[Bug]: Sidebar dropdown does not close when the arrow is clicked again

Summary

In the left sidebar navigation, when a section with sub-items is expanded (for example, "Installation"), clicking its downward-facing arrow again does not collapse the section. Users naturally expect an expand/collapse toggle arrow to work in both directions — even if this is intentional (e.g., to keep the active page's context visible), it hurts the UX since users assume clicking again will close the dropdown.

Page

https://react.dev/learn/installation

Details

To Reproduce

1. Go to any documentation page with an expanded sidebar section, such as https://react.dev/learn/installation.
2. Locate the left sidebar navigation.
3. Click on the downward-facing arrow next to the currently active/expanded section (e.g., "Installation").
4. Notice that the section does not collapse as expected.

Expected behavior
Clicking the dropdown arrow of an already expanded section should toggle it back to a collapsed state.

Desktop:

• OS: Windows
• Browser: Chrome

Additional context
Observed on the main react.dev documentation site. Fixing this would make the navigation feel more intuitive and aligned with standard UI patterns for accordions/dropdowns.

https://react.dev/learn/installation のサイドバーから始め、ドキュメントサイトで展開されたセクションの矢印のクリックハンドラーを追跡します。Chrome で動作を再現し、続けて矢印をもう一度クリックするとセクションが折りたたまれ、ナビゲーションとアクティブページのコンテキストが引き続き機能することを確認します。