fix(livechat): collapse theatre full-bleed chat container when live chat is hidden (#3664)

[yu2971512385-ui]

What this fixes

Fixes #3664 — in theatre mode, when Live chat is set to Hidden, the player stays offset to the left as if the sidebar/chat were still there, leaving an empty column on the right.

Root cause

The existing rule only hides the chat frame itself:

html[it-livechat='hidden'] ytd-live-chat-frame#chat { display: none !important; }

In theatre mode YouTube reparents the live chat into #panels-full-bleed-container. Setting only #chat to display:none removes the chat contents but the surrounding #panels-full-bleed-container keeps reserving its width, so the theatre player never reclaims that space. (This matches the community uBlock workaround ###panels-full-bleed-container mentioned in the issue.)

A repo-wide search confirms #panels-full-bleed-container was not handled anywhere before this change.

The fix

Collapse #panels-full-bleed-container while it is hosting the hidden chat, scoped to theatre mode:

html[it-livechat='hidden'] ytd-watch-flexy[theater] #panels-full-bleed-container:has(ytd-live-chat-frame#chat):not(:has(ytd-engagement-panel-section-list-renderer[visibility='ENGAGEMENT_PANEL_VISIBILITY_EXPANDED'])) {
    display: none !important;
}

• Scoped to ytd-watch-flexy[theater] so the normal (non-theatre) layout is untouched.
• :has(ytd-live-chat-frame#chat) — only act on the container that hosts the chat.
• :not(:has(...EXPANDED)) — if the user explicitly opens an engagement panel (transcript, chapters, …) in theatre mode, the container stays visible so those panels remain usable.

Tests

Added tests/unit/livechat-theater-offset.test.js, mirroring the existing hide-sidebar-transcript.test.js CSS-content style. npx jest passes locally.

How to reproduce / verify

1. Set Live chat → Hidden
2. Open a live stream, switch to theatre mode
3. Before: player is offset with an empty right column. After: player fills the width.
4. Open a transcript/chapters panel in theatre mode → still shows (not affected).

I've verified the rule loads via the manifest content-script CSS and that unit tests pass; a second visual confirmation from anyone on the issue is very welcome.

[yu2971512385-ui]

Tested locally as an unpacked extension on a live stream in theatre mode:

• Before: the player was offset with an empty column on the right.
• After: the player fills the full width.
• Confirmed that an opened transcript panel still shows in theatre mode.

Happy to adjust the selector scope if you'd prefer.