Examine source code of No BS Ruler

{
  "manifest_version": 3,
  "name": "No BS Ruler",
  "version": "1.1",
  "description": "Measure pixels without the usual nonsense. Drag, align, and get exact distances. Device toolbar friendly.",
  "browser_specific_settings": {
    "gecko": {
      "id": "nobstruler@jannator.extension",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "permissions": [
    "activeTab",
    "scripting"
  ],
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "action": {
    "default_popup": "ui/popup.html",
    "default_title": "No BS Ruler",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content/state.js",
        "content/measurements.js",
        "content/overlay.js",
        "content/interaction.js",
        "content/content.js"
      ],
      "run_at": "document_idle"
    }
  ]
}