fix(content-blog): resolve anchor links in truncated blog previews

[devteamaegis]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Blog - Relative links in blog truncated content (list view) are breaking #9731) and the maintainers have approved on my working plan.

This is a small, focused bug fix rather than a new API or substantial change, so the last box is left unchecked. It closes #9731.

Motivation

Closes #9731 (see also #10287).

In blog paginated list views (/blog, tags pages, author pages), a post preview is rendered under the list page URL, not the post permalink. An in-page anchor link written above the truncate marker, e.g.

Jump to [the details](#details) below.

{/* truncate */}

## Details

was left relative and therefore resolved against the list page — /blog#details — instead of the post, so clicking it in a preview leads nowhere. On the full post page the same link works, because it resolves against the post URL. onBrokenAnchors also reports these as broken.

The fix

The blog markdown loader already truncates the preview inside its ?truncated=true branch — the one place in the pipeline that knows it is producing a list-view preview (the full post page is a separate compilation with no query). There we now rebase bare #anchor link targets to the current post permalink, using the source -> permalink map the plugin already maintains for Markdown link resolution.

Only bare #anchor targets are rewritten. Other relative links (./x, ../x, x.md) are intentionally left untouched — they are handled later in the MDX pipeline (resolveMarkdownLinks / transformLinks) — and absolute/external links are ignored. The full post page is not affected, so same-page anchors there remain native <a href="#…"> links.

Changed files (all in docusaurus-plugin-content-blog):

• blogUtils.ts — new resolveTruncatedAnchorLinks(content, permalink) helper.
• markdownLoader.ts — call it in the truncated branch, resolving the current post permalink from this.resourcePath.
• index.ts / types.ts — pass siteDir and the sourceToPermalink map to the loader.

Test Plan

Unit tests (blogUtils.test.ts, describe('resolveTruncatedAnchorLinks')): 5 cases covering bare anchors, a following title, <…>-wrapped targets, multiple anchors, and that non-anchor/absolute/external links are left untouched. yarn test blogUtils (vitest) is green; ESLint and the formatter pass.

End-to-end on a fresh classic site with a post containing [the details section](#details) above the truncate marker and a ## Details heading below:

	before	after
List view /blog	<a href="#details"> (→ /blog#details, broken)	<a href="/blog/…/post#details">
Author page	<a href="#details"> (broken)	<a href="/blog/…/post#details">
Post page	<a href="#details">	<a href="#details"> (unchanged)
onBrokenAnchors build report	reports /blog → #details broken	no broken anchors

Test links

Deploy preview: https://deploy-preview-12364--docusaurus-2.netlify.app/

Related issues/PRs

Closes #9731. Related: #10287.

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