javafx.scene.Group的疑问setAutoSizeChildren

javafx.scene.Group如果设置setAutoSizeChildren为fasle，那么下面例子中的按钮将不能正常显示

package group;import javafx.application.Application;import javafx.scene.Group;import javafx.scene.Scene;import javafx.scene.control.Button;import javafx.stage.Stage;public class TestGroup3 extends Application {@Overridepublic void start(Stage stage) throws Exception {Group g = new Group();Button btn = new Button("btn");g.getChildren().add(btn);g.setAutoSizeChildren(false);// 如果注释掉这句话，那么按钮将不会显示stage.setScene(new Scene(g));stage.show();btn.setMinSize(20, 20);btn.setPrefSize(30, 30);}public static void main(String[] args) {launch(args);}}