<zk>
<html><![CDATA[
<ol>
<li>Drag the scroll bar several times.</li>
<li>Sometimes the listbox show a empty content.</li>
</ol>
]]></html>
<zscript><![CDATA[
import java.util.*;
import org.zkoss.zul.*;
List list = new ArrayList();
for (int i = 0; i < 200; i++)
list.add("item " + i);
ListModelList model = new ListModelList(list);
]]></zscript>
<hlayout>
<listbox model="${model}" width="300px"
height="250px"/>
</hlayout>
</zk>
using follow script could 100% with given zul reproduce it.
var num = [932,1734,3028,2484,1630,750,25,260,1114,2434,3443,5150,3856];
var k = setInterval(function(){
if(num.length > 0 ){
zk.Widget.$(jq("@listbox")).ebody.scrollTop = num.shift();;
}else{
clearInterval(k);
}
},2000);
Fix since 2011/06/20.