Parallel wvtt subtitle cues are not rendered

[tobbee]

• Link to playable MPD file: https://192-46-234-23.ip.linodeusercontent.com/livesim2/timesubswvtt_se/timesubsreg_2/testpic_2s/Manifest.mpd
• Dash.js version: v4.7.4
• Browser name/version: Chrome 123.0.6312.107
• OS name/version: macOS Sonoma 14.4.1

Steps to reproduce

1. Play the link from test instance of Livesim2

Observed behavior

The player is showing one left-aligned subtitle on line2, but not the other right-aligned subtitle on cue 10.

Expected behavior

The player should always show two cues with the same text. A top left-aligned one on line2, and a lower right-aligned one on line 10.

These are in two parallel "vttc" boxes in the same mp4 wvtt sample. I ran into this problem when implementing
styled subtitles in Shaka-packager. Only one line was visible in wvtt. although they are both in the sample.
The same styling for stpp (TTML) worked (that output is not in livesim2 yet, though).

The subtitle cues content of a wvtt segment can be listed using the mp4ff-subslister tool at https://github.com/Eyevinn/mp4ff/tree/master/cmd/mp4ff-subslister
Lacking support for multiple simultaneous cues was recently reported there as well, but has now been added.

[tobbee]

I just made a simple VOD test asset with a side-loaded WebVTT to see if that worked better, but it did not.

It always have two simultaneous cues like:

00:00:00.000 --> 00:00:00.900 line:2 align:left
line 2 left: 00:00:00

00:00:00.000 --> 00:00:00.900 line:10 align:right
line 10 right: 00:00:00

and should be displayed with an upper left-aligned text followed by a lower right-aligned time.

Only the upper one is shown.

From a WebVTT point of view. the two alignments need to be different cues.

The VoD test asset is:
https://192-46-234-23.ip.linodeusercontent.com/vod/testpic_2s/webvtt_subs.mpd

I can also make a VoD asset for the wvtt case, if that is desired for easier development/testing?