[Button] size jumps

developers build of 8.0 (less then 8.0b01)

Code:
    @Override
    public void start(Stage stage) throws Exception {
        final Button button = new Button("WWWWWW");
        final ObjectProperty<Font> font = new SimpleObjectProperty<Font>(new Font(20));
        button.setOnAction(new EventHandler<ActionEvent>(){

            @Override
            public void handle(ActionEvent t) {
                button.fontProperty().bind(font);
            }
        });
        Pane pane = new Pane();
        pane.getChildren().addAll(button);
        Scene scene = new Scene(pane, 300, 300);
        stage.setScene(scene);
        stage.show();
    }

Steps : click button.
move cursor out of the button.
move cursor on the button.
move cursor out of the button

Look at the attached movie.