894556 - Multiple video tracks in a MediaStream are not reflected on the videoTracks object on the video element

Paul Kinlan
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The first issue I have found trying to build a video editor on the web.

I have multiple video streams (desktop and web cam) and I wanted to be able to toggle between the video streams on one video element so that I can quickly switch between the web cam and the desktop and not break the MediaRecorder.

It looks like you should be able to do it via toggling the selected property on the videoTracks object on the <video> element, but you can’t, the array of tracks contains only 1 element (the first video track on the MediaStream).

What steps will reproduce the problem? (1) Get two MediaStreams with video tracks (2) Add them to a new MediaStream and attach as srcObject on a videoElement (3) Check the videoElement.videoTracks object and see there is only one track

Demo at https://multiple-tracks-bug.glitch.me/

What is the expected result? I would expect videoElement.videoTracks to have two elements.

What happens instead? It only has the first videoTrack that was added to the MediaStream.

Read full post.

Repro case.

window.onload = () => {
  if('getDisplayMedia' in navigator) warning.style.display = 'none';

  let blobs;
  let blob;
  let rec;
  let stream;
  let webcamStream;
  let desktopStream;

  captureBtn.onclick = async () => {

       
    desktopStream = await navigator.getDisplayMedia({video:true});
    webcamStream = await navigator.mediaDevices.getUserMedia({video: { height: 1080, width: 1920 }, audio: true});
    
    // Always 
    let tracks = [...desktopStream.getTracks(), ... webcamStream.getTracks()]
    console.log('Tracks to add to stream', tracks);
    stream = new MediaStream(tracks);
    
    console.log('Tracks on stream', stream.getTracks());
    
    videoElement.srcObject = stream;
    
    console.log('Tracks on video element that has stream', videoElement.videoTracks)
    
    // I would expect the length to be 2 and not 1
  };

};

Paul Kinlan

Trying to make the web and developers better.

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