MD Settings: Adding some unit tests for <settings-main>.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Fast out, slow in.
 var EASING_FUNCTION = 'cubic-bezier(0.4, 0, 0.2, 1)';
 var EXPAND_DURATION = 350;
 
 /**
  * Calls |readyTest| repeatedly until it returns true, then calls
@@ skipping 23 matching lines @@
    * @type {string} Selector to get the sections. Derived elements
    *     must override.
    */
   sectionSelector: '',
 
   /** @type {?Element} The scrolling container. */
   scroller: null,
 
   /** @override */
   attached: function() {
-    this.scroller = this.domHost && this.domHost.parentNode.$.mainContainer;
+    if (this.domHost && this.domHost.parentNode.tagName == 'PAPER-HEADER-PANEL')
+      this.scroller = this.domHost.parentNode.$.mainContainer;
+    else
+      this.scroller = document.body; // Used in unit tests.

[michaelpg, 2016/07/30 05:38:44]

could this just be assert(this.parentElement)? if we're attached but we're not
in a shadow tree we must be a normal DOM child.

it can equate to document.body in tests, but it also means we could parent this
thing with a <div> to test the scrolling behaviors without messing with the
entire document.body.

[michaelpg, 2016/08/02 20:45:20]

or better, see my new suggestion

[dpapad, 2016/08/02 22:33:15]

Previous suggestion does not work.
this.parentElement is null at the time the attached() callback fires.
this.parentNode is [object ShadowRoot] (still causes tests to fail).

I would rather not spend too much time tweaking this line. Instead the root
issue https://bugs.chromium.org/p/chromium/issues/detail?id=631281 should be
addressed. See also my response to your newest suggestion.

   },
 
   /**
    * Hides or unhides the sections not being expanded.
    * @param {string} sectionName The section to keep visible.
    * @param {boolean} hidden Whether the sections should be hidden.
    * @private
    */
   toggleOtherSectionsHidden_: function(sectionName, hidden) {
     var sections = Polymer.dom(this.root).querySelectorAll(
@@ skipping 370 matching lines @@
         this.$$('[section=' + section + ']'));
   },
 };
 
 
 /** @polymerBehavior */
 var RoutableBehavior = [
   MainPageBehavior,
   RoutableBehaviorImpl
 ];