How to make StackPane fill a BorderPane in JavaFX

I have a simple JavaFX class that for this question is structured like this.

BorderPane
     StackPane
         FlowPane

BorderPane contains StackPane which contains FlowPane.

Now I've been trying for days to make that StackPane fill its parent, BorderPane, but nothing seems to work. The final result should be StackPane having its background set to "red".

The code:

import javafx.application.Application;
import javafx.geometry.Orientation;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.image.ImageView;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.FlowPane;
import javafx.scene.layout.Pane;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

/**
 *
 * @author A4XX-COLCRI
 */
public class LogicGatesSimulator extends Application {
    /**
     * The root node of the application.
     */
    Group root;
    /**
     * This is the top-level container child of root.
     */
    BorderPane mainContainer;
    /**
     * Contains the canvas and represents the drawing area.
     */
    Pane canvasContainer;
    /**
     * Defines where all the shapes will be drawn.
     */
    Canvas canvas;
    /**
     * Defines the toolbox pane.
     */
    StackPane toolboxPane;
    /**
     * Defines the toolbox flowpane.
     */
    FlowPane toolbox;
    /**
     * Defines the top bar menu.
     */
    MenuBar menu;


    @Override
    public void start(Stage primaryStage) {
        // Inizializzazione degli elementi grafici
        root = new Group();
        mainContainer = new BorderPane();
        canvasContainer = new Pane();
        canvas = new Canvas();
        toolboxPane = new StackPane();
        toolboxPane.setStyle("-fx-background-color: red; ");
        toolbox = new FlowPane(Orientation.VERTICAL);
        toolbox.setPrefWrapLength(250);
        toolbox.minWidth(Region.USE_COMPUTED_SIZE);
        toolbox.minHeight(Region.USE_COMPUTED_SIZE);
        toolbox.maxWidth(Region.USE_COMPUTED_SIZE);
        toolbox.maxHeight(Region.USE_COMPUTED_SIZE);
        toolbox.prefHeight(Region.USE_COMPUTED_SIZE);
        //toolbox.setStyle("-fx-background-color: red;");

        // Inizializzazione scena
        Scene scene = new Scene(root, 1280, 800);
        primaryStage.setTitle("Logic Gates Simulator");
        primaryStage.setScene(scene);

        // Menu
        menu = new MenuBar();
        Menu menuFile = new Menu("File");
        Menu menuEdit = new Menu("Edit");
        Menu menuView = new Menu("View");
        Menu menuAbout = new Menu("About");
        menu.getMenus().addAll(menuFile, menuEdit, menuView, menuAbout);

        // Porte logiche nella toolbox 
        ImageView andImage = new ImageView(getClass().getResource("images/and.png").toExternalForm());
        ImageView nandImage = new ImageView(getClass().getResource("images/nand.png").toExternalForm());
        ImageView orImage = new ImageView(getClass().getResource("images/or.png").toExternalForm());
        ImageView norImage = new ImageView(getClass().getResource("images/nor.png").toExternalForm());
        ImageView xorImage = new ImageView(getClass().getResource("images/xor.png").toExternalForm());
        ImageView xnorImage = new ImageView(getClass().getResource("images/xnor.png").toExternalForm());
        ImageView notImage = new ImageView(getClass().getResource("images/not.png").toExternalForm());

        // Strutturazione
        toolboxPane.getChildren().add(toolbox);
        mainContainer.setLeft(toolboxPane);
        mainContainer.setRight(canvasContainer);
        toolbox.getChildren().addAll(andImage, nandImage, orImage, norImage, xorImage, xnorImage, notImage);
        canvasContainer.getChildren().add(canvas);
        root.getChildren().add(mainContainer);

        // Mostra la scena
        primaryStage.show();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }
}     

A photo of my application running

So basically this left area should be filling entirely the left side of my BorderPane, thus the background should cover correctly its area. I also tried changing it from a StackPane to an AnchorPane and setting its TopAnchor, LeftAnchor etc. to 0 but it's not working. Thank you.

You're omitting the part causing the behavior from the structure description:

The Group.

A Group resizes it's children to their preferred size. It's not resizeable so resizing the stage has no effect on the Group or any of it's descendants...

Simply remove the Group and use the mainContainer BorderPane as the scene root.