Open just folder menu on middle clicking chevron button.

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

Index: chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm
diff --git a/chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm b/chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm
index 685c00b4fbba6ac07dbeb5646403cb7e904c5d0d..b5f17bbc80a192eb30a924243514284ae99bf0a3 100644
--- a/chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm
+++ b/chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm
@@ -716,13 +716,28 @@ void RecordAppLaunch(Profile* profile, GURL url) {
     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];