feat(theme): add accordion mode for sidebar

[Copilot]

Adds sidebar.accordion option to collapse sibling sections when expanding a top-level sidebar group, keeping documentation sidebars compact for sites with many sections.

Changes

• Type definitions: Added accordion?: boolean to Sidebar and NormalizedSidebar interfaces
• Normalization: Preserve accordion flag through config normalization, filter it from path matching
• Component behavior: Modified SidebarGroup.toggleCollapse() to collapse sibling top-level groups when accordion enabled
  • Only affects depth-0 groups
  • Respects collapsible: false flag

Usage

export default defineConfig({
  themeConfig: {
    sidebar: {
      accordion: true,
      '/': [
        { text: 'Guide', items: [...] },
        { text: 'API Reference', items: [...] }
      ]
    }
  }
});

Screenshots

Initial state with all sections expanded:

Accordion mode - only Guide section expanded:

Accordion mode - only API Reference section expanded:

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Feature]: Accordion mode for sidebar (single section expanded at a time)</issue_title>
<issue_description>### What problem does this feature solve?

It would be useful to have an option for accordion-style sidebar behavior where expanding one section automatically collapses others.

For documentation sites with many sidebar sections (e.g., multiple products or large changelogs), the sidebar can become very long when multiple sections are expanded. Accordion mode would keep the sidebar compact and focused.

What does the proposed API look like?

  // In rspress.config.ts
  themeConfig: {
    sidebar: {
      accordion: true, // Only one section expanded at a time
    }
  }

…or per-path:

  sidebar: {
    '/docs/': {
      accordion: true,
      items: [...]
    }
  }

Alternatives considered:

• CSS/JS manipulation in custom theme - doesn't persist because Rspress manages sidebar state internally
• Always starting sections collapsed (collapsed: true) - requires manual expand each time, poor UX

Prior art:

• https://getbootstrap.com/docs/5.3/components/accordion/ - uses data-bs-parent attribute
• [Accordion] shows only one expanded section at a time carbon-design-system/carbon#2141 - similar feature request</issue_description>

<agent_instructions>修改 useSidebarDynamic 并实现这个 accordion，仅实现

sidebar: {
accordion: true
}</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature]: Accordion mode for sidebar (single section expanded at a time) #2989

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