[Webkit-unassigned] [Bug 212274] New: MediaRecorder stopRecorder() returns empty Blob after first use

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=212274

            Bug ID: 212274
           Summary: MediaRecorder stopRecorder() returns empty Blob after
                    first use
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Macintosh
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Media
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: majo at squadcast.fm

I started working on fixing this bug https://bugs.webkit.org/show_bug.cgi?id=202233 but found weird behaviour with stopRecording(). 
It returns an empty blob when you press stop after you press start, then stop and the start again.

To replicate:

 1. Open Safari
 2. Open the Develop Menu
 3. Hover over the Experimental Features list item
 4. Click on MediaRecorder to Enable it
 5. Navigate to https://codepen.io/majov5/pen/KKdEpKZ
 6. Click on the blue Start Test button at the bottom
 7. Allow access to Microphone & Camera
 8. Click on the Record button at the bottom
 9. Wait a few second and click Stop.

In the console of Codepen will we something like

[object Blob] {
   size: 18201,
   slice: function slice() {...},
   type: "video/mp4"
}

 10. Click on the Record button again.
 11. Wait a few second and click Stop.

Now in the console we have empty Blob

[object Blob] {
   size: 0,
   slice: function slice() {...},
   type: ""
}

Is this supposed to happen?

I have test it in Chrome and Firefox, and I always get a Blob with content.

I have test it in Safari 13.1, Safari Technology Preview (Release 106 (Safari 13.2, WebKit 15610.1.12.2)) and the MiniBrowser and is the same result as explained in the 11 steps.

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