为什么在用Java开发的应用程序中有一个透明但很小的顶部面板？

Question

look at top panel:

I am developing a application like alfred using javafx. As you can see,there is a transparent top panel.There is also a same problem when using swing. My development environment is as follows.

• OS: Arch Linux
• Kernel: x86_64 Linux 4.8.13-1-ARCH
• WM: i3
• Java: "1.8.0_102"

package sample;

import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

public class Main extends Application {

  @Override
  public void start(Stage primaryStage) throws Exception{
    Parent root = new Parent() {
      };
    primaryStage.initStyle(StageStyle.UTILITY);
    primaryStage.setTitle("Hello World");
    primaryStage.setScene(new Scene(root, 500, 275));
    primaryStage.show();
  }


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

Answer 1

I had found the cause that compton,a compositor for X,has a feature "Titlebar/frame transparency". – z10030313335