         didSet {
             guard let textView = self.textView else { return }
           
-            // observe text selection change to update outline menu selection
             // -> DO NOT use block-based KVO for NSTextView sublcass
             //    since it causes application crash on OS X 10.11 (but ok on macOS 10.12 and later 2018-02)
             textView.addObserver(self, forKeyPath: #keyPath(NSTextView.layoutOrientation), options: .initial, context: nil)
             
+            // observe text selection change to update outline menu selection
             NotificationCenter.default.addObserver(self, selector: #selector(invalidateOutlineMenuSelection), name: NSTextView.didChangeSelectionNotification, object: textView)
         }
     }

        
        return paragraphStyle
    }()
    
    
    /// set outline menu selection

    }
    
    
    /// can select prev item in outline menu?
    var canSelectPrevItem: Bool {
        
