Handling of enabling/displaying multiple text tracks at once

[OwenEdwards]

• Some browsers which have native support for text tracks with HTML5 video allow the display of both captions and subtitles simultaneously
• It's not clear if video.js text track handling allows enabling both subtitles and captions at the same time
• It's not clear if vtt.js supports being passed cues from different text tracks at the same time
• There isn't a standard way to style captions and subtitles so that it is obvious to the user which is which, nor is it clear whether this is necessary.
• Adding support for text descriptions tracks (#2343) ideally requires handling of enabling/displaying multiple text tracks at once,

(This is based on a discussion related to an earlier PR to provide support for text descriptions tracks (#2328))

[heff]

Before the vtt.js switch, subtitles were in white text and captions were in yellow text. I feel like I saw that style somewhere else. It does seem unlikely that you'd need both subtitles and captions, or that it would at least be hard to keep up with reading both.

[OwenEdwards]

Right about displaying both captions and subtitles; since the vtt.js switch I've seen a situation where video.js allows both to be displayed when using native text tracks with HTML (in Chrome, I think), where video.js was handling the controls but not the display. OTOH, when video.js was doing both controls and display (via vtt.js), then it seems to restrict display to one of subtitles or captions, but the controls (which indicate which one is currently selected) don't get updated (e.g. English captions are still highlighted as selected on the menu, even after English subtitles are selected).

So let's have this issue relate to having those behaviors consistent?

Displaying captions/subtitles and descriptions at the same time needs a little more work, since, as you say, the cognitive load of reading both could get too high if they aren't authored well, but in general the use case will be to only enable one at a time. I just need to handle that error case in a clean way.

Note that displaying both captions/subtitles and descriptions at the same time is something that a transcript plugin ought to support; I'm working on that too.

[heff]

but the controls (which indicate which one is currently selected) don't get updated

That's interesting. So either our custom tracks system isn't passing both track's cues, or VTT.js is ignoring one of them. But yeah, it'd be good if it were consistent. When Chrome's display is used do the captions and subtitles overlap or stack?

Video.js Transcript looks great!

[vwkd]

Would be great to support dual subtitles by showing one subtitle at the bottom and a second one at the top.

This is especially useful for language learning to show subtitles for both the source and target language at the same time.