How to use MediaRecorder as MediaSource

As an exercise in learning WebRTC I am trying to to show the local webcam and side by side with a delayed playback of the webcam. In order to achieve this I am trying to pass recorded blobs to a BufferSource and use the corresponding MediaSource as source for a video element.

// the ondataavailable callback for the MediaRecorder
async function handleDataAvailable(event) {
  // console.log("handleDataAvailable", event);
  if (event.data && event.data.size > 0) {
    recordedBlobs.push(event.data);
  }

  if (recordedBlobs.length > 5) {
    if (recordedBlobs.length === 5)
      console.log("buffered enough for delayed playback");
    if (!updatingBuffer) {
      updatingBuffer = true;
      const bufferedBlob = recordedBlobs.shift();
      const bufferedAsArrayBuffer = await bufferedBlob.arrayBuffer();
      if (!sourceBuffer.updating) {
        console.log("appending to buffer");
        sourceBuffer.appendBuffer(bufferedAsArrayBuffer);
      } else {
        console.warn("Buffer still updating... ");
        recordedBlobs.unshift(bufferedBlob);
      }
    }
  }
}

// connecting the media source to the video element
recordedVideo.src = null;
recordedVideo.srcObject = null;
recordedVideo.src = window.URL.createObjectURL(mediaSource);
recordedVideo.controls = true;
try {
  await recordedVideo.play();
} catch (e) {
  console.error(`Play failed: ${e}`);
}

All code: https://jsfiddle.net/43rm7258/1/

When I run this in Chromium 78 I get an NotSupportedError: Failed to load because no supported source was found. from the play element of the video element.

I have no clue what I am doing wrong or how to proceed at this point.

This is about something similar, but does not help me: MediaSource randomly stops video

This example was my starting point: https://webrtc.github.io/samples/src/content/getusermedia/record/

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