[MD Bookmarks] Add hover border to sidebar splitter.

ui/webui/resources/js/cr/ui/splitter.js

 // Copyright (c) 2012 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 This implements a splitter element which can be used to resize
  * elements in split panes.
  *
  * The parent of the splitter should be an hbox (display: -webkit-box) with at
  * least one previous element sibling. The splitter controls the width of the
@@ skipping 215 matching lines @@
       this.endDrag_();
     },
 
     /**
      * Handles start of the splitter dragging. Saves current width of the
      * element being resized.
      * @protected
      */
     handleSplitterDragStart: function() {
       // Use the computed width style as the base so that we can ignore what
-      // box sizing the element has.
+      // box sizing the element has. Add the difference between offset and
+      // client widths to account for any scrollbars.
       var targetElement = this.getResizeTarget_();
       var doc = targetElement.ownerDocument;
       this.startWidth_ =
-          parseFloat(doc.defaultView.getComputedStyle(targetElement).width);
+          parseFloat(doc.defaultView.getComputedStyle(targetElement).width) +
+          targetElement.offsetWidth - targetElement.clientWidth;
     },
 
     /**
      * Handles splitter moves. Updates width of the element being resized.
      * @param {number} deltaX The change of splitter horizontal position.
      * @protected
      */
     handleSplitterDragMove: function(deltaX) {
       var targetElement = this.getResizeTarget_();
       var newWidth = this.startWidth_ + this.calcDeltaX_(deltaX);
@@ skipping 12 matching lines @@
       var doc = targetElement.ownerDocument;
       var computedWidth =
           parseFloat(doc.defaultView.getComputedStyle(targetElement).width);
       if (this.startWidth_ != computedWidth)
         cr.dispatchSimpleEvent(this, 'resize');
     },
   };
 
   return {Splitter: Splitter};
 });