import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

public class TransparentStage extends Application {

    @Override
    public void start(Stage primaryStage) { 
        Circle circle = new Circle(100,100,100); 
        StackPane root = new StackPane(); 
        root.getChildren().add(new Button("Hallo")); 
        Scene scene = new Scene(root, 200, 200); 
        root.setShape(circle); 
        primaryStage.setScene(scene); 
        primaryStage.initStyle(StageStyle.TRANSPARENT); 
        scene.setFill(Color.TRANSPARENT); 
        primaryStage.show(); 
    } 

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