Add SideSheet component, refactor TOC and AIChat to use it

[zenoachtig]

This pull request introduces a new SideSheet component and refactors the TOC and AIChat to use it.

SideSheet Component & Usage Updates:

• Added a new SideSheet component and refactored both the Table of Contents (TOC) and AIChat to use it, improving the user experience for side panels. (.changeset/curly-eels-carry.md [1] AIChat.tsx [2] [3] [4]
• Updated layout components (CustomizationRootLayout, SiteDynamicLayout, SiteStaticLayout) to accept and apply htmlClassName and bodyClassName props, supporting new sheet-related classes. (CustomizationRootLayout.tsx [1] [2] [3]; layout.tsx packages/gitbook/src/app/sites/dynamic/[mode]/[siteURL]/[siteData]/(content)/layout.tsxL26-R27, packages/gitbook/src/app/sites/static/[mode]/[siteURL]/[siteData]/(content)/layout.tsxL22-R26)

UI/UX and Responsive Improvements:

• Improved handling of safe area insets for components like CookiesToast and Header, ensuring proper spacing on devices with notches or rounded corners. (CookiesToast.tsx [1] [2]; Header.tsx [3]
• Added viewportFit: 'cover' to the viewport meta tag for better fullscreen support on mobile devices. (SiteLayout.tsx packages/gitbook/src/components/SiteLayout/SiteLayout.tsxR104)

Table of Contents and Section List Enhancements:

• Updated active prop handling for TOC and section lists to use hash-prefixed IDs, improving navigation and highlighting. (AIChat.tsx [1]; SiteSectionList.tsx [2]; SiteSectionTabs.tsx [3]
• Refined layout and header logic for TOC and section lists in SpaceLayout, improving visibility and responsiveness. (SpaceLayout.tsx [1] [2] [3] [4] [5]

Minor UI and Codebase Cleanups:

• Simplified and cleaned up the mobile menu toggle logic in HeaderMobileMenu, removing unnecessary state and scroll tracking. (HeaderMobileMenu.tsx [1] [2]
• Fixed minor UI issues in AnnouncementBanner and other components for better alignment and interaction. (AnnouncementBanner.tsx [1] [2]
• Miscellaneous class name and style adjustments for consistency and improved transitions. (EmbeddableDocsPage.tsx packages/gitbook/src/components/Embeddable/EmbeddableDocsPage.tsxL84-R84)

These changes collectively provide a more modern, consistent, and maintainable user interface, especially around side panels and responsive layouts.

[linear]

RND-7803 Better support for docs assistant on smaller screens

RND-7450 Mobile TOC + Chat layout

[changeset-bot]

🦋 Changeset detected

Latest commit: aa86711

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
gitbook	Patch

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-d5czwcvpq-gitbook.vercel.app	✅
Cloudflare	https://43baf39d-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-d5czwcvpq-gitbook.vercel.app/url/gitbook.com/docs	https://43baf39d-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-d5czwcvpq-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://43baf39d-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	⚠️ Changes detected (Review)	78 changed	Dec 11, 2025, 6:40 PM
v2-cloudflare (Inspect)	⚠️ Changes detected (Review)	327 changed	Dec 11, 2025, 6:45 PM
v2-vercel (Inspect)	⚠️ Changes detected (Review)	333 changed	Dec 11, 2025, 6:44 PM