onprogress doesn't return response/responseText, even when it's available

[qsniyg]

Expected Behavior

The script below should output "Good" a few times, as responseText should be available if loaded > 0. This is how Violentmonkey behaves.

Actual Behavior

The script outputs "Bad", as responseText isn't available

Specifications

• Chromium: 80.0.3987.163
• TM: 4.9
• OS: Linux

Script

// ==UserScript==
// @name         New Userscript
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://www.google.com/
// @grant        GM_xmlhttpRequest
// ==/UserScript==

(function() {
    'use strict';

    GM_xmlhttpRequest({
        url: "https://www.google.com/logos/doodles/2020/thank-you-public-transportation-workers-6753651837108759-law.gif",
        method: "GET",
        onprogress: function(resp) {
            if (resp.loaded) {
                console.log(resp.responseText ? "Good" : "Bad");
            }
        }
    });
})();

[derjanb]

1278 might help here.