[Firefox] GM_xmlhttpRequest/GM.xmlHttpRequest regression in v2.12.12+

[chocolateboy]

What is the problem?

GM_xmlhttpRequest and GM.xmlHttpRequest don't work in some cases in Violentmonkey >= v2.12.12, but work in v2.12.11 and v2.12.10.

How to reproduce it?

1. Install the script below
2. Run it on an IMDb movie page (e.g.) with no other scripts running on the page
3. On the legacy/current site, an alert should appear showing details of the response
4. On the new (React) site, the request is not executed and the alert is not shown

What is the expected result?

alert("response: 200 OK (1256 bytes)")

What is the actual result?

No alert. The request is not initiated; no progress, state-change, error or load events are fired; no error is logged.

Works in

• Violentmonkey v2.12.11
• Tampermonkey 4.12.6132
• Greasemonkey 4.11 (just GM.xmlHttpRequest)

Doesn't work in

• Violentmonkey 2.12.12 - 2.12.14

Script

// ==UserScript==
// @name          XHR Test
// @description   Test XHR regression in Violentmonkey on the new IMDb site
// @version       0.0.1
// @namespace     test
// @include       https://www.imdb.tld/title/*
// @grant         GM.xmlHttpRequest
// @noframes
// ==/UserScript==

const request = {
    method: 'GET',
    url: 'https://www.example.com/',
    onerror: console.error,
    onload (res) {
        const { length } = res.responseText
        alert(`response: ${res.status} ${res.statusText} (${length} bytes)`)
    },
}

GM.xmlHttpRequest(request)

Environment

• Browser: Firefox 87.0
• Violentmonkey: 2.12.14
• OS: Linux (Arch)

[gera2ld]

Introduced by #1235 .

In Firefox with released version of Violentmonkey, the script is injected even before the page loading event is triggered, as a result, the initiated requests are immediately canceled when loading event is triggered.