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

js&css/extension/www.youtube.com/appearance/sidebar/sidebar.css

@@ -160,6 +160,17 @@ html[it-thumbnails-hide='true'] ytd-watch-next-secondary-results-renderer ytd-pl
 html[it-livechat='hidden'] ytd-live-chat-frame#chat {
 	  display: none !important;
 }
+
+/* #3664 - In theatre mode YouTube reparents the live chat into
+   #panels-full-bleed-container. Hiding only ytd-live-chat-frame#chat leaves
+   this container reserving its width, so the theatre player stays offset as
+   if the sidebar were still present. Collapse the container when the live
+   chat is the panel it hosts, while leaving any engagement panel the user
+   explicitly opened (transcript, chapters, ...) untouched. */
+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;
+}
+
 html[it-livechat='collapsed'] #chat-container:not([it-activated])::before {
 	content: 'Show live chat' !important;
 }

tests/unit/livechat-theater-offset.test.js

@@ -0,0 +1,31 @@
+// Test for Issue #3664: Theatre mode stays offset when live chat is hidden
+
+const fs = require('fs');
+const path = require('path');
+
+describe('Hidden live chat + theatre offset (#3664)', () => {
+	let sidebarCssContent;
+
+	beforeAll(() => {
+		const filePath = path.join(__dirname, '../../js&css/extension/www.youtube.com/appearance/sidebar/sidebar.css');
+		sidebarCssContent = fs.readFileSync(filePath, 'utf8');
+	});
+
+	test('should still hide the live chat frame itself when livechat is hidden', () => {
+		expect(sidebarCssContent).toContain("html[it-livechat='hidden'] ytd-live-chat-frame#chat");
+	});
+
+	test('should collapse the theatre full-bleed panel container that hosts the hidden chat', () => {
+		expect(sidebarCssContent).toMatch(
+			/html\[it-livechat='hidden'\] ytd-watch-flexy\[theater\] #panels-full-bleed-container/
+		);
+	});
+
+	test('should keep an explicitly opened engagement panel visible in theatre mode', () => {
+		// The rule must exclude an expanded engagement panel so hiding chat does
+		// not also hide a transcript/chapters panel the user opened.
+		expect(sidebarCssContent).toContain(
+			":not(:has(ytd-engagement-panel-section-list-renderer[visibility='ENGAGEMENT_PANEL_VISIBILITY_EXPANDED']))"
+		);
+	});
+});