[akira28]

The API Topics sidebar heading now starts expanded on every page, so the topic pages are visible without a click. A sidebar group opts in through a new marker interface; every other group renders exactly as before.

Affects: Site UI, API reference

Prompt summary: Expand the Topics heading of the API sidebar by default. The PR that adds the Topics heading, #4628, is already large, so this change comes as its own PR on top of it.

Stack: 2 of 2, on top of #4628. Merge #4628 first.

Why

A sidebar group only opens on pages inside it. On the product overview or an operation page, the Topics heading is collapsed, so readers do not see pages such as Authentication or Rate limiting unless they open it. There is no way for a group to start open.

What

Groups that start expanded

A navigation node that implements ISidebarExpandedNavigationItem renders with its toggle checked and its subtree open, in the default and the preview navigation. The API Topics heading is the only group that opts in.

Unchanged output for every other group

The new attributes are written only for an expanded group, on the same line as the toggle's previous attribute. The flag also enters the sidebar content hash only when it is set. A collapsed group and the sidebar tree id are therefore byte-identical to main.

Client-side collapse

Changing to another section collapses every folder in the sidebar. A group marked data-nav-default-open now stays open through that reset.

Verify

dotnet test tests/Elastic.ApiExplorer.Tests/
# Generate_XTopics_WritesTopicPagesWithoutRedirects — on the landing page only the Topics toggle is checked
cd src/Elastic.Documentation.Site && npm run test
# collapseAllFolders: keeps a default-open group expanded

To confirm the flag-off site is unchanged:

cd src/Elastic.Documentation.Site
npm run visual:seed
npm run visual:check

Check on the preview

• Cloud API overview — Topics is expanded although the page is not inside it; the endpoint groups stay collapsed.
• Cloud: Get deployments — on an operation page, both Topics and the operation's own group are open.
• Elasticsearch API overview — control page: no Topics heading, so every group is collapsed as before.