[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 19 matching lines @@
 
     /** @private */
     advancedToggleExpanded_: {
       type: Boolean,
       value: false,
     },
 
     /** @private */
     inSubpage_: Boolean,
 
+    /** @private */
+    overscroll_: {
+      type: Number,
+      observer: 'overscrollChanged_',
+    },
+
     /**
      * Controls which main pages are displayed via dom-ifs.
      * @type {!{about: boolean, basic: boolean, advanced: boolean}}
      * @private
      */
     showPages_: {
       type: Object,
       value: function() {
         return {about: false, basic: false, advanced: false};
       },
@@ skipping 21 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));
+
+  },
+
+  /** @private */
+  overscrollChanged_: function() {
+    if (!this.overscroll_ && this.boundScroll_) {
+      this.parentNode.scroller.removeEventListener('scroll', this.boundScroll_);
+      this.boundScroll_ = null;
+    } else if (this.overscroll_ && !this.boundScroll_) {
+      this.boundScroll_ = this.scrollEventListener_.bind(this);
+      this.parentNode.scroller.addEventListener('scroll', this.boundScroll_);
+    }
+  },
+
+  /** @private */
+  scrollEventListener_: function() {
+    var scroller = this.parentNode.scroller;
+    var overscroll = this.$.overscroll;
+    var visibleBottom = scroller.scrollTop + scroller.clientHeight;
+    var overscrollBottom = overscroll.offsetTop + overscroll.scrollHeight;
+    // How much of the overscroll is visible (may be negative).
+    var visibleOverscroll = overscroll.scrollHeight -
+                            (overscrollBottom - visibleBottom);
+    this.overscroll_ = Math.max(0, visibleOverscroll);

[Dan Beam, 2016/08/03 22:04:59]

if this could share more with overscrollHeight_ that's cool, i.e. why can't this
be more similar to

  this.overscroll_ = this.overscrollHeight_();

?

[dschuyler, 2016/08/03 22:26:03]

I think the math looks similar off the cuff. I don't think
they'd combine well. The common bits appear to be

foo: function(a, b, c, d, e) {
  return Math.max(0, e - ((a + b) - (c + b)));
}

and then call foo() with two sets of five parameters. Would
adding a function like foo() help?

   },
 
   /**
    * @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 30 matching lines @@
             this.pageVisibility.advancedSettings !== false);
         this.advancedToggleExpanded_ = true;
       }
     }
 
     // Wait for any other changes prior to calculating the overflow padding.
     setTimeout(function() {
       // Ensure any dom-if reflects the current properties.
       Polymer.dom.flush();
 
-      this.$.overscroll.style.paddingBottom = this.overscrollHeight_() + 'px';
+      this.overscroll_ = this.overscrollHeight_();
     }.bind(this));
   },
 
   /**
    * Return the height that the overscroll padding should be set to.
    * This is used to determine how much padding to apply to the end of the
    * content so that the last element may align with the top of the content
    * area.
    * @return {number}
    * @private
@@ skipping 71 matching lines @@
 
   /**
    * @param {(boolean|undefined)} visibility
    * @return {boolean} True unless visibility is false.
    * @private
    */
   showAdvancedSettings_: function(visibility) {
     return visibility !== false;
   },
 });