Chrome扩展Manifest v3内容脚本无法修改iframe的UserAgent问题

我正在开发一款Chrome扩展，已发布Manifest v2版本，同时也开发了Manifest v3版本，但两者存在一处功能差异：在特定测试页面中，v2能修改主窗口及iframe的UserAgent，v3仅能修改主窗口的UserAgent。

共用内容脚本 #

var AGScript = `

    var setAgent = function(win)
    {
        try
        {
            Object.defineProperty(win.navigator, "userAgentData", {value:undefined});
            Object.defineProperty(win.navigator, "platform", {value:undefined});
            Object.defineProperty(win.navigator, "vendor", {value:undefined});
            Object.defineProperty(win.navigator, "userAgent", {value:"xxx"});
            Object.defineProperty(win.navigator, "appVersion", {value:"xxx"});
        } catch(e) {}
    };

    document.addEventListener("DOMContentLoaded", function(event) { setAgent(window); });
    document.addEventListener("DOMNodeInserted", function(event) { if (event.target.tagName == "IFRAME") setAgent(event.target.contentWindow); });
    setAgent(window);

`;

var script = document.createElement("script");
script.type = "text/javascript";
script.textContent = AGScript;
document.documentElement.appendChild(script);
script.remove();

Manifest v2配置 #

"content_scripts": [ {
    "all_frames": true,
    "js": [ "content-ag.js" ],
    "match_about_blank": true,
    "matches": [ "http://*/*", "https://*/*", "file:///*" ],
    "run_at": "document_start"
} ]

Manifest v3配置 #

"content_scripts": [ {
    "all_frames": true,
    "js": [ "content-ag.js" ],
    "matches": [ "<all_urls>" ],
    "run_at": "document_start",
    "world": "MAIN"
} ]

测试差异与初步判断 #

• 在javascript测试页面，v2版本可修改主窗口及iframe.contentWindow的UserAgent，但v3版本仅能修改主窗口的UserAgent。尝试调整多种清单配置仍无法解决。
• 在useragent测试页面，两个版本均可修改所有内容（包括iframe），因此判断这是一个时序问题。

更新后的问题 #

按照建议添加了"match_origin_as_fallback": true，并将内容脚本修改为自执行函数形式：

(() => {

    console.log("here!");

    var setAgent = function(win)
    {
        try
        {
            Object.defineProperty(win.navigator, "userAgentData", {value:undefined});
            Object.defineProperty(win.navigator, "platform", {value:undefined});
            Object.defineProperty(win.navigator, "vendor", {value:undefined});
            Object.defineProperty(win.navigator, "userAgent", {value:"xxx"});
            Object.defineProperty(win.navigator, "appVersion", {value:"xxx"});
        } catch(e) {}
    };

    document.addEventListener("DOMContentLoaded", function(event) { setAgent(window); });
    document.addEventListener("DOMNodeInserted", function(event) { if (event.target.tagName == "IFRAME") setAgent(event.target.contentWindow); });
    setAgent(window);

})()

但在useragent测试页面测试时，UserAgent仍未被覆盖，控制台已打印"here!"，我是否哪里操作有误？（我在Manifest v2中测试，理论上不应有差异）

contentWindow拦截器相关问题 #

这段代码导致useragent测试网站报错：Cannot read properties of undefined (reading 'navigator')，报错代码如下：

function getUAs() {
    const frame = document.createElement("iframe");
    frame.setAttribute("sandbox", "allow-same-origin"),
    frame.src = "javascript:false",
    frame.classList.add("hidden"),
    document.body.appendChild(frame);
    const {userAgent: userAgent, appVersion: appVersion} = frame.contentWindow.navigator; // 报错位置
    return frame.remove(),
    [navigator.userAgent, _navigator.userAgent, "Mozilla/" + _navigator.appVersion, headerUA, userAgent, "Mozilla/" + appVersion]
}

当我将const fp = wnd.HTMLIFrameElement.prototype;替换为const fp = wnd.HTMLElement.prototype;时，报错解决，但无法正常覆盖UserAgent。

内容的提问来源于stack exchange，提问作者Escape75