Combobox in modal window moves viewport

<zk xmlns="http://www.zkoss.org/2005/zul">

	<zscript><![CDATA[
		List filler = new LinkedList();
		
		for (int x=0; x<500; x++) {
			filler.add("line" + x);
		}
	]]></zscript>
	
	<div forEach="${filler}">
		<label value="${each}"/>
	</div>
	 
	<window title="Comboboxtest" width="200px" height="200px">
	
		<attribute name="onCreate">
			self.doModal();
			self.setPosition("center");
		</attribute>
		
		<combobox width="40px" readonly="true" text="item 3">
			<comboitem label="item 1"/>
			<comboitem label="item 2"/>
			<comboitem label="item 3"/>
			<comboitem label="item 4"/>
			<comboitem label="item 5"/>
			<comboitem label="item 6"/>
		</combobox>
		
	</window>
			
</zk>