The "rules for updating the text track rendering" are not run when mode changes from hidden to showing

[domenic]

Porting from https://www.w3.org/Bugs/Public/show_bug.cgi?id=28236, where @foolip says:

The "time marches on" algorithm runs the "rules for updating the text track rendering" only for "affected tracks", i.e. tracks where the the active cues have changed. This misses cues in tracks that have gone from hidden to showing. Per spec, those cues would only become visible when another cue in the same track changes its active state.

This has been worked around in Blink by also running the "rules for updating the text track rendering" when a track goes from hidden to showing. It could possibly be spec'd as part of the "time marches on" algorithm, but that would only work because the idea is that this algorithm is run all the time, where in an implementation you would only run it when you have reason to think that any cues have changed their active state.

Perhaps @foolip would like to work on a PR/tests? :)

[foolip]

I found where the code in question is at least:
https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/html/media/html_media_element.cc?l=3817&rcl=54f759642aa8824f98f73889161efeb9c9a14218

[foolip]

The comment was added by me in https://chromium.googlesource.com/chromium/src/+/f252a000237623fb2acb3aa2c0eb9d638a619680 and the code itself traces back into WebKit through a bunch of changes, but probably originally by @eric-carlson, and I'd bet Safari also still updates text track rendering when the mode changes.

@cpearce @jyavenard do you know who knows this code in Gecko?

At the end of the day, I suspect all that needs to happen here is a small spec change. I'm gonna call it good first bug and hope :)

[jyavenard]

@alastor0325 is the one currently maintaining this code in gecko

[alastor0325]

We (gecko) do a similar thing to call rules for updating the text track rendering here when the text track changes between showing and hidden.