ComboBox strange behavior

Hello all !

When i click on comboBox button my GridLayout expand. Is that normal ?

public class LoginForm extends VerticalLayout {
	
	Panel form = new Panel();
	Embedded icon = new Embedded("", new ThemeResource("../imagens/cadeado.jpg"));
	TextField txtUsuario = new TextField("Usuário");
	TextField txtLogin = new TextField("Login");
	ComboBox cbNivelAcesso = new ComboBox("Nível de Acesso");
	Button btnLogin = new Button("OK");
	
	public LoginForm() {
		
		form.setWidth("350px");
		form.setHeight("400px");
		form.addComponent(icon);
		
		GridLayout layout = new GridLayout(3, 5);
		layout.setWidth("100%");
		layout.setSpacing(true);
		form.addComponent(layout);
		layout.setWidth("100%");
		addComponent(form);
		setSizeFull();
		setComponentAlignment(form, Alignment.MIDDLE_CENTER);
		
		layout.addComponent(txtUsuario, 1, 1, 2, 1);
		txtUsuario.setWidth("100%");
		layout.addComponent(txtLogin, 1, 2, 2, 2);
		txtLogin.setWidth("100%");
		layout.addComponent(cbNivelAcesso, 1, 3, 2, 3);
		cbNivelAcesso.setWidth("100%");
		layout.addComponent(btnLogin, 2, 4);
		btnLogin.setWidth("100%");
		
		
	}

}