Navigator7 refresh in multiple tabs

I created a simple page with a button. Each time you press the button a new label is created. If you open two tabs in you browser and click on button several times everything works as expected. In every tab labels are created independently. If you refresh the first tab nothing changed, all created labels stay in place. But when refreshing second tab, all labels disappear, page is created from the beginning. Is it normal behaviour? And how can it be altered?

public class TestPage extends VerticalLayout {

	private int num = 0;

        public TestPage()  {
		Button button = new Button("add", new ClickListener() {
			public void buttonClick(ClickEvent event) {
				addComponent(new Label(Integer.toString(num)));