feat(client): add nested sections support with auto-expansion in sidebar

[Venkat-Entropik]

Description

This PR updates the sidebar to support nested sections (sub-sections inside groups).

Now the sidebar:

• Supports multiple levels (not just a flat list)
• Highlights the current page correctly

This makes navigation clearer, especially for deeply structured docs.

---

Validation

• Checked that parent sections expand when visiting pages inside them
• Checked that nested sections also expand correctly
• Verified it works for multiple levels
• Confirmed current page is highlighted properly

---

Related Issues

closes #8804

---

Check List

• I have read the Contributing Guidelines and followed commit message rules.
• I have run pnpm format.
• I have run pnpm test.
• I have run pnpm build.
• Added tests if needed.

[cursor]

PR Summary

Medium Risk
Adds recursive rendering and state (auto-open) to the shared Sidebar/SidebarGroup components, which could affect navigation UX and active-link behavior across docs pages. Risk is moderate due to new nested data shapes and path-matching logic, but changes are localized to sidebar UI.

Overview
Adds nested sidebar section support end-to-end. withSidebar now maps the site navigation tree into a recursive items structure instead of flattening one level.

Updates SidebarGroup to render nested items recursively via <details>/<summary>, auto-expanding parent sections when the current pathname is inside them, and styles the nested UI accordingly. The mobile select now flattens nested items so all leaf links remain selectable, and Storybook gains a NestedGroup example.

^{Reviewed by Cursor Bugbot for commit ab79c46. Bugbot is set up for automated code reviews on this repo. Configure here.}

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
nodejs-org	Ready	Preview	Apr 8, 2026 2:34am

[github-actions]

👋 Codeowner Review Request

The following codeowners have been identified for the changed files:

Team reviewers: @nodejs/nodejs-website

Please review the changes when you have a chance. Thank you! 🙏

[avivkeller]

Can you update the stories?

[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO	Report
/en	🟢 99	🟢 93	🟢 96	🟢 100	🔗
/en/about	🟢 100	🟢 94	🟢 96	🟠 88	🔗
/en/about/previous-releases	🟢 97	🟢 97	🟢 100	🟢 100	🔗
/en/download	🟢 97	🟢 96	🟠 81	🟢 100	🔗
/en/download/archive/current	🟢 100	🟢 97	🟢 100	🟢 100	🔗
/en/blog	🟢 100	🟢 97	🟢 96	🟢 100	🔗

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.25%. Comparing base (9345c76) to head (a632529).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8806      +/-   ##
==========================================
+ Coverage   74.22%   74.25%   +0.02%     
==========================================
  Files         105      105              
  Lines        8870     8870              
  Branches      328      328              
==========================================
+ Hits         6584     6586       +2     
+ Misses       2284     2282       -2     
  Partials        2        2              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit ab79c46. Configure here.}

[bmuenzenmeyer]

Can someone show me where this code is actually being applied? I looked at the code, the preview, and the issue and couldn't easily find validation examples

[avivkeller]

Can someone show me where this code is actually being applied? I looked at the code, the preview, and the issue and couldn't easily find validation examples

It would be used in the learn repository, I can set up a demo using this PR if it helps

[avivkeller]

It'll be used in nodejs/learn. Would you like me to set up a demo?