Selects the horizontal direction
+::  Selects the horizontal direction.
 
 :   <code>vertical</code>
-::  Selects the vertical direction
+::  Selects the vertical direction.
 
 <div class="issue">
 
 Should the physical directions ("horizontal" and "vertical") be removed, leaving 
 only the logical directions ("block" and "inline")?
 
-We should add an "auto" value here that means, "If only one direction is
-scrollable use that direction, otherwise use the block direction".
-
-What about a value that means, "the longest scroll direction." That would be
+What about a value that means, "the longest scroll direction"? That would be
 more reliable than "auto" for the case where layout differences could mean that,
 although normally you only expect the inline direction to be scrollable, on
 some devices you end up with a small scrollable range in the block direction
@@ -494,7 +496,7 @@ The {{ScrollTriggerKind}} enumeration specifies the kind of a {{ScrollTrigger}}.
 dictionary ScrollTriggerOptions {
   Element scrollSource;
   ScrollTriggerKind kind = "offset";
-  ScrollDirection orientation;
+  ScrollDirection orientation = "auto";
   DOMString scrollOffset = "auto";
   DOMString endScrollOffset = "auto";
 };
@@ -542,13 +544,6 @@ element.
 ::  Determines the direction of scrolling which drives the activation and deactivation
     of the trigger.
 
-    If a value for {{orientation}} is not specified, and {{scrollSource}} is only scrollable
-    in one direction, that direction is used; if {{scrollSource}} is scrollable
-    in both directions, the {{block}} direction is used.
-
-    Issue: I think we should simply define a suitable "auto" value and make that
-    the default value in the {{ScrollTriggerOptions}} dictionary.
-
 :   <dfn attribute for=ScrollTrigger>scrollOffset</dfn>
 ::  The scroll offset, in the direction specified by {{orientation}}, that triggers 
     activation of the trigger.