Maximizing Portlet

Hi everybody,

I'm new to Vaadin framework and facing a strange difficulty. I'm developing a little portlet with eclipse Helios deployed on GateIn. When i start the application everything is fine but when I maximize the portlet, the application content becomes empty !

Here is the code that I use in the init method of the application

setTheme("runo");
		Window mainWindow = new Window("Vaadinproject Application");
		HorizontalLayout hl = new HorizontalLayout();
		Button button1 = new Button("My Button1");
		button1.addListener(this);
		Button button2 = new Button("My Button2");
		button2.addListener(this);
		Button button3 = new Button("My Button3");
		button3.addListener(this);
		Button button4 = new Button("My Button4");
		button4.addListener(this);
		hl.addComponent(button1);
		hl.addComponent(button2);
		hl.addComponent(button3);
		hl.addComponent(button4);
		Label label = new Label("Hello Vaadin USER");
		hl.addComponent(label);
		mainWindow.addComponent(hl);
		setMainWindow(mainWindow);
		getMainWindow().addListener(this);

You can see images before and after maximization. After this maximization Application content becomes always invisible

Anybody has any idea ?

Thanks a lot for your help