feat: add Radix UI slot component and enhance Markdown rendering

[MyPrototypeWhat]

• Added @radix-ui/react-slot dependency to package.json for improved component composition.
• Introduced a new Loader component with various loading styles to enhance user experience during asynchronous operations.
• Updated PlaceholderBlock to utilize the new Loader component, improving loading state representation.
• Enhanced Markdown component to support smooth fade animations based on streaming status, improving visual feedback during content updates.
• Refactored utility functions to include a new cn function for class name merging, streamlining component styling.

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[claude]

Code Review for PR #10391

Thank you for your contribution! I've reviewed the changes to add Radix UI slot component and enhance Markdown rendering. Here's my comprehensive feedback:

✅ Strengths

• Good initiative to improve loading state UX with a dedicated Loader component
• Comprehensive loader component with 12 different variants provides flexibility
• Proper use of React.memo() for performance optimization
• Clean migration from styled-components in PlaceholderBlock (aligns with project's UI migration goals)

🔍 Code Quality & Best Practices

1. Memory Leak Risk in Markdown Component 🚨

The SmoothFade HOC creates new component instances on every render, which could lead to memory issues:

// Current implementation creates new components on every render
p: SmoothFade((props) => <p {...props} />, isStreaming),

Recommendation: Move SmoothFade component creation outside the render cycle or use React.memo.

2. Missing Dependencies Installation ⚠️

The PR adds @radix-ui/react-slot but it's not actually used in any of the changed files. Consider removing it if unused, or document its intended usage.

3. Utility Function Addition

The new cn() utility function is added but clsx and tailwind-merge dependencies might already exist elsewhere in the codebase. Verify these aren't duplicated.

🐛 Potential Bugs

1. Animation Cleanup Issue

In SmoothFade, the animation cleanup removes classes that might be needed:

e.currentTarget.classList.remove('animate-[fadeIn_500ms_ease-out_forwards]')

This could cause visual glitches if the component re-renders during streaming.

2. Missing Style Injection Safety

The fadeStyle is injected directly without checking for duplicate injections or cleanup on unmount.

⚡ Performance Considerations

1. Excessive Re-renders: The Markdown component will re-render frequently during streaming. Consider using React.useMemo for the SmoothFade wrapped components.

2. Animation Performance: Multiple simultaneous fade animations could impact performance on lower-end devices. Consider using CSS will-change property or GPU-accelerated transforms.

3. Large Loader Component: The loader.tsx file is 375 lines with many inline styles. Consider:

   • Splitting into separate component files
   • Using CSS modules or external stylesheets for better performance
   • Lazy loading variants that aren't immediately needed

🔐 Security Concerns

The code appears secure with no obvious vulnerabilities. The Markdown component properly sanitizes content through the existing ReactMarkdown implementation.

🧪 Test Coverage Recommendations

Consider adding tests for:

1. Loader component variants rendering correctly
2. SmoothFade animation lifecycle (start, end, cleanup)
3. PlaceholderBlock integration with new Loader
4. Memory leak prevention in streaming scenarios

📝 Additional Suggestions

1. Documentation: Add JSDoc comments for the Loader component props and variants
2. Accessibility: Ensure all loader variants properly announce loading state to screen readers
3. Consistency: The project is migrating to HeroUI - verify if HeroUI already provides similar loading components
4. Type Safety: Consider adding more specific TypeScript types for the animation event handlers

🎯 Action Items

Required:

• Fix the memory leak risk in SmoothFade implementation
• Add proper cleanup for injected styles
• Consider performance optimizations for streaming scenarios

Recommended:

• Split the large loader.tsx file into smaller modules
• Add comprehensive tests
• Remove unused @radix-ui/react-slot dependency or document its usage
• Verify alignment with HeroUI component library

Overall, this is a good improvement to the user experience, but needs some refinements for production readiness. The animation enhancements are visually appealing but require careful implementation to avoid performance issues.