2056714 - Česká televize captions do not display in the Picture-in-Picture window

Status: RESOLVED FIXED

(Toolkit :: Picture-in-Picture, defect)

Description

[Lukáš Lipinský]

Found in:

• Firefox Nightly 155.0a1 (Build ID 20260721185201; local mozilla-central build)
• Linux x86_64, X11/Xfce
• Tested from the Czech Republic

Steps to reproduce:

1. Open https://www.ceskatelevize.cz/zive/ct1/ while a programme offering captions is playing. Stream availability may be region-dependent.
2. Start playback.
3. Enable captions in the České televize player and verify that they appear in the source page.
4. Open the video in Firefox Picture-in-Picture.

Actual result:
The source player continues to render captions over the original video's placeholder, but the PiP window contains no captions. The caption control in the PiP window cannot make them appear.

Expected result:
Captions enabled in the source player are rendered in the PiP window and can be shown or hidden using the PiP caption control.

Technical diagnosis:
The video is hosted in an embedded https://player.ceskatelevize.cz/... document. It provides WebVTT text tracks, but the selected caption track uses TextTrack.mode == "hidden" because the site renders activeCues through its own overlay. PictureInPictureChild normally selects only tracks whose mode is "showing"; hidden tracks require a site-specific showHiddenTextTracks override.

Runtime validation:
Adding the following entry to PictureInPicture:SiteOverrides and reopening PiP made the same captions render directly in the PiP window:

"https://player.ceskatelevize.cz/*": {
showHiddenTextTracks: true,
}

Without that override the problem is reproducible. This is analogous to bug 1818054 (Vimeo) and uses the site-specific mechanism introduced in bug 1754549.

A data-only patch is ready for browser/extensions/pictureinpicture/data/picture_in_picture_overrides.js. It passes git diff --check, the targeted ESLint check, and the PiP text-track browser test (44/44).

Comment 1

[Lukáš Lipinský]

Attachment: Bug 2056714 - Display Česká televize captions in Picture-in-Picture. r=mconley

The Česká televize player keeps its active WebVTT track in hidden mode and renders captions in its own page overlay. Picture-in-Picture therefore does not select the track.

Treat hidden tracks as showing for the player iframe origin, as already done for TED and Vimeo.

Comment 2

[Lukáš Lipinský]

Additional reproduction: https://ct24.ceskatelevize.cz/video/zive-vysilani uses the same hidden active Czech WebVTT track, but its video is owned directly by the ct24.ceskatelevize.cz top-level document rather than the player.ceskatelevize.cz iframe. D313325 has been updated to add https://ct24.ceskatelevize.cz/* to the same site override. End-to-end testing in a clean Firefox profile confirmed that the pattern matches and the Czech cue is rendered inside the actual PiP window while the source track remains hidden.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/75eb9e887d90
https://hg.mozilla.org/integration/autoland/rev/31b81ee7f92a
Display Česká televize captions in Picture-in-Picture. r=mconley

Comment 4

[pstanciu]

https://hg.mozilla.org/mozilla-central/rev/31b81ee7f92a