<zk>
    <tabbox xmlns:w="client">
<!--     render tab2 from start, to avoid having to select tab2 before checking visible. -->
    <custom-attributes org.zkoss.zul.client.rod="false"/>
        <tabs>
            <tab label="Tab 1" />
            <tab label="Tab 2" />
        </tabs>
        <tabpanels>
            <tabpanel><button w:onClick="alert('Textbox isRealVisible returns '+zk('$tx1').$().isRealVisible())"/></tabpanel>
            <tabpanel><textbox id="tx1" /></tabpanel>
        </tabpanels>
    </tabbox>
</zk>