Progressbox not show with long operation

the following zul works with ZK 6.5.0 but not ZK 6.5.1. tested on Chrome, FF, and IE 9

<zk>
<button label="Long Operation">
<attribute name="onClick">
System.out.println("start long operation");
Thread.sleep(5000);
System.out.println("end long operation");
</attribute>
</button>
</zk>