IE11 file download triggers onBeforeUnload

Steps to Reproduce

Run fiddle in IE11:
http://zkfiddle.org/sample/1qah9ee/2-ie11-iframe-onbeforeunload-download

trigger onBeforeunload by closing the current tab (or using ctrl+W)
click file download button

Current Result

file download triggers onBeforeUnload message ("Are you sure you want to quit")

Expected Result

file download should not trigger this message

Debug Info

In pure html, IE 11 triggers onBeforeUnload event on the main window if an Iframe in the page puts a request and receive a file response with content-header "content-disposition:attachment"
Can reproduce in pure html

Root Cause

IE11 wrongly triggers onBeforeUnload on main window when iframe content is changed

Workaround

• not perfect since if the server replies for longer than 1000ms, download will still trigger onBeforeUnload
• Page can be closed without warning for 1000ms after receiving the download instruction

  <script><![CDATA[
  	zk.afterLoad('zul', function() {
  		var x = {};
  		zk.override(zAu.cmd0, x ,{
  			download : function() {
  				var sbu = zk.skipBfUnload;
  				zk.skipBfUnload = true;
  				var result = x.download.apply(this, arguments);
  				setTimeout(function(){zk.skipBfUnload = sbu;},1000);
  				return result;
  			}
  		});//zk.override
  	});//zk.afterLoad
  	]]></script>