Greasemonkey-like JavaScript injection does not work with HTML documents loaded from .qrc resources

Background

Since Qt 5.8, QtWebEngine supports injecting scripts to HTML documents in a Greasemonkey way, for example:

// ==UserScript==
// @include http://*.qt.io/*
// @exclude http://wiki.qt.io/*
// ==/UserScript==
//
// alert("a script was injected to some pages")

Problem description

However, injection does not work with pages loaded from .qrc resources. For example, given QWebEngine instance loads an HTML document from qrc:/res/page.html, following script will not be injected, despite of variety of @include and @match directives:

// ==UserScript==
// @match qrc:///res/page.html*
// @match qrc://res/page.html*
// @match qrc:/res/page.html*
// @match *
// @include qrc:///res/page.html*
// @include qrc://res/page.html*
// @include qrc:/res/page.html*
// @include *
// @run-at document-end
// ==/UserScript==

alert("it works")

It appears that this way of injection does not work with qrc URL scheme at all.

Original Greasemonkey also whitelists only certain URL shemes by default, however this is configurable: https://wiki.greasespot.net/Include_and_exclude_rules#Greaseable_schemes.

Expected resolution

1. At minimum, docs should be clear about this.
2. A better solution is to allow qrc URL scheme in @include and @match directives.
3. Perhaps the best soution is to make configurable which URL schemes are supported.

Existing workarounds

Currently, the easiest workarounds are:

1. Inject scripts with C++ API. However it adds maintenance burden, especially if some scripts are loaded in a Greasemonkey way, whereas others via C++ API.
2. Inline scripts in HTML document. However, this makes maintenance more difficult, and prevents script from being reused.
3. Remove ==UserScript== header, and filter pages in JavaScript by constructs like if (location === ...). This is also harder to maintain, and may come with some performance penalty.