JavaFX: Why does stage.setResizable(false) cause additional margins?

This small JavaFX test application

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class ApplicationWithNonResizableStage extends Application {

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

    @Override
    public void start(final Stage primaryStage) throws Exception {
        final Rectangle rectangle = new Rectangle(200, 100, Color.POWDERBLUE);
        final BorderPane pane = new BorderPane(rectangle);
        final Scene scene = new Scene(pane);
        primaryStage.setScene(scene);
        primaryStage.setResizable(false);
        primaryStage.show();
    }
}

produce a window with unwanted padding:

Removing the call primaryStage.setResizable(false) also removes the effect:

What is going wrong?