Tree scroll position didn't clear when setting new model and then invalidate

<zk>
    <window border="normal" title="hello">
<zscript><![CDATA[
DefaultTreeNode root = new DefaultTreeNode("root", new ArrayList());
for (int i = 0; i < 99; i++) {
    root.add(new DefaultTreeNode("item " + i));
}
DefaultTreeModel model = new DefaultTreeModel(root);
void invalidate() {
    root.getChildren().clear();
    root.getChildren().add(new DefaultTreeNode("New item"));
    tree.setModel(new DefaultTreeModel(root));
    tree.invalidate();
}
]]></zscript>
        <button id="btn" label="Switch to smaller model" onClick="invalidate()" />
        <vlayout height="100px" width="200px">
            <tree id="tree" model="${model}" vflex="1">
                <treecols>
                    <treecol label="Treecol" />
                </treecols>
            </tree>
        </vlayout>
    </window>
</zk>