To reproduce:

1. Click "Show/hide root" button. Root will disappear.
2. Click the button again. It is expected that root item will appear again but it won't.


public class JavaFXApplication22 extends Application {
    
    TreeView treeView;
    
    @Override
    public void start(Stage primaryStage) {
        
        HBox root = new HBox(10.0);
        Scene scene = new Scene(root, 300, 250);
        
        treeView = getTreeView();
        
        ToggleButton btnShowRoot = new ToggleButton("Show/hide root");
        btnShowRoot.setSelected(true);
        btnShowRoot.selectedProperty().addListener(
        new ChangeListener<Boolean>() {
            @Override
            public void changed(ObservableValue<? extends Boolean> ov,
                Boolean oldVal, Boolean newVal) {
                
                treeView.setShowRoot(newVal.booleanValue());
            }
        });
        
        root.getChildren().addAll(treeView, btnShowRoot);
        
        primaryStage.setTitle(VersionInfo.getRuntimeVersion());
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }

    private TreeView getTreeView() {
        TreeView tv = new TreeView();
        
        TreeItem root = new TreeItem("Root");
        tv.setRoot(root);
        
        root.setExpanded(true);
        
        for (int i = 0; i < 10; i++) {
            TreeItem it = new TreeItem(String.format("Tree item - %d", i));
            root.getChildren().add(it);
        }
        
        return tv;
    }
}