         
         return paragraphStyle
     }()
+    
+    
+    /// update enabilities of jump buttons
+    private func updatePrevNextButtonEnabled() {
+        
+        self.prevButton!.isEnabled = self.canSelectPrevItem
+        self.nextButton!.isEnabled = self.canSelectNextItem
+    }
     
     
     /// set outline menu selection

        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)
            
            NotificationCenter.default.addObserver(self, selector: #selector(invalidateOutlineMenuSelection), name: NSTextView.didChangeSelectionNotification, object: textView)
        }
    }

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