Final video not playable in gradio UI?

The scripts all finish and video creation all works.
The "Finished Frames" has the 5 second seek bar, but no video is shown.
When you right-click and "open in new tab" it complains "Video cannot be played because the file is corrupt."
If I browse to the file in explorer and double-click the mp4 it plays fine in my default media player.

Reproduce the issue in the Gradio UI and compare the Finished Frames video there with the same generated MP4 in a desktop player. Investigate why browser playback reports the file as corrupt, and consider the issue done when the final video displays and plays in the UI.

Written by the indexing model from the issue text.