feat(platform): streaming markdown line buffer and split rendering

[larryro]

Summary

• Add a line-buffer utility that cleanly handles stream chunk buffering, ensuring markdown is only emitted at complete line boundaries
• Split streaming markdown into stable (already-completed blocks) and streaming (in-progress block) portions — stable blocks are parsed once and memoized, reducing per-frame rendering work from O(full_text) to O(last_block)
• Improve remendMarkdown with tilde fence support, better trailing marker detection for underscores and CJK text, and a targetCPS=0 guard in useStreamBuffer

Test plan

• Unit tests added for line-buffer, find-block-split, and updated remend-markdown tests
• Unit tests updated for use-stream-buffer and incremental-markdown
• Manually verify streaming chat messages render smoothly without flicker
• Verify completed markdown blocks don't re-render during streaming
• Verify code fences (backtick and tilde) render correctly mid-stream

Summary by CodeRabbit

• New Features

  • Added smooth drain animation phase when text streaming completes, gradually revealing buffered content
  • Enhanced markdown streaming stability to prevent partial syntax flicker (code fences, emphasis markers)

• Improvements

  • Improved accessibility with aria-busy attributes for animated elements
  • Refined cursor rendering to ensure single cursor display during streaming animation
  • Better handling of special markdown constructs (code blocks, thematic breaks, emphasis) during character-by-character reveal

[greptile-apps]

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[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

This PR refactors the streaming markdown reveal mechanism from anchor-based to block-based splitting. It removes the anchorPosition export from useStreamBuffer and replaces it with findBlockSplitPoint, which determines stable split boundaries using double-newline delimiters. New line-buffer utilities (isAmbiguousPartialLine, isAtTrailingEmptyMarker, findSyntaxSkipEnd) prevent partial syntax rendering during incremental reveals. The drain behavior on stream-end changes from immediate reveal to animated draining at 3× the character-reveal rate. IncrementalMarkdown now accepts aria-busy for accessibility and enforces a single-cursor guarantee via useLayoutEffect. remendMarkdown extends tilde fence support and relocates trailing whitespace before closing emphasis markers. Supporting test suites are refactored to focus on drain and line-buffer behaviors, and vite.config.ts adds explicit proxy routes for storage and action API endpoints.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~65 minutes

Possibly related PRs

• feat(platform): simplify chat auto-scroll and fix streaming markdown flicker #830: Modifies streaming markdown stabilization with remendMarkdown and incremental-markdown rendering changes for revealed-slice handling.
• fix(platform): stabilize chat scroll and layout during streaming drain #611: Directly modifies useStreamBuffer anchor semantics and drain behavior with corresponding test suite changes.
• feat(platform): improve chat markdown rendering and scroll behavior #843: Extends remend-markdown fence and formatting-marker handling, overlapping with tilde fence and whitespace relocation logic in this PR.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 70.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat(platform): streaming markdown line buffer and split rendering' clearly and concisely summarizes the main changes: introduction of streaming markdown line buffering and split rendering logic.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/chat-streaming-line-buffer

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