It looks like GStreamer has supported this for a few years: https://gstreamer.freedesktop.org/documentation/isomp4/GstBaseQTMux.html?gi-language=c#GstBaseQTMux:

It looks like GStreamer has supported this for a few years: https://gstreamer.freedesktop.org/documentation/isomp4/GstBaseQTMux.html?gi-language=c#GstBaseQTMux:fragment-mode https://gstreamer.freedesktop.org/documentation/isomp4/GstBa... I always forget about GStreamer but I think I have a perfect application for it. Hopefully it’s easier to use as a library than MediaFoundation or FFMpeg.