To reproduce the bug execute the code snippet (you can find below) with the latest distrib (I used zk*-6.5.1.FL.20121109.jar) and press "Access page" button.

Bug: "Processing..." progress bar not appears as you can see at the attached image.

<zk xmlns="http://www.zkoss.org/2005/zul">
    <div>
        <button label="Access page" onClick="Thread.sleep(10000L);"/>
    </div>
</zk>