Run code:

    @Override
    public void start(Stage stage) throws Exception {

        final Accordion acc = new Accordion();

        final TitledPane pane1 = new TitledPane();
        pane1.setGraphic(new Label("title 1\nLong text long text"));
        pane1.setContent(new Button("Some button"));
        acc.getPanes().addAll(pane1);

        final TitledPane pane2 = new TitledPane();
        pane2.setGraphic(new Label("title 1\nLong text long text"));
        pane2.setContent(new Button("Some button"));
        acc.getPanes().addAll(pane2);

        final TitledPane pane3 = new TitledPane();
        pane3.setGraphic(new Label("title 1\nLong text long text"));
        pane3.setContent(new Button("Some button"));
        acc.getPanes().addAll(pane3);

        final ToggleButton visible = new ToggleButton("Visible");

        pane2.visibleProperty().bindBidirectional(visible.selectedProperty());

        VBox pane = new VBox();
        pane.getChildren().addAll(visible, acc);
        Scene scene = new Scene(pane, 300, 700);
        stage.setScene(scene);
        stage.show();
    }

There is a toggle button, which switch visibility of the second Titled pane. when titled pane is invisible, there is just an empty space in accordion. I think, visible property should be tracked by accordion.