test instant subpage load with animation duration 0

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.
 
 /**
  * Responds to route changes by expanding, collapsing, or scrolling to sections
  * on the page. Expanded sections take up the full height of the container. At
  * most one section should be expanded at any given time.
  * @polymerBehavior MainPageBehavior
  */
@@ skipping 18 matching lines @@
       type: Boolean,
       value: false,
       observer: 'inSearchModeChanged_',
     },
   },
 
   /** @type {?HTMLElement} The scrolling container. */
   scroller: null,
 
   listeners: {
-    'neon-animation-finish': 'onNeonAnimationFinish_'
+    'neon-animation-finish': 'onNeonAnimationFinish_',
+    'subpage-expand': 'onSubpageExpand_',
+  },
+
+  onSubpageExpand_: function() {
+    debugger;
+    if (this.oldDuration_) {
+      settings.animation.Timing.DURATION = this.oldDuration_;
+      document.body.querySelector('settings-ui::shadow #container').style.visibility = 'visible';
+      this.oldDuration_ = undefined;
+    }
   },
 
   /** @override */
   attached: function() {
     this.scroller = this.domHost ? this.domHost.parentNode : document.body;
   },
 
   /**
    * Remove the is-animating attribute once the animation is complete.
    * This may catch animations finishing more often than needed, which is not
@@ skipping 37 matching lines @@
 
     // TODO(dschuyler): This doesn't set the flag in the case of going to or
     // from the main page. It seems sensible to set the flag in those cases,
     // unfortunately bug 708465 happens. Figure out why that is and then set
     // this flag more broadly.
     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)
+    if (!oldRoute) {
+      if (newRoute.isSubpage()) {
+        this.oldDuration_ = settings.animation.Timing.DURATION;
+        settings.animation.Timing.DURATION = 0;
+        document.body.querySelector('settings-ui::shadow #container').style.visibility = 'hidden';

[michaelpg, 2017/05/05 20:42:44]

using visibility: hidden instead of display: none so the section still has a
size, otherwise various errors occur because things have no size and
tryTransitionToSection enters an infinite loop

[michaelpg, 2017/05/22 22:10:51]

(Copying scottchen's response to this thread:)
I'm not sure. The code is already too complicated, because we keep adding
conditions that weren't planned for. But you're right that setting the duration
to 0 globally is hacky (and possibly risky if for some reason we don't set it
back).

I wonder if a compromise would be to have the DURATION always start at 0, and
only set DURATION to the normal value upon receiving some event to tell us that
the important stuff on the page has finished loading?

We might still have to fiddle with showing/hiding some things, but maybe
changing the start conditions instead of trying a bunch of async stuff would be
simpler.
Can we simplify your CL with a one-off event we fire that causes settings-ui to
show/hide the container instead of data binding?

+      }
       setTimeout(this.tryTransitionToSection_.bind(this, scrollToSection));
+    }
     else
       this.tryTransitionToSection_(scrollToSection);
   },
 
   /**
    * When exiting search mode, we need to make another attempt to scroll to
    * the correct section, since it has just been re-rendered.
    * @private
    */
   inSearchModeChanged_: function(inSearchMode) {
@@ skipping 240 matching lines @@
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];