When using the tab key to change focus of component the focus never position on combobutton.

Example:
<zk>
<hbox pack="center" align="top" width="100%" height="400px">
<style src="/widgets/combobox/simple_combobox/css/simple_combo.css" />
<div apply="org.zkoss.bind.BindComposer" height="278px" width="443px"
viewModel="@id('vm') @init('com.teste.comboButton.comboButton.MyViewModel')">
<separator spacing="40px" />
<hbox>
<vlayout>
<separator/>

<textbox value="teste1" focus="true"></textbox>

<label value="Color" />
<combobutton id="cmbColor" width="150px" label="escolhe cor" >
<menupopup children="@load(vm.names)">
<template name="children" var="node">
<menuitem label="${node}" ></menuitem>
</template>
</menupopup>

</combobutton>

<textbox value="test2"></textbox>

</vlayout>

<vlayout>
<separator/>

<textbox value="test3"></textbox>

<label value="Color2" />
<combobutton id="cmbColor2" width="150px" label="color" >
<menupopup children="@load(vm.names)">
<template name="children" var="node2">
<menuitem label="${node2}" ></menuitem>
</template>
</menupopup>

</combobutton>

<textbox value="test4"></textbox>

</vlayout>
<vlayout>
<separator/>

<textbox value="test5"></textbox>

<label value="Color2" />
<combobutton id="cmbColor3" width="150px" label="color:" >

</combobutton>

<textbox value="test6"></textbox>
</vlayout>

</hbox>
</div>
</hbox>
</zk>