MP4 TTML segments with multiple samples are not correctly handled

[julijane]

Have you read the FAQ and checked for duplicate open issues?
yes

If the problem is related to FairPlay, have you read the tutorial?

n/a

What version of Shaka Player are you using?

v4.13.2

Can you reproduce the issue with our latest release version?
yes

Can you reproduce the issue with the latest code from main?
yes

Are you using the demo app or your own custom app?
custom app

If custom app, can you reproduce the issue using our demo app?
not tried, but as the issue is clearly not implemented handling logic, there is no need to

What browser and OS are you using?
Chrome on Linux, not that it matters.

For embedded devices (smart TVs, etc.), what model and firmware version are you using?
n/a

What are the manifest and license server URIs?

n/a

What configuration are you using? What is the output of player.getNonDefaultConfiguration()?

n/a

What did you do?

Attempted to playback a DASH presentation which has a caption track where the segments have multiple samples. This is allowed as per ISO14496-12 and ISO23000-19. gpac creates such segments.

What did you expect to happen?
Subtitles to be displayed.

What actually happened?

Errors in the console about not being able to parse the TTML XML.

Are you planning to send a PR to fix it?
yes, the fix is already implemented.