Open just folder menu on middle clicking chevron button.

chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm

 // 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.
 
 #import "chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.h"
 
 #include "base/mac/bundle_locations.h"
 #include "base/mac/mac_util.h"
 #include "base/metrics/histogram.h"
 #include "base/prefs/pref_service.h"
@@ skipping 698 matching lines @@
 // Common function to open a bookmark folder of any type.
 - (void)openBookmarkFolder:(id)sender {
   DCHECK([sender isKindOfClass:[BookmarkButton class]]);
   DCHECK([[sender cell] isKindOfClass:[BookmarkButtonCell class]]);
 
   // Only record the action if it's the initial folder being opened.
   if (!showFolderMenus_)
     RecordBookmarkFolderOpen([self bookmarkLaunchLocation]);
   showFolderMenus_ = !showFolderMenus_;
 
-  if (sender == offTheSideButton_)
+  // Middle click on chevron should not open bookmarks under it, instead just
+  // open its folder menu.
+  if (sender == offTheSideButton_) {
     [[sender cell] setStartingChildIndex:displayedButtonCount_];
-
+    NSEvent* event = [NSApp currentEvent];
+    if ([event type] == NSOtherMouseUp) {
+      [self openOrCloseBookmarkFolderForOffTheSideButton];
+      return;
+    }
+  }
   // Toggle presentation of bar folder menus.
   [folderTarget_ openBookmarkFolderFromButton:sender];
 }
 
+- (void)openOrCloseBookmarkFolderForOffTheSideButton {
+  // If clicked on already opened folder, then close it and return.
+  if ([folderController_ parentButton] == offTheSideButton_)
+    [self closeBookmarkFolder:self];
+  else
+    [self addNewFolderControllerWithParentButton:offTheSideButton_];
+}
+
 // Click on a bookmark folder button.
 - (IBAction)openBookmarkFolderFromButton:(id)sender {
   [self openBookmarkFolder:sender];
 }
 
 // Click on the "off the side" button (chevron), which opens like a folder
 // button but isn't exactly a parent folder.
 - (IBAction)openOffTheSideFolderFromButton:(id)sender {
   [self openBookmarkFolder:sender];
 }
@@ skipping 2062 matching lines @@
 - (id<BookmarkButtonControllerProtocol>)controllerForNode:
     (const BookmarkNode*)node {
   // See if it's in the bar, then if it is in the hierarchy of visible
   // folder menus.
   if (bookmarkModel_->bookmark_bar_node() == node)
     return self;
   return [folderController_ controllerForNode:node];
 }
 
 @end