New added header is invisible when all headers have fixed width

<zk>
    <listbox>
        <listhead id="listhead">
            <listheader label="1" width="200px"/>
            <listheader label="2" width="200px"/>
        </listhead>
        <listitem id="listitem">
            <listcell label="1"/>
            <listcell label="2"/>
        </listitem>
    </listbox>
    <button label="append">
        <attribute name="onClick">
            listhead.appendChild(new Listheader("3"));
        </attribute>
    </button>
</zk>