[Mouse] getX() and getY() for MouseEvent may return -1

Compile and run next sample:

/*
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package javafxapplication9;

import java.util.List;
import javafx.application.Application;
import javafx.css.CssMetaData;
import javafx.css.Styleable;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.CornerRadii;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

/**
 *
 * @author Ladislav Török
 */
public class JavaFXApplication9 extends Application {

    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnMouseMoved((EventHandler<? super MouseEvent>) event -> {
            System.out.println("gteX(), getY(): " + event.getX() + ", " + event.getY());
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);

        Scene scene = new Scene(root, 300, 250);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }

}

Move mouse cursor over button, set cursor to top left corner of button.