Use style like "position:absolute;left:20px;top:8px;" for component(textbox/datebox/combobox and so on) in tabpanel, the top value haven't subtract the height of component 'tab'(tab is on the top).
There is the same problem while this zul file is used with popup. But that's OK while this zul is used with west/center/east and so on.

ZUL File(sample.zul):
<?xml version="1.0" encoding="UTF-8" ?>
<window xmlns="http://www.zkoss.org/2005/zul" 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" xmlns:w="http://www.zkoss.org/2005/zk/client" width="100%" height="100%">
<label value="message" style="position:absolute;left:20px;top:8px;"/>
<textbox value="" style="position:absolute;left:50px;top:8px;" />
</window>