Using this ZUL page
<zk>
<div>
<datebox
format="short"
timeZonesReadonly="true"
displayedTimeZones="UTC"
/>
<button label="Test #1"></button>
<button label="Test #2"></button>
</div>
</zk>

generates the site attached as screenshot. Both buttons are inside the
datebox's popup. If I remove the 'displayedTimeZones="UTC"' part everything
works as expected.