问JavaFx渲染不正确

EN

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;


public class MainGUI extends Application{

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

    }

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

        Screen firstScreen = new Screen();
        Scene mainScene = new Scene(firstScreen );
        pStage.setScene(mainScene);
        pStage.setTitle("TestProg");
        pStage.show();
    }
}

import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;



public class Screen extends VBox{
    private final Label L1 = new Label("Label 1");
    private final Label L2 = new Label("Label 2");
    private final TextField F1 = new TextField();
    private final TextField F2 = new TextField();
    private final Button B1 = new Button("This is a button");

    Screen(){
        super();
        showStuff();
    }
    private void showStuff(){
        this.setSpacing(5);
        this.setAlignment(Pos.CENTER);
        this.setPadding(new Insets(10,0,0,10));
        HBox Box1 = new HBox(L1,F1);
        HBox Box2 = new HBox(L2,F2);
        this.getChildren().addAll(Box1,Box2,B1);
    }
}