Steps to reproduce

1. load the zul below

   <zk>
   	<zscript><![CDATA[
   		List items = new ArrayList();
   	    for(int i = 0; i <= 250; i++) {
   	    	items.add("Item " + i);
   	    }
   	    //model.setMultiple(true);
   	]]></zscript>
   
   	<hlayout>
   		<vlayout>
   			//single selection works as expected
   			<listbox width="300px" mold="paging" pageSize="100" height="300px" checkmark="true" pagingPosition="top" multiple="false">
   				<listhead>
   					<listheader label="col"/>
   				</listhead>
   				<listitem forEach="${items}" label="${each}"/>
   			</listbox>
   		</vlayout>
   	</hlayout>
   </zk>
   

   or http://zkfiddle.org/sample/1qejeug/2-scrolling-position

2. select "item 199" at the 2nd page
3. press down key (or page down key)

result