Support scrollbar snapback on Windows

Summary: See D11472. I eyeballed the "140" number by screenshotting / measuring in Paint.

Test Plan: Made the snapback thing return `true` and got snapback on OSX.

Reviewers: chad

Reviewed By: chad

Subscribers: avivey, epriestley

Differential Revision: https://secure.phabricator.com/D11485

resources/celerity/map.php

@@ -199,7 +199,7 @@ return array(
     'rsrc/externals/javelin/lib/Resource.js' => '44959b73',
     'rsrc/externals/javelin/lib/Routable.js' => 'b3e7d692',
     'rsrc/externals/javelin/lib/Router.js' => '29274e2b',
-    'rsrc/externals/javelin/lib/Scrollbar.js' => 'a89e4fc3',
+    'rsrc/externals/javelin/lib/Scrollbar.js' => '8ebeb833',
     'rsrc/externals/javelin/lib/URI.js' => '6eff08aa',
     'rsrc/externals/javelin/lib/Vector.js' => 'cc1bd0b0',
     'rsrc/externals/javelin/lib/WebSocket.js' => '3f840822',
@@ -673,7 +673,7 @@ return array(
     'javelin-resource' => '44959b73',
     'javelin-routable' => 'b3e7d692',
     'javelin-router' => '29274e2b',
-    'javelin-scrollbar' => 'a89e4fc3',
+    'javelin-scrollbar' => '8ebeb833',
     'javelin-stratcom' => '8b0ad945',
     'javelin-tokenizer' => '7644823e',
     'javelin-typeahead' => '70baed2f',
@@ -1468,6 +1468,12 @@ return array(
       'javelin-stratcom',
       'javelin-behavior',
     ),
+    '8ebeb833' => array(
+      'javelin-install',
+      'javelin-dom',
+      'javelin-stratcom',
+      'javelin-vector',
+    ),
     '8ef9ab58' => array(
       'javelin-behavior',
       'javelin-dom',
@@ -1550,12 +1556,6 @@ return array(
       'javelin-stratcom',
       'javelin-dom',
     ),
-    'a89e4fc3' => array(
-      'javelin-install',
-      'javelin-dom',
-      'javelin-stratcom',
-      'javelin-vector',
-    ),
     'a8d8459d' => array(
       'javelin-behavior',
       'javelin-dom',

webroot/rsrc/externals/javelin/lib/Scrollbar.js

@@ -188,7 +188,7 @@ JX.install('Scrollbar', {
       e.kill();
 
       // Store the position where the drag started.
-      this._dragOrigin = JX.$V(e).y;
+      this._dragOrigin = JX.$V(e);
 
       // Store the original position of the handle.
       this._scrollOrigin = this._viewport.scrollTop;
@@ -203,14 +203,35 @@ JX.install('Scrollbar', {
         return;
       }
 
-      var offset = (JX.$V(e).y - this._dragOrigin);
+      var p = JX.$V(e);
+      var offset = (p.y - this._dragOrigin.y);
       var ratio = offset / JX.Vector.getDim(this._bar).y;
       var adjust = ratio * JX.Vector.getDim(this._content).y;
 
+      if (this._shouldSnapback()) {
+        if (Math.abs(p.x - this._dragOrigin.x) > 140) {
+          adjust = 0;
+        }
+      }
+
       this._viewport.scrollTop = this._scrollOrigin + adjust;
     },
 
 
+    /**
+     * Should the scrollbar snap back to the original position if the user
+     * drags the mouse away to the left or right, perpendicular to the
+     * scrollbar?
+     *
+     * Scrollbars have this behavior on Windows, but not on OSX or Linux.
+     */
+    _shouldSnapback: function() {
+      // Since this is an OS-specific behavior, detect the OS. We can't
+      // reasonably use feature detection here.
+      return (navigator.platform.indexOf('Win') > -1);
+    },
+
+
     /**
      * When the user releases the mouse after a drag, stop moving the
      * viewport.