Applying an offset to VTT captions

[riksagar]

I have a vtt caption file, and the timestamps all match the timestamps of the audio/video representations.
But now I'm using the video/audio in a multiperiod manifest, so the audio/video has a @start=... set to about 5 minutes (example: <Period id="15" start="PT5M40.773S" duration="PT1H30M59.036S"> ... </Period>)

The vtt parser in dash-js doesn't seem to be taking the offset of the <Period ... /> into account. When dash-js plays the period, the rendered captions are from 5:40 into the VTT file. I expected the captions would get adjusted. Similar to how the timeline for the video is adjusted.

Is there a recommended way to adjust the timing of a VTT text track when used in multiperiod? (without re-generating the VTT)?

Looking at the code: dash-js is creating Cue objects (here) when the caption text is added. The function that adds the caption text (here) has an option to pass in a timeOffset, but it's hard coded to zero. Seems the timeOffset is only used for Microsoft Smooth support (here).

Thanks in advance for your advice.
Rik.