[MD Bookmarks] Right click on bookmark items open context menu.

chrome/browser/resources/md_bookmarks/list.js

 // Copyright 2016 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.
 
 Polymer({
   is: 'bookmarks-list',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 32 matching lines @@
   },
 
   /**
    * @param {Event} e
    * @private
    */
   onOpenItemMenu_: function(e) {
     this.menuItem_ = e.detail.item;
     var menu = /** @type {!CrActionMenuElement} */ (
         this.$.dropdown);
-    menu.showAt(/** @type {!Element} */ (e.detail.target));
+    if (e.detail.targetElement) {
+      menu.showAt(/** @type {!Element} */ (e.detail.targetElement));
+    } else {
+      menu.showAtPosition({
+        top: e.detail.y,
+        left: e.detail.x,
+      });
+    }
   },
 
   /** @private */
   onEditTap_: function() {
     this.closeDropdownMenu_();
     /** @type {BookmarksEditDialogElement} */ (this.$.editDialog.get())
         .showEditDialog(this.menuItem_);
   },
 
   /** @private */
@@ skipping 12 matching lines @@
         // TODO(jiaxi): Add toast later.
       }.bind(this));
     } else {
       chrome.bookmarks.removeTree(this.menuItem_.id, function() {
         // TODO(jiaxi): Add toast later.
       }.bind(this));
     }
     this.closeDropdownMenu_();
   },
 
+  /**
+   * @param {Event} e

[tsergeant, 2017/04/13 01:15:47]

I think you should add a small comment here to show why this is needed

[calamity, 2017/04/13 04:56:46]

Done.

+   * @private
+   */
+  onMenuMousedown_: function(e) {
+    if (e.path[0] != this.$.dropdown)
+      return;
+
+    this.$.dropdown.close();
+  },
+
   /** @private */
   closeDropdownMenu_: function() {
     var menu = /** @type {!CrActionMenuElement} */ (
         this.$.dropdown);
     menu.close();
   },
 
   /** @private */
   getEditActionLabel_: function() {
     var label = this.menuItem_.url ? 'menuEdit' : 'menuRename';
     return loadTimeData.getString(label);
   },
 
   /** @private */
   emptyListMessage_: function() {
     var emptyListMessage = this.searchTerm_ ? 'noSearchResults' : 'emptyList';
     return loadTimeData.getString(emptyListMessage);
   },
 
   /** @private */
   isEmptyList_: function() {
     return this.displayedList_.length == 0;
   },
 
   /** @private */
   deselectItems_: function() {
     this.dispatch(bookmarks.actions.deselectItems());
   },
 });