[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.

从 https://react.dev/learn/installation 的侧边栏开始，跟踪文档站点中已展开部分的箭头点击处理程序。先在 Chrome 中复现该行为，然后验证再次点击箭头会折叠该部分，同时导航和当前页面上下文仍然正常工作。