steps to reproduce

1. open chrome to simulate iPad with dev tool
2. run the code below:

   <zk>
   <button label="Download " onClick="download()">
   </button>
   <zscript><![CDATA[
           import org.apache.poi.util.*;
           public void download() throws java.io.IOException{
           	org.zkoss.zul.Filedownload.save(IOUtils.toByteArray(support.issue.IssueVM.class.getResourceAsStream("test.pdf")), "application/pdf", "test.pdf");
           }
   ]]>
   </zscript>
   </zk>
   You can replace the file with any file path
   

3. click the button to download a file

result

root cause

f (zk.mobile) {
	zAu.cmd0.download = function (url) {
		if (url) {
			var link = document.createElement('a'),
				evt = document.createEvent('Event');
			link.id = 'zk_download';
			link.href = url;
			if (!(zk.ios >= 7)) 
				link.target = '_blank';
			evt.initEvent('click', true, false);
			link.dispatchEvent(evt);
		}
	};
}

firing a click event on an anchor element will trigger onbeforeunload event, so client engine sends remove desktop au. Then the target resource becomes unavailable.

debug info

• this issue doesn't happen in 7.x
• a user reported chrome and safari on iPad2 (iOS 9.2.1) also has this issue. (But I cannot reproduce this with physical iPad)
• He said if he visits the link generated by the AU response e.g. http://localhost:8080/zk8support/zkau/view/z_t59/dwnmed-0/ck7/test.pdf, he gets a 410 HTTP error on both iOS and Android. (But I cannot reproduce this issue)
• Android phones don't have such issue in the example above.

workaround

<script defer="true"><![CDATA[
zAu.cmd0.download = function (url) {
 if (url) {
  var ifr = jq('#zk_download')[0];
  if (ifr) {
   ifr.src = url; 
  } else {
   var html = '<iframe src="' + url
   + '" id="zk_download" name="zk_download" style="display:none;width:0;height:0;border:0" frameborder="0"></iframe>';
   jq(document.body).append(html);
  }
 }
};
]]>
</script>