treeitem的visible bug？

<?xml version="1.0" encoding="UTF-8"?>
<?page zscriptLanguage="GroovyGrails"?>

<zk xmlns="http://www.zkoss.org/2005/zul"
    xmlns:h="http://www.w3.org/1999/xhtml"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.zkoss.org/2005/zul http://www.zkoss.org/2005/zul/zul.xsd">

  <window>

    <button label="button1">
      <attribute name="onClick">
        assert tree.treechildren.items.size() == 4
        tree.treechildren.items*.visible = false
        assert tree.treechildren.items.findAll{it.visible}.size() == 0
      </attribute>
    </button>

    <tree id="tree" rows="12">
      <treechildren>
        <treeitem>
          <treerow>
            <treecell>
              <label value="item1"/>
            </treecell>
          </treerow>
          <treechildren>
            <treeitem>
              <treerow>
                <treecell>
                  <label value="item2"/>
                </treecell>
              </treerow>
            </treeitem>
            <treeitem>
              <treerow>
                <treecell>
                  <label value="item3"/>
                </treecell>
              </treerow>
            </treeitem>
          </treechildren>
        </treeitem>
        <treeitem>
          <treerow>
            <treecell>
              <label value="item4"/>
            </treecell>
          </treerow>
        </treeitem>
      </treechildren>
    </tree>
  </window>
</zk>