listbox with percentage height doesn't have a vertical scrollbar

<zk>
    <zscript><![CDATA[
ListModelList model = new ListModelList(Locale.getAvailableLocales());
    ]]></zscript>
    <div height="400px">
        <listbox model="${model}" height="80%">
            <listhead>
                <listheader label="dual listbox"/>
            </listhead>
            <template name="model">
                <listitem>
                    <listcell label="${each.language}"/>
                </listitem>
            </template>
        </listbox>
    </div>
</zk>