chromium/chrome/test/data/extensions/api_test/side_panel/extension/manifest.json

{
  "name": "Extension",
  "version": "1.0",
  "manifest_version": 3,
  "permissions": ["sidePanel"],
  "side_panel": {"default_path": "default_path.html"},
  "background": {"service_worker": "service_worker.js"}
}