GM.xmlHttpRequest doesn't support onprogress event

[ccloli]

// ==UserScript==
// @name        GM.xmlHttpRequest
// @version     1.0
// @description GM.xmlHttpRequest
// @match       <all_urls>
// @include     *
// @connect     *
// @grant       GM_xmlhttpRequest
// @grant       GM.xmlHttpRequest
// ==/UserScript==

var GM_xmlhttpRequest = GM_xmlhttpRequest || GM.xmlHttpRequest;

GM_xmlhttpRequest({
  method: 'GET',
  url: 'http://localhost/100MB.bin',
  responseType: 'arraybuffer',
  onprogress: res => console.log(res),
  onload: res => console.log('loaded', res.response.byteLength, res.responseHeaders),
  onerror: () => console.log('error')
});
console.log('request started');

The log when using GreaseMonkey (onprogress event is not working):

The log when using Tampermonkey (expected, despite the annoying ArrayBuffer converter):

[arantius]

This is in build 4.1beta5 ( https://addons.mozilla.org/firefox/downloads/file/793791/greasemonkey-4.1beta5-an+fx.xpi?src=devhub ). Testing and confirming would be appreciated!