[MD settings] shrink overscroll when scrolling up

chrome/browser/resources/settings/settings_main/settings_main.js

 // Copyright 2015 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.
 
 /**
  * @fileoverview
  * 'settings-main' displays the selected settings page.
  */
 Polymer({
   is: 'settings-main',
@@ skipping 60 matching lines @@
     },
   },
 
   /** @override */
   attached: function() {
     document.addEventListener('toggle-advanced-page', function(e) {
       this.advancedToggleExpanded_ = e.detail;
       settings.navigateTo(this.advancedToggleExpanded_ ?
           settings.Route.ADVANCED : settings.Route.BASIC);
     }.bind(this));
+
+    var mainContainer = this.domHost.$$('paper-header-panel').$.mainContainer;

[Dan Beam, 2016/08/03 01:30:09]

can we not poke into paper-header-panel's local DOM?

[dschuyler, 2016/08/03 19:23:08]

I changed it to something a bit more direct.
I'm not sure how to get the scroll event and
data without looking at the #mainContainer.

Done.

+    var overscroll = this.$.overscroll;
+    mainContainer.addEventListener('scroll', function() {
+      // TODO(dschuyler): This works okay, but not perfectly with page-up/down
+      // and home/end. It may have to do with the order in which the resize
+      // occurs (relative to the scroll action).
+      var visibleBottom = mainContainer.scrollTop + mainContainer.clientHeight;
+      var overscrollBottom = overscroll.offsetTop + overscroll.scrollHeight;
+      // How much of the overscroll is visible (may be negative).
+      var visibleOverscroll = overscroll.scrollHeight -
+                              (overscrollBottom - visibleBottom);
+      if (visibleOverscroll > 0)

[Dan Beam, 2016/08/03 01:30:09]

so why are we only setting this when overscroll > 0?  shouldn't we just be doing
Math.min(0, visibleOverscroll)?

[dschuyler, 2016/08/03 19:23:08]

Done.

That would be logically the same, but function
calls have a high overhead compared to the 
conditional. Since it was logically the same 
and equally readable, I was going for the one 
with less overhead.

+        overscroll.style.paddingBottom = visibleOverscroll + 'px';
+    }.bind(this));
   },
 
   /**
    * @param {boolean} opened Whether the menu is expanded.
    * @return {string} Which icon to use.
    * @private
    */
   arrowState_: function(opened) {
     return opened ? 'settings:arrow-drop-up' : 'cr:arrow-drop-down';
   },
@@ skipping 117 matching lines @@
 
   /**
    * @param {(boolean|undefined)} visibility
    * @return {boolean} True unless visibility is false.
    * @private
    */
   showAdvancedSettings_: function(visibility) {
     return visibility !== false;
   },
 });