For any simplest application the client area is not updated at application startup (until resize).



Sample to check:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class App extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        Button control = new Button("Button");
        Scene scene = new Scene(new VBox(control), 100, 100);
        stage.setScene(scene);
        stage.show();
    }
}