Why each JavaFX application flash on my system?

Question

I had created simple app using NetBeans wizard but it was flashing while creating alert I recreated it but same thing happen.

When I click on button it does flash rectangular region at top left corner of the screen and then load Alert.

Here is code :

public class MyAlert extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("FXMLDocument.fxml"));

        Scene scene = new Scene(root);

        stage.setScene(scene);
        stage.show();
    }

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

}  

FXML.fxml:

<AnchorPane id="AnchorPane" prefHeight="200" prefWidth="320" xmlns:fx="http://javafx.com/fxml/1" fx:controller="myalert.FXMLDocumentController">
    <children>
        <Button layoutX="126" layoutY="90" text="Click Me!" onAction="#handleButtonAction" fx:id="button" />
    </children>
</AnchorPane>  

ControllerClass:

public class FXMLDocumentController implements Initializable {

    @FXML
    private void handleButtonAction(ActionEvent event) {
        Alert alert = new Alert(Alert.AlertType.ERROR);
        alert.setTitle("Message");
        alert.setHeaderText("Error Message");
        alert.show();
    }  

I'm using JDK 1.8.0 u131
With old processor

• Processor: 2* Pentium@ Dual-Core CPU E5300 @ 2.60GH

• Memory: 1.9 GiB of RAM

• OS Kubuntu 18.04

Don't know if this is case.

Answer 1

I've checked on windows 7 if getting same behavior, but application run smoothly without flash as I've mentioned in my question.

I'm using Kubuntu 18.04 and Desktop Effects are enabled and day ago Compositor was failed and that was the reason of flashing screen when launching Frame or Alert dialog.

The Compositor can be found in the System Settings under Display and Monitor > Compositor.

I re-enabled and problem is solved.

This question is helpful only for KDE Plasma Desktop users.