Examine source code of ScreenRuler

{
  "manifest_version": 2,
  "name": "ScreenRuler",
  "version": "1.2.0",
  "description": "On-screen ruler to measure distances and element sizes in pixels",
  "browser_specific_settings": {
    "gecko": {
      "id": "{d65ecd2b-8cb4-431d-8606-35e651e4bfd9}",
      "strict_min_version": "96.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "icons": {
    "16": "pix/x16.png",
    "32": "pix/x32.png",
    "48": "pix/x48.png",
    "96": "pix/x96.png",
    "128": "pix/x128.png"
  },
  "browser_action": {
    "default_popup": "screen/screen.html",
    "default_icon": {
      "16": "pix/x16.png",
      "32": "pix/x32.png",
      "48": "pix/x48.png",
      "96": "pix/x96.png",
      "128": "pix/x128.png"
    }
  },
  "background": {
    "scripts": [
      "agent/tracker.js"
    ]
  },
  "permissions": [
    "tabs",
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "ruler.js"
      ],
      "run_at": "document_idle"
    }
  ]
}