MP4 Files Unplayable on Windows

There's something non-standard about the way the MP4 files are saved. At least on Windows, I cannot play them without first re-encoding them in Handbrake.

Reproduce the MP4 playback failure on Windows and compare the saved file with one that plays without HandBrake re-encoding. The issue names no implementation file or test, so first locate the MP4-saving entry point and identify the output-format difference. Done means generated MP4 files play directly on Windows and the behavior has a regression check.

Written by the indexing model from the issue text.