ScrollPane not showing as needed, FlowPane content

Have a look at the code below and tell me if that's what you want to achieve. I am still not sure what cause the problem, I will have to look the documentation of ScrollPane to find out. My suspicion is at setFitToWidth & setFitToHeight methods. Although I still believe it's not a bug.

import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.scene.Scene;
import javafx.scene.control.ScrollPane;
import javafx.scene.layout.FlowPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Main extends Application {

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        FlowPane flow = new FlowPane();
        flow.setStyle("-fx-border-color: red");

        addPanes(flow, 16);

        ScrollPane scroll = new ScrollPane(flow);
        scroll.setStyle("-fx-border-color: green");

        // Apparently this cause the issue here.
        // scroll.setFitToHeight(true);
        // scroll.setFitToWidth(true);


        // Instead just make the flow pane take the dimensions of the ScrollPane
        // the -5 is to not show the Bars when both of panes have the same dimensions  
        flow.prefWidthProperty().bind(Bindings.add(-5, scroll.widthProperty()));
        flow.prefHeightProperty().bind(Bindings.add(-5, scroll.heightProperty()));

        Scene scene = new Scene(scroll, 450, 450);
        primaryStage.setScene(scene);
        primaryStage.show();

    }

    public void addPanes(FlowPane root, int panes) {
        for (int i = 0; i < panes; i++) {
            HBox filler = new HBox();
            filler.setStyle("-fx-border-color: black");
            filler.setPrefSize(100, 100);
            root.getChildren().add(filler);
        }
    }
}

Looking documentation of the ScrollPane, and in specific the setFitToHeight you will find that :

Property description: If true and if the contained node is a Resizable, then the node will be kept resized to match the height of the ScrollPane's viewport. If the contained node is not a Resizable, this value is ignored.

And because the node inside the ScrollPane will be kept resized to match the width and height of the ScrollPane's viewport thats why the Vertical ScrollBar will never appear.

You can add the code below to always show your vertical scrollbar.

scroll.setVbarPolicy(ScrollPane.ScrollBarPolicy.ALWAYS);