Phase 21 Plan 02: Chat Markdown Renderer with Syntax Highlighting Summary

One-liner: rehype-highlight markdown renderer with theme-aware hljs CSS overrides and ChatCodeBlock copy button using navigator.clipboard.writeText.

Completed Tasks

Task	Name	Commit	Files
1	Install rehype-highlight and add hljs theme CSS overrides	3e2bc1ae	ui/package.json, ui/src/index.css
2 (RED)	Add failing tests for ChatMarkdownMessage	732032a6	ui/src/components/ChatMarkdownMessage.test.tsx
2 (GREEN)	Create ChatCodeBlock and ChatMarkdownMessage components	576e302a	ui/src/components/ChatCodeBlock.tsx, ui/src/components/ChatMarkdownMessage.tsx

What Was Built

ChatMarkdownMessage

Markdown renderer component wrapping react-markdown with:

rehypePlugins={[rehypeHighlight]} — applies hljs token classes to code blocks
remarkPlugins={[remarkGfm]} — GitHub Flavored Markdown
components={{ pre: ChatCodeBlock }} — custom code block with toolbar
paperclip-markdown class — picks up existing prose styles from index.css

ChatCodeBlock

Pre-element override for react-markdown that provides:

Language label extracted from language-xxx className on the child <code> element
Copy button using navigator.clipboard.writeText (Copy/Check icon toggle with 1500ms success state)
Plain pre fallback when no code child present (e.g. plain preformatted text)
Typed using HTMLAttributes<HTMLPreElement> & ExtraProps to satisfy react-markdown's ComponentType constraint

Highlight.js Theme CSS (index.css)

Added 60 lines of CSS overrides covering 15 token types across three themes:

.dark .hljs* — Catppuccin Mocha palette (#cba6f7 keywords, #a6e3a1 strings, etc.)
.theme-tokyo-night .hljs* — Tokyo Night palette (#bb9af7 keywords, #9ece6a strings, etc.)
:root .hljs* — Catppuccin Latte palette (#8839ef keywords, #40a02b strings, etc.)

No external CSS file imports — all overrides are scoped custom properties, avoiding FOUC and theme conflicts.

Deviations from Plan

Auto-fixed Issues

1. [Rule 1 - Bug] Fixed ChatCodeBlock TypeScript type signature

Found during: Task 2 — TypeScript type check after implementation
Issue: [key: string]: unknown index signature incompatible with ClassAttributes<HTMLPreElement> & HTMLAttributes<HTMLPreElement> & ExtraProps from react-markdown
Fix: Changed component props type to HTMLAttributes<HTMLPreElement> & ExtraProps (importing ExtraProps from react-markdown)
Files modified: ui/src/components/ChatCodeBlock.tsx
Commit: 576e302a (included in same commit)

Verification Results

pnpm --filter @paperclipai/ui exec -- tsc --noEmit — PASS (0 errors)
pnpm vitest run ui/src/components/ChatMarkdownMessage.test.tsx — PASS (4/4 tests)
rehype-highlight present in ui/package.json — PASS
.dark .hljs-keyword { color: #cba6f7; } in index.css — PASS
.theme-tokyo-night .hljs-keyword { color: #bb9af7; } in index.css — PASS
:root .hljs-keyword { color: #8839ef; } in index.css — PASS
navigator.clipboard.writeText in ChatCodeBlock — PASS
aria-label="Copy code" / "Copied!" in ChatCodeBlock — PASS
Language extraction from language-xxx pattern — PASS

Self-Check: PASSED

All files created and commits verified.

Known Stubs

None — both components are fully wired. ChatMarkdownMessage uses rehype-highlight for real syntax highlighting; ChatCodeBlock calls navigator.clipboard.writeText for real copy functionality.

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