Imported from curiositech/some_claude_skills (.claude/skills/reactive-dashboard-performance/SKILL.md). Install upstream with npx skills add curiositech/some_claude_skills --skill reactive-dashboard-performance. Copyright stays with the author.

Reactive Dashboard Performance

Expert in building production-grade reactive dashboards that load in <100ms and have comprehensive test coverage.

Core Expertise

Performance Patterns (Linear, Vercel, Notion-grade)

1. Skeleton-First Loading

   • Render skeleton immediately (0ms perceived load)
   • Stream in data progressively
   • Never show spinners for <200ms loads

2. Aggressive Caching

   • React Query with staleTime: 5min, cacheTime: 30min
   • Optimistic updates for mutations
   • Prefetch on hover/mount

3. Code Splitting

   • Route-based splitting (Next.js automatic)
   • Component-level lazy() for heavy widgets
   • Preload critical paths

4. Memoization Strategy

   • useMemo for expensive computations
   • React.memo for pure components
   • useCallback for stable references

Testing Reactive Dashboards

1. Mock Strategy

   • Mock at service boundary (React Query, analytics)
   • Never mock UI components (test real DOM)
   • Use MSW for API mocking when possible

2. Async Handling

   // WRONG - races with React
   render(<Dashboard />);
   const element = screen.getByText('Welcome');
   
   // RIGHT - waits for async resolution
   render(<Dashboard />);
   const element = await screen.findByText('Welcome');

3. Timeout Debugging

   • Timeouts mean: missing mock, wrong query, or component not rendering
   • Use screen.debug() to see actual DOM
   • Check console for unmocked errors

4. Test Wrapper Pattern

   const TestProviders = ({ children }) => (
     <QueryClientProvider client={testQueryClient}>
       <AuthProvider>
         {children}
       </AuthProvider>
     </QueryClientProvider>
   );

Real-World Examples

• Linear Dashboard: Skeleton → Stale data → Fresh data (perceived <50ms)
• Vercel Dashboard: Prefetch on nav hover, optimistic deploys
• Notion Pages: Infinite cache, local-first, sync in background

Diagnostic Protocol

Integration Test Timeouts

1. Check what's actually rendering

   render(<Component />);
   screen.debug(); // See actual DOM

2. Find unmocked dependencies

   • Check console for "not a function" errors
   • Look for network requests in test output
   • Verify all contexts are provided

3. Fix async queries

   • Use findBy* instead of getBy*
   • Increase timeout if needed: waitFor(() => {...}, { timeout: 3000 })
   • Mock React Query properly

4. Simplify component tree

   • Test widgets individually first
   • Add full integration tests last
   • Use data-testid for complex queries

Performance Optimization

Dashboard Load Budget

React Query Config

const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 5 * 60 * 1000, // 5min
      cacheTime: 30 * 60 * 1000, // 30min
      refetchOnWindowFocus: false,
      refetchOnMount: false,
      retry: 1,
    },
  },
});

Skeleton Pattern

function Dashboard() {
  const { data, isLoading } = useQuery('dashboard', fetchDashboard);

  // Show skeleton immediately, no loading check
  return (
    <div>
      {data ? <RealWidget data={data} /> : <SkeletonWidget />}
    </div>
  );
}

Common Pitfalls

1. Spinners for fast loads - Use skeletons instead
2. Unmemoized expensive computations - Wrap in useMemo
3. Testing implementation details - Test user behavior
4. Mocking too much - Mock at boundaries only
5. Synchronous test expectations - Everything is async

When debugging test timeouts, ALWAYS start with screen.debug() to see what actually rendered.