right clicking doesn't open a custom context menu

Steps to reproduce

1. load attached zul
2. right click a cell
   open a custom context menu
3. right click another cell

actual result

the custom context menu just disappears and do not open again

expected result

open the custom context menu again after it disappears.
When you right click the "testing area", each right click open the context menu after it disappears.

debug info

1. sometimes the issue doesn't happen, try to left click anywhere on the page and reproduce again
2. even we open a context menu manually in Java (like the code below), the issue still happens.

   	@Listen("onCellRightClick = #ss")
   	public void doContext(CellMouseEvent event) {
   		myContext.open(event.getClientx(), event.getClienty());
   	}
   

workaround

		<script><![CDATA[
		//workaround
		jq(document).mousedown(function(event){
			if (event.which == 3){ //right click
				zk.Widget.$('$ss').focus();
			}
		});
		]]></script>