VTT captions not loading/displaying immediately on iPhone, despite being within a visible timeframe

[daniemo2]

What do you want to do with Hls.js?

Opening as a question as I'm not sure if this is a bug or simply something I can configure.

Reproducible on this demo page: https://hlsjs.video-dev.org/demo/?src=https%3A%2F%2Fmtoczko.github.io%2Fhls-test-streams%2Ftest-group%2Fplaylist.m3u8&demoConfig=eyJlbmFibGVTdHJlYW1pbmciOnRydWUsImF1dG9SZWNvdmVyRXJyb3IiOnRydWUsInN0b3BPblN0YWxsIjpmYWxzZSwiZHVtcGZNUDQiOmZhbHNlLCJsZXZlbENhcHBpbmciOi0xLCJsaW1pdE1ldHJpY3MiOi0xfQ==

With the above stream, it has a VTT playlist configured to continuously show a caption of '1080p' for a couple of seconds every second. On Chrome, if I enable captions, it will immediately request the playlist.m3u8 file, the next few .vtt files and then start showing captions.

On iOS, it loads the playlist.m3u8 file however doesn't request the .vtt files until after the next .ts file is requested, which could be a few seconds. Therefore the captions don't show until after a few seconds. Ideally it would mirror the behaviour on Chrome.

Is this something I can configure? A browser restraint? A bug?

Thanks!

What have you tried so far?

No response

[robwalch]

Thanks for reporting this issue. This was a bug in how ManagedMediaSource streaming events were handled. All loading of segments including subtitles was stopped on "endStreaming" (30 seconds in the buffer) and not resumed until "startStreaming" is dispatched (when the buffer is down to 10 seconds or less).

A fix has been released in patch v1.5.18. The workaround would be to call hls.resumeBuffering() changing subtitle tracks.

[daniemo2]

Thanks a lot for the quick turnaround @robwalch! Much appreciated.