[MD settings] hide scrollbar during animation to subpage

chrome/browser/resources/settings/settings_page/main_page_behavior.js

Index: chrome/browser/resources/settings/settings_page/main_page_behavior.js
diff --git a/chrome/browser/resources/settings/settings_page/main_page_behavior.js b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
index 6e7fe98eea12454d25a57f389bd44ffe39f58c40..1c8f066f8395676e03c89b2d9f88989b5ca1b505 100644
--- a/chrome/browser/resources/settings/settings_page/main_page_behavior.js
+++ b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
@@ -11,6 +11,16 @@
 var MainPageBehaviorImpl = {
   properties: {
     /**
+     * Help CSS to alter style during the horizontal swipe animation.
+     * Note that this is unrelated to the |currentAnimation_| (which refers to
+     * the vertical exapand animation).
+     */
+    isSubpageAnimating: {
+      reflectToAttribute: true,
+      type: Boolean,
+    },
+
+    /**
      * Whether a search operation is in progress or previous search results are
      * being displayed.
      * @private {boolean}
@@ -25,6 +35,10 @@ var MainPageBehaviorImpl = {
   /** @type {?HTMLElement} The scrolling container. */
   scroller: null,
 
+  listeners: {
+    'neon-animation-finish': 'onNeonAnimationFinish_'
+  },
+
   /** @override */
   attached: function() {
     if (this.domHost && this.domHost.parentNode.tagName == 'PAPER-HEADER-PANEL')
@@ -34,6 +48,17 @@ var MainPageBehaviorImpl = {
   },
 
   /**
+   * Remove the is-animating attribute once the animation is complete.
+   * This may catch animations finishing more often than needed, which is not
+   * known to cause any issues (e.g. when animating from a shallower page to a
+   * deeper page; or when transitioning to the main page).
+   * @private
+   */
+  onNeonAnimationFinish_: function() {
+    this.isSubpageAnimating = false;
+  },
+
+  /**
    * @param {!settings.Route} newRoute
    * @param {settings.Route} oldRoute
    */
@@ -57,6 +82,9 @@ var MainPageBehaviorImpl = {
         !settings.lastRouteChangeWasPopstate() || oldRouteWasSection ||
         oldRoute == settings.Route.BASIC;
 
+    if (oldRoute && (oldRoute.isSubpage() || newRoute.isSubpage()))
+      this.isSubpageAnimating = true;
+
     // For previously uncreated pages (including on first load), allow the page
     // to render before scrolling to or expanding the section.
     if (!oldRoute || this.scrollHeight == 0)