onbeforeunload is cancelling all event

<?xml version="1.0" encoding="utf-8"?>

<zk xmlns="http://www.zkoss.org/2005/zul"
	xmlns:h="http://www.w3.org/1999/xhtml"
	xmlns:n="http://www.zkoss.org/2005/zk/native"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.zkoss.org/2005/zul http://www.zkoss.org/2005/zul/zul.xsd">

	<window width="200px" height="150px" border="normal" title="Test Javascript 3">
        
        <!-- SERVER -->
        <zscript>
        	<![CDATA[
            	private void cleanSession() {
            		alert("clean session...");
            	}
        	]]>
        </zscript>
        
        <!-- JAVASCRIPT -->
        <script type="text/javascript" >
        	<![CDATA[
        	zk.listen(window, "beforeunload", showMsg);
        	
			function showMsg() {
				if (confirm("You are trying exit or refresh.\nDo you want exit?")) {
					var btn1 = document.getElementsByTagName("button")[0];
					comm.sendEvent(btn1, "onClick");
				}
			}
			
			function cleanSession3() {
				var btn1 = document.getElementsByTagName("button")[0];
				comm.sendEvent(btn1, "onClick");
			}
			]]>
		</script>
        
        <button id="btn1" label="BTN 1" onClick="cleanSession()" />
        <separator />
        <button id="btn2" label="BTN 2" action="onclick: cleanSession3()" />	
        
    </window> 

</zk>