code: public final Node getDisclosureNode() {
          if (disclosureNode.get() == null) {
              final StackPane disclosureNode = new StackPane();
              disclosureNode.getStyleClass().setAll("tree-disclosure-node");
              disclosureNode.setMouseTransparent(true);
  
              final StackPane disclosureNodeArrow = new StackPane();
              disclosureNodeArrow.getStyleClass().setAll("arrow");
              disclosureNode.getChildren().add(disclosureNodeArrow);
              setDisclosureNode(disclosureNode);
          }
          return disclosureNode.get();
      }
docs: Returns the current disclosure node set in this cell.