mp4 file changes size during playback

[David Ing]

I have an mp4 file which changes size during playback.  But I noticed that
gst-discoverer-1.0 does not report the fact that the size changes.

I have two questions.

   1. In theory, what is the fastest way to scan the file to determine
   where the size changes?
   2. In practice, what is the easiest way (given the current state of
   gstreamer) to scan the file and determine where the size changes?

Here is the file which changes size during playback:

https://drive.google.com/drive/folders/1f4pGNDhzCvsg1xXWj4y9DS3vXkaglOvZ?usp=sharing

Here is the gst-discoverer-1.0 output:

$ *gst-discoverer-1.0 -v screencap.m4v *
Analyzing file:///home/ding/mess/repro/screencap.m4v
Done discovering file:///home/ding/mess/repro/screencap.m4v

Topology:
  container: video/quicktime, variant=(string)iso
    video: video/x-h264, stream-format=(string)avc, alignment=(string)au,
level=(string)4.2, profile=(string)main,
codec_data=(buffer)014d402affe1002b674d402a965402a835f35602d40404050000030001000003003ce060007d0000030177009a9503b4244d4001000468ce3520,
width=(int)996, height=(int)632, pixel-aspect-ratio=(fraction)1/1,
framerate=(fraction)30/1, interlace-mode=(string)progressive,
chroma-format=(string)4:2:0, bit-depth-luma=(uint)8,
bit-depth-chroma=(uint)8, parsed=(boolean)true
      Tags:
        video codec: H.264 / AVC
        bitrate: 309812
        datetime: 2019-11-01T17:40:33Z
        title: n/a
        container format: ISO MP4/M4A

      Codec:
        video/x-h264, stream-format=(string)avc, alignment=(string)au,
level=(string)4.2, profile=(string)main,
codec_data=(buffer)014d402affe1002b674d402a965402a835f35602d40404050000030001000003003ce060007d0000030177009a9503b4244d4001000468ce3520,
width=(int)996, height=(int)632, pixel-aspect-ratio=(fraction)1/1,
framerate=(fraction)30/1, interlace-mode=(string)progressive,
chroma-format=(string)4:2:0, bit-depth-luma=(uint)8,
bit-depth-chroma=(uint)8, parsed=(boolean)true
      Additional info:
        None
      Stream ID:
14c5bd3783191c8af25f30b00a879f8f7bdaa4367c8ecbd954b38e5367507175/001
      Width: 1350
      Height: 846
      Depth: 24
      Frame rate: 30/1
      Pixel aspect ratio: 1/1
      Interlaced: false
      Bitrate: 309812
      Max bitrate: 0

Properties:
  Duration: 0:00:34.861322222
  Seekable: yes
  Live: no
  Tags:
      video codec: H.264 / AVC
      bitrate: 309812
      datetime: 2019-11-01T17:40:33Z
      title: n/a
      container format: ISO MP4/M4A
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