North hides when i refresh databind

package j1ekeig4$v2;
public class Item {
	private Integer id;
	private String descricao;
	public Item(Integer id, String descricao) {
		super();
		this.id = id;
		this.descricao = descricao;
	}
	public Item() {
	}
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getDescricao() {
		return descricao;
	}
	public void setDescricao(String descricao) {
		this.descricao = descricao;
	}
}

package j1ekeig4$v2;
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import org.zkoss.bind.AnnotateBinder;
import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.NotifyChange;
import org.zkoss.zk.ui.Component;
import org.zkoss.zk.ui.util.GenericForwardComposer;
import org.zkoss.zul.Tab;
public class TestComposer extends GenericForwardComposer<Component> {
	/**
	 * 
	 */
	private static final long serialVersionUID = 7548168648624725680L;
	private List<Item> items=new ArrayList<Item>();
	private Item selectedItem;
	private Tab tabItems;
 
	public void doAfterCompose(Component comp) throws Exception {
		super.doAfterCompose(comp);
 
		AnnotateBinder binder=new AnnotateBinder();
 
		getItems().add(new Item(1, "Miguel"));
		getItems().add(new Item(2, "Nelson"));
 
		comp.setAttribute("vm", this);
 
		binder.init(comp, this);
 
		binder.loadComponent(comp, true);
	}
 
	@NotifyChange("items")
	@Command
	public void showItems() {
		tabItems.setSelected(true);
 
		getItems().add(new Item(getItems().size()+1, UUID.randomUUID().toString()));
	}
	public List<Item> getItems() {
		return items;
	}
	public void setItems(List<Item> items) {
		this.items = items;
	}
	public Item getSelectedItem() {
		return selectedItem;
	}
	public void setSelectedItem(Item selectedItem) {
		this.selectedItem = selectedItem;
	}
}

<zk>
	<window title="Hello World!!" border="normal" width="800px"
		height="500px" apply="j1ekeig4$v2.TestComposer">
		<tabbox hflex="1" vflex="1">
			<tabs>
				<tab label="tab 1" />
				<tab label="Items" id="tabItems" />
			</tabs>
			<tabpanels>
				<tabpanel>
					<button label="Show Items"
						onClick="@command('showItems')" />
				</tabpanel>
				<tabpanel>
					<borderlayout width="100%" height="100%">
						<north size="30px" flex="true" splittable="true" minsize="30" maxsize="30" >
							<combobox />
						</north>
						<center autoscroll="true">
							<listbox model="@load(vm.items)">
								<listhead>
									<listheader label="id" />
									<listheader label="descricao" />
								</listhead>
								<template name="model" var="item">
									<listitem value="@load(item)">
										<listcell
											label="@load(item.id)" />
										<listcell
											label="@load(item.descricao)" />
									</listitem>
								</template>
							</listbox>
						</center>
					</borderlayout>
				</tabpanel>
			</tabpanels>
		</tabbox>
	</window>
</zk>