Bugzilla

Very quick diagnosis from poking in GDB...

It looked to me like the track has a mode of `disabled` by default -- and when a track has that mode, it bails from `MaybeDispatchLoadResource()` without actually dispatching a task to load the resource, here:
https://searchfox.org/firefox-main/rev/98bf4b92d3f5d7a9855281df4bf333210bcfbbc4/dom/media/webvtt/HTMLTrackElement.cpp#249,252-253,255-257
```cpp
void HTMLTrackElement::MaybeDispatchLoadResource() {
...
  // step2, if the text track's text track mode is not set to one of hidden or
  // showing, then return.
...
  if (mTrack->Mode() == TextTrackMode::Disabled && !resistFingerprinting) {
    LOG("Do not load resource for disable track");
    return;
```

The **very first track** manages to avoid that problem, because we get a call to `HonorUserPreferencesForTrackSelection` which changes the mode of all disabled tracks here:
https://searchfox.org/firefox-main/rev/98bf4b92d3f5d7a9855281df4bf333210bcfbbc4/dom/media/webvtt/TextTrackManager.cpp#328-331,341-352
```cpp
void TextTrackManager::HonorUserPreferencesForTrackSelection() {
  if (performedTrackSelection || !mTextTracks) {
    return;
  }
...
  // Step 4: Set all TextTracks with a kind of metadata that are disabled
  // to hidden.
  for (uint32_t i = 0; i < mTextTracks->Length(); i++) {
    RefPtr<TextTrack> track = (*mTextTracks)[i];
    if (track->Kind() == TextTrackKind::Metadata && TrackIsDefault(track) &&
        track->Mode() == TextTrackMode::Disabled) {
      track->SetMode(TextTrackMode::Hidden);
    }
  }

  performedTrackSelection = true;
}
```

I think later tracks trigger that same call to `HonorUserPreferencesForTrackSelection`, but they skip the important parts of that function because `performedTrackSelection` is now `true`  which makes them take the early-return at the start of the function.

Very quick diagnosis from poking in GDB...

It looked to me like each `track` element has a `Mode()` of `TextTrackMode::Disabled` by default -- and when a track has that `Disabled` mode, it bails from `MaybeDispatchLoadResource()` **without actually dispatching any task to load the resource**, here:
https://searchfox.org/firefox-main/rev/98bf4b92d3f5d7a9855281df4bf333210bcfbbc4/dom/media/webvtt/HTMLTrackElement.cpp#249,252-253,255-257
```cpp
void HTMLTrackElement::MaybeDispatchLoadResource() {
...
  // step2, if the text track's text track mode is not set to one of hidden or
  // showing, then return.
...
  if (mTrack->Mode() == TextTrackMode::Disabled && !resistFingerprinting) {
    LOG("Do not load resource for disable track");
    return;
```

The **very first track** manages to avoid that problem, because we get a call to `HonorUserPreferencesForTrackSelection` which changes the mode of all disabled tracks here:
https://searchfox.org/firefox-main/rev/98bf4b92d3f5d7a9855281df4bf333210bcfbbc4/dom/media/webvtt/TextTrackManager.cpp#328-331,341-352
```cpp
void TextTrackManager::HonorUserPreferencesForTrackSelection() {
  if (performedTrackSelection || !mTextTracks) {
    return;
  }
...
  // Step 4: Set all TextTracks with a kind of metadata that are disabled
  // to hidden.
  for (uint32_t i = 0; i < mTextTracks->Length(); i++) {
    RefPtr<TextTrack> track = (*mTextTracks)[i];
    if (track->Kind() == TextTrackKind::Metadata && TrackIsDefault(track) &&
        track->Mode() == TextTrackMode::Disabled) {
      track->SetMode(TextTrackMode::Hidden);
    }
  }

  performedTrackSelection = true;
}
```

I think later tracks trigger that same call to `HonorUserPreferencesForTrackSelection`, but they skip the important parts of that function because `performedTrackSelection` is now `true`  which makes them take the early-return at the start of the function.