Solve user agent camera/microphone double-mute

[jan-ivar]

User agent mute-toggles for camera & mic can be useful, yielding enhanced privacy (no need to trust site), and quick access (a sneeze coming on, or a family member walking into frame?)

• Safari has pause/resume in its URL bar
• Firefox has global cam/mic mute toggles behind a pref (set privacy.webrtc.globalMuteToggles in about:config)
• Chrome has recently opened an issue discussing it.

It's behind a pref in Firefox because:

1. The double-mute problem: site-mute + ua-mute = 4 states, where 3 produce no sound ("Can you hear me now?")
2. UA-mute of microphone interferes with "Are you talking?" features
3. Some sites (Meet) stop camera to work around crbug 642785 in Chrome, so there's no video track to UA-mute

This image is titled: "Am I muted?"

This issue is only about (1) the double-mute problem.

We determined we can only solve the double-mute problem by involving the site, which requires standardization.

The idea is:

1. If the UA mutes or unmutes, the site should update its button to match.
2. If the user unmutes using the site's button, the UA should unmute(!)

The first point requires no spec change: sites can listen to the mute and unmute events on the track (but they don't).

The second point is key: if the user sees the site's button turn to "muted", they'll expect to be able to click it to unmute.

This is where it gets tricky, because we don't want to allow sites to unmute themselves at will, as this defeats any privacy benefits.

The proposal here is:

partial interface MediaStreamTrack {
  undefined unmute();
}

It would throw InvalidStateError unless it has transient activation, is fully active, and has focus. User agents may also throw NotAllowedError for any reason, but if they don't then they must unmute the track (which will fire the unmute event).

This should let user agents that wish to develop UX without the double-mute problem.