To reproduce try to resize any simple application by dragging corner.

package main;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.scene.text.Text;
import javafx.stage.Stage;

public class Main extends Application {

    VBox root;

    @Override
    public void start(Stage stage) {
        stage.setTitle(this.getClass().getSimpleName());
        stage.setScene(new ToolBarAppScene());
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class, args);
    }

    public class ToolBarAppScene extends Scene {
        public ToolBarAppScene() {
            super(root = new VBox());

            Text label = new Text("Label");

            root.getChildren().add(label);
        }
    }
}