After scroll to bottom, then scroll to top, the scroll will move to middle after each sort (or invalidate())

<zk>
<zscript><![CDATA[
Object[] o = new Object[30];
]]></zscript>
<button onClick="lb.invalidate();" label="invalidate" />

<listbox id="lb" width="100px" height="300px">
<listitem label="item" forEach="${o}" />
</listbox>

</zk>