title: Constantly resizing .webm file
body: <p>I've been messing with webm file headers (messing with the header data for file length etc.) and in the process I stumbled upon a webm that behaves weirdly. I reuploaded it <a href="https://www.webmshare.com/play/mK53Y" rel="noreferrer">here on webmshare</a>. It constantly resizes itself which makes it hard to impossible to close/pause it. The same behavior can be observed with vlc media player and mpv player. It still does this on discord, but the player itself doesn't rescale, just the image does.</p>
  <p>After some digging I found the apparent source of the .webm, a youtube video by the creator <a href="https://youtu.be/X7_4voIvKAM" rel="noreferrer">youtube link here</a>.</p>
  <p>This leads to my suspicion and explains the relatively large filesize of the original webm:</p>
  <p>Apparently a command to change the webm size/aspect ratio is given between every frame of the webm. The original creator seems to add this programmatically between frames.</p>
  <p>Now to my question:</p>
  <p>How does this happen? What information is being added to change the aspect ratio while playing? Where is it added and how? Is it a HEX value edit or something else?</p>
  <p>Any hints towards where to look or explanations are much appreciated!</p>