Examine source code of showWindowSize

{
  "manifest_version": 2,
  "name": "showWindowSize",
  "description": "Utility for displaying browser window size",
  "version": "1.0.0",
  "permissions": [
    "tabs"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{15b25e21-b72a-43db-9106-82bbc989a0d6}"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "js/pd-size.js"
      ],
      "css": [
        "css/pd-view.css"
      ]
    }
  ]
}