Examine source code of Default Container Handler

{
  "manifest_version": 2,
  "name": "Default Container Handler",
  "description": "Opens external links in a default container, with graceful handling when Zen reassigns the tab to a different container.",
  "version": "1.0.1",
  "icons": {
    "48": "icon.svg",
    "96": "icon.svg"
  },
  "content_security_policy": "script-src 'self'; object-src 'none';",
  "browser_specific_settings": {
    "gecko": {
      "id": "default-container-handler@sarcasticbird.com",
      "strict_min_version": "59.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "protocol_handlers": [
    {
      "protocol": "ext+container",
      "name": "Default Container Handler",
      "uriTemplate": "/opener.html#%s"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "contextualIdentities",
    "cookies",
    "tabs",
    "webNavigation"
  ]
}