Irregular fmp4 fragment durations with "progressive" setting

What do you want to do with Hls.js?

Hi, given a HLS VOD with fragmented mp4s (6s fragments, 2s chunks) using byterange requests, I'm wondering why the loaded fragments are of different durations when using the fetch-loader vs xhr-loader.

Given this stream: https://hlsjs.video-dev.org/demo/?src=https%3A%2F%2Fd3bao5x7qmqzgm.cloudfront.net%2Fchunked%2Fmaster.m3u8&demoConfig=eyJlbmFibGVTdHJlYW1pbmciOnRydWUsImF1dG9SZWNvdmVyRXJyb3IiOnRydWUsInN0b3BPblN0YWxsIjpmYWxzZSwiZHVtcGZNUDQiOmZhbHNlLCJsZXZlbENhcHBpbmciOi0xLCJsaW1pdE1ldHJpY3MiOi0xfQ==

Example with xhr-loader showing consistent ~6s fragments for audio and video in the Timeline:

{
  "debug": true,
  "enableWorker": true,
  "backBufferLength": 90,
  "progressive": false
}

Example with fetch-loader showing fragments of varying durations in the Timeline:

{
  "debug": true,
  "enableWorker": true,
  "backBufferLength": 90,
  "progressive": true
}

The reason I am interested in using fetch-loader is to reduce startup time by allowing playback as soon as a chunk is downloaded rather than a full fragment. Is there a less experimental way to achieve this that you recommend?

Thank you

What have you tried so far?

• Different chunk durations (500ms, 1s, 2s)
• A custom fetch-loader, but the magic comes from more than just using fetch it is actually appending the chunks as they come which I have not re-implemented

I wanted to try next:

• fmp4 fragments of equal length as the chunk (6s fragments, 6s chunk with single moof+mdat)

Reproduce the stream in the linked Hls.js demo with progressive set to false and true, then compare the fetch-loader and xhr-loader paths and their timeline durations. Start with the loader behavior described in the issue; done means determining whether the varying fMP4 durations are expected or identifying a focused fix or recommendation for chunked playback.