perf(markdown): cache completed streaming blocks

[c121914yu]

Summary

• split streaming Markdown into root-level blocks and memoize completed blocks
• keep the existing tail-only fade on the active block
• preserve cross-block reference links and GFM footnotes by falling back to full-document rendering

Verification

• pnpm exec vitest run -c vitest.config.ts test/components/Markdown/streamMarkdownBlocks.test.ts test/components/Markdown/rehypeStreamAnimated.test.ts test/components/Markdown/utils.test.ts --run --coverage=false
• app test suite: 150 test files passed; the 8 initial sandbox-adapter resolution failures passed after building the local workspace package
• targeted ESLint and Prettier checks passed

No documentation changes.

[c121914yu]

Superseded by clean cache-only PR #7306 based on latest main.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	18.43%	1632 / 8855
🔵	Statements	18.41%	1713 / 9300
🔵	Functions	16.21%	342 / 2109
🔵	Branches	15.1%	759 / 5024

File Coverage

No changed files found.

Generated in workflow #1406 for commit 106bb4d by the Vitest Coverage Report Action

[github-actions]

✅ Admin Preview Image Ready!

ghcr.io/labring/fastgpt-pr:admin_106bb4d9bb0773e3f271b275bbd04772c85fa2e8

🕒 Time: 2026-07-14 21:10:37 (UTC+8)

[Copilot]

Pull request overview

This PR improves streaming Markdown rendering performance in the app by splitting the streamed Markdown into root-level blocks and memoizing completed blocks so only the actively-growing tail block re-renders (while preserving cross-block reference/footnote correctness via a full-document fallback).

Changes:

• Added a unified/remark-based splitMarkdownBlocks helper to split Markdown by root node source ranges and detect reference/footnote cases that require full-document rendering.
• Updated the Markdown renderer to render per-block ReactMarkdown instances via a memoized MarkdownStreamBlock, applying the streaming tail animation only to the active (last) block.
• Updated the stream-tail fade-in behavior and CSS fallback, and added unit tests covering splitting behavior and rendering equivalence.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
projects/app/test/components/Markdown/streamMarkdownBlocks.test.ts	Adds coverage for block splitting behavior, offsets, reference/footnote fallback, and render equivalence.
projects/app/src/components/Markdown/streamMarkdownBlocks.ts	Introduces root-level Markdown block splitting using unified + remark-parse (+ math/gfm) with safe fallbacks.
projects/app/src/components/Markdown/index.tsx	Switches streaming rendering to memoized per-block rendering; keeps tail-only animation on the last block.
projects/app/src/components/Markdown/index.module.scss	Adds CSS keyframes/props to provide a fade-in fallback for .stream-tail.
projects/app/package.json	Adds direct dependencies needed for block parsing (remark-parse, unified).
pnpm-lock.yaml	Locks newly added dependencies for the app workspace.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file

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