Files.app: Prevent a scroolbar from overshooting the border of the view.

chrome/browser/resources/file_manager/js/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.
 
 'use strict';
 
 /**
  * Creates a new scroll bar element.
  * @extends {HTMLDivElement}
  * @constructor
@@ skipping 134 matching lines @@
   if (!event.which) {
     this.onMouseUp_(event);
     return;
   }
   var clientSize = this.clientHeight_;
   var totalSize = this.scrollHeight_;
   var buttonSize = Math.max(50, clientSize / totalSize * clientSize);
 
   var buttonPosition = this.buttonPressedPosition_ +
       (event.screenY - this.buttonPressedEvent_.screenY);
+  // Ensures the scrollbar is in the view.
+  buttonPosition =
+      Math.max(0, Math.min(buttonPosition, clientSize - buttonSize));
   var scrollPosition = totalSize * (buttonPosition / clientSize);
 
   this.scrollTop_ = scrollPosition;
   this.view_.scrollTop = scrollPosition;
   this.redraw_();
 };
 
 /**
  * Handles changed in Dom by redrawing the scrollbar. Ignores consecutive calls.
  * @private
@@ skipping 35 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;
 };