# Streaming UI Optimization - Implementation Summary

## Overview

Implemented comprehensive streaming UI optimization patterns to ensure smooth, animated streaming with per-step updates at 30-60fps without layout thrashing.

## Files Created

### 1. Custom Hooks (`src/components/FastAgentPanel/hooks/`)

#### `useStreamingBuffer.ts`
- **Purpose**: Frame-aligned streaming buffer for batching updates
- **Key Features**:
  - Accumulates tokens, steps, and status updates in a ring buffer
  - Flushes on `requestAnimationFrame` or at configurable intervals (default: 33ms ≈ 30fps)
  - Auto-flushes when buffer exceeds max size (default: 50 updates)
  - Prevents UI blocking by batching updates
  - Lifecycle management: `start()`, `stop()`, `flush()`
  - Development logging support

#### `useSmartAutoScroll.ts`
- **Purpose**: Intelligent auto-scroll respecting user scroll position
- **Key Features**:
  - Only auto-scrolls when user is near bottom (configurable threshold, default: 80px)
  - Pauses auto-scroll when user scrolls up
  - Resumes when user scrolls back to bottom
  - Uses scroll anchoring to prevent layout thrash
  - Respects `prefers-reduced-motion` for accessibility
  - Methods: `autoScroll()`, `scrollToBottom()`, `reset()`, `isNearBottom()`

#### `index.ts`
- Exports both hooks for easy importing

### 2. Memoized Components

#### `StepTimelineItem.tsx`
- **Purpose**: Memoized timeline step component for smooth rendering
- **Optimizations**:
  - Uses `React.memo` with custom comparison function
  - Only re-renders when step data or expanded state changes
  - Animations use `transform` and `opacity` (GPU-accelerated)
  - Smooth transitions with `cubic-bezier(0.4, 0, 0.2, 1)` timing
  - Reduces commit time per step update
  - Scales well with many steps

### 3. CSS Animations

#### `FastAgentPanel.animations.css` (Enhanced)
Added GPU-accelerated animations:
- `@keyframes fadeInSmooth` - Smooth fade-in with translateY
- `@keyframes expandHeight` - Expand/collapse with opacity
- `@keyframes stepCountPulse` - Step counter increment animation
- `@keyframes typingDot` - Typing indicator dots
- `.animate-fadeIn` - Apply smooth fade-in
- `.animate-expandHeight` - Apply expand animation
- `.timeline-item` - GPU-accelerated hover effects
- `.streaming-message-container` - Containment for layout optimization
- `.tool-result-popover` - Smooth popover appearance

### 4. Updated Components

#### `FastAgentPanel.UIMessageStream.tsx`
- **Integrated Hooks**:
  - `useSmartAutoScroll` for intelligent auto-scroll
  - `useDeferredValue` for deferred message filtering
  - `useTransition` for smooth state transitions
- **Optimizations**:
  - Defers heavy message filtering to prevent UI blocking
  - Smart auto-scroll respects user scroll position
  - Smooth scroll anchor at end of messages
  - Improved performance during streaming

### 5. Tests

#### `streaming-optimization.test.tsx`
- Tests for `useStreamingBuffer` hook
- Tests for `useSmartAutoScroll` hook
- Performance tests for streaming UI
- Uses Vitest (`vi.fn()`) instead of Jest
- Validates buffer accumulation, flushing, and lifecycle
- Validates auto-scroll behavior and user scroll detection
- Validates frame rate maintenance (≥30fps)

### 6. Documentation

#### `STREAMING_OPTIMIZATION.md`
- Comprehensive guide to streaming optimization patterns
- Architecture overview
- Usage examples for each hook
- CSS animation reference
- Performance targets
- Best practices
- Monitoring and troubleshooting

## Performance Targets

- **Frame Rate**: 30-60fps during streaming
- **Commit Time**: <8ms average per update
- **Buffer Flush**: Every 33ms (30fps) or when buffer full
- **Auto-Scroll**: Smooth with no jank
- **Memory**: Bounded buffer prevents memory leaks

## Key Improvements

### Before
- Direct state updates on every token → excessive re-renders
- Auto-scroll fighting with user scroll
- Layout thrashing from height/position animations
- Heavy filtering blocking UI during streaming
- No batching of updates

### After
- Batched updates on animation frames → 30-60fps
- Smart auto-scroll respects user position
- GPU-accelerated animations (transform/opacity only)
- Deferred filtering prevents UI blocking
- Smooth, responsive streaming experience

## Integration Points

1. **UIMessageStream**: Uses `useSmartAutoScroll` and `useDeferredValue`
2. **StepTimeline**: Uses memoized `StepTimelineItem` components
3. **Animations**: CSS provides smooth, GPU-accelerated transitions
4. **Hooks**: Available for use in any streaming component

## Usage Example

```tsx
// In a streaming component
const { autoScroll, reset } = useSmartAutoScroll(scrollRef, {
  nearBottomThreshold: 80,
  enableLogging: false,
});

useEffect(() => {
  autoScroll();
}, [messages.length, autoScroll]);

useEffect(() => {
  reset();
}, [reset]);
```

## Testing

Run tests:
```bash
npm run test:run -- streaming-optimization.test.tsx
```

## Next Steps

1. Monitor performance in production with React Profiler
2. Adjust buffer size and flush interval based on actual usage
3. Consider Web Worker offload for heavy markdown parsing
4. Add performance monitoring dashboard
5. Gather user feedback on streaming smoothness

## References

- React 18 Hooks: useTransition, useDeferredValue
- CSS Animations Performance Guide
- Scroll Anchoring Specification
- GPU-Accelerated Animations Best Practices

