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M5 Audio Download Handoff

Scope

Implemented an opt-in, explicit audio download action using the existing credentialless ANDROID_VR acquisition path and the privileged Firefox downloads API.

Behavior

  • downloadEnabled defaults off and instantly controls a small in-player download button.
  • A click performs a fresh player request, selects itag 251 before 140, and derives a bounded sanitized .webm or .m4a filename from the video title.
  • MAIN-to-content download data uses nonce-authenticated JSON-string CustomEvent details.
  • Content and background validate the selected URL and filename; production accepts only HTTPS Googlevideo hosts.
  • Background prefers direct downloads.download and uses a credentialless Blob fallback only if direct download fails.
  • Any acquisition, bridge, validation, API, or fallback error reports failure to the control and leaves playback untouched.

Validation

Unit coverage includes filename sanitization, extension mapping, and URL allowlisting. The packaged bench includes download-disabled and download-enabled cases while retaining all earlier milestone cases. See the implementation handoff for exact gate output.