Scroll the attached view when wheel spinned on top of a scroll button.

ui/file_manager/file_manager/foreground/js/ui/scrollbar.js

 // Copyright (c) 2013 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.
 
 /**
  * Creates a new scroll bar element.
  * @extends {HTMLDivElement}
  * @constructor
  */
 var ScrollBar = cr.ui.define('div');
@@ skipping 33 matching lines @@
  * Initializes the DOM structure of the scrollbar.
  */
 ScrollBar.prototype.decorate = function() {
   this.classList.add('scrollbar');
   this.button_ = util.createChild(this, 'scrollbar-button', 'div');
   this.mode = ScrollBar.Mode.VERTICAL;
   this.idleTimerId_ = 0;
 
   this.button_.addEventListener('mousedown',
                                 this.onButtonPressed_.bind(this));
+  this.button_.addEventListener('mousewheel', this.onWheel_.bind(this));

[fukino, 2016/07/11 10:09:52]

mousewheel event is deprecated, so please use the standard wheel event.

[yamaguchi, 2016/07/11 10:17:20]

Done.

   window.addEventListener('mouseup', this.onMouseUp_.bind(this));
   window.addEventListener('mousemove', this.onMouseMove_.bind(this));
 };
 
 /**
  * Initialize a scrollbar.
  *
  * @param {Element} parent Parent element, must have a relative or absolute
  *     positioning.
  * @param {Element=} opt_scrollableArea Element with scrollable contents.
@@ skipping 15 matching lines @@
   this.view_.addEventListener('scroll', this.onScroll_.bind(this));
   this.view_.addEventListener('relayout', this.onRelayout_.bind(this));
   this.domObserver_ = new MutationObserver(this.onDomChanged_.bind(this));
   this.domObserver_.observe(
       this.view_,
       /** @type {MutationObserverInit} */ ({subtree: true, attributes: true}));
   this.onRelayout_();
 };
 
 /**
+ * Handles scroll by a mouse wheel.
+ *
+ * @param {Event} event Mouse event.
+ * @private
+ */
+ScrollBar.prototype.onWheel_ = function(event) {
+  var scrollPosition = this.view_.scrollTop + event.deltaY;
+  // Ensure the scrollbar is in the view.
+  var scrollBottom =
+      Math.max(this.view_.scrollHeight - this.view_.clientHeight, 0);
+  scrollPosition = Math.max(Math.min(scrollPosition, scrollBottom), 0);
+

[fukino, 2016/07/11 10:09:52]

Please add a comment about the difference between native scroll and this manual
scroll.

[yamaguchi, 2016/07/11 10:17:20]

Done.

+  this.scrollTop_ = scrollPosition;
+  this.view_.scrollTop = scrollPosition;
+  this.redraw_();
+}
+
+/**
  * Scroll handler.
  * @private
  */
 ScrollBar.prototype.onScroll_ = function() {
   this.scrollTop_ = this.view_.scrollTop;
   this.redraw_();
 
   // Add class 'scrolling' to scrollbar to make it visible while scrolling.
   this.button_.classList.add('scrolling');
 
@@ skipping 126 matching lines @@
     requestAnimationFrame(function() {
       this.hidden = hidden;
       this.button_.style.top = buttonTop + 'px';
       this.button_.style.height = buttonSize + 'px';
     }.bind(this));
   }
 
   this.lastButtonTop_ = buttonTop;
   this.lastButtonSize_ = buttonSize;
 };