为什么带有declarativeNetRequest的modifyHeaders只能在iframe中工作?

我正在编写一个Chrome扩展,它使用declarativeNetRequest API修改user-agent头。有趣的是,用户代理只针对iframe中的请求进行修改,如devtools中所示。我一定错过了什么/误解了什么。

manifest.json

{
    "name": "My extension",
    "version": "1.0.0",
    "manifest_version": 3,
    "description": "My extension",
    "homepage_url": "http://example.com",
    "content_scripts": [
     {
         "matches": ["http://*/*", "https://*/*"],
         "css": ["style.css"],
         "js": ["script.js"]
     }
    ],
    "declarative_net_request": {
    "rule_resources": [{
      "id": "modify-user-agent",
      "enabled": true,
      "path": "rules.json"
    }]
  },
    "icons": {
        "16": "icons/icon16.png",
        "48": "icons/icon48.png",
        "128": "icons/icon128.png"
    },
    "web_accessible_resources": [{
        "resources": [
            "redirect.html"
        ],
        "matches": [
            "*://*/*"
        ]
    }],
    "action":
    {
        "default_icon": "icons/icon48.png",
        "default_popup": "./src/index.html"
    },
    "default_locale": "en",
    "permissions": [
        "contentSettings",
        "tabs",
        "storage",
        "declarativeNetRequest",
        "declarativeNetRequestWithHostAccess"
    ],
    "host_permissions": [
    "*://*/*"
  ]
}

rules.json

[{
    "id": 1,
    "priority": 1,
    "action": {
        "type": "modifyHeaders",
        "requestHeaders": [{
            "header": "User-Agent",
            "operation": "set",
            "value": "My User Agent"
      }]
    },
    "condition": {
        "urlFilter": "*://*/*"
    }
}]

更新:我修改了规则,并将resourceType包含在条件中:

"resourceTypes": ["main_frame", "sub_frame"]

这在主框架和iframe中都有效。但我仍然不明白为什么默认情况下它只适用于iframe。如果没有指定,我找不到任何关于resourceTypes默认值的文档。