http://www.zkoss.org/forum/listComment/17353/1/20
try following case
1.select a listitem,
2.shift-select another item, you will see the selection is now incorrect.

<zk>
<zscript>
ListModel strset = new org.zkoss.zksandbox.grid.FakeListModel(20000);
</zscript>
<listbox id="list" width="200px" rows="10" model="${strset}" onSelect="list.invalidate()" multiple="true">
<listhead>
<listheader label="Load on Demend" sort="auto"/>
</listhead>
</listbox>
<button id="btn" label="focus"/>
</zk>