hflex fail in combobox while select options

only happens when select option first time
<zk>
<grid hflex="1">
<rows>
<row>
<combobox hflex="1"
onChange='self.setStyle("color:green;");'>
<comboitem label="14068250 (14068250,2,FCY111)" />
</combobox>
</row>
<row>
<combobox hflex="1" />
</row>
</rows>
</grid>
</zk>