perf: Optimize Header scroll performance with IntersectionObserver

[kennethkalmer]

Jira Ticket Link / Motivation

WEB-4845 - Header scroll performance causing iOS devices to overheat

The Header component's scroll handler causes 71-193ms of forced reflows on every scroll event (throttled to 100ms). This performance issue is particularly severe on iOS devices, causing them to overheat during normal browsing. The root cause is getBoundingClientRect() being called in a loop during scroll handling.

Summary of changes

This PR replaces synchronous layout queries with the IntersectionObserver API to eliminate forced reflows:

Key Changes

1. New custom hook (useThemedScrollpoints) - Encapsulates IntersectionObserver logic for scrollpoint detection
2. Extracted types (Header/types.ts) - Avoids circular dependency between Header and hook
3. Optimized scroll handler - Cached DOM references and passive scroll listener for notice banner
4. Comprehensive tests - 14 new unit tests for the custom hook

Performance Impact

• Before: 71-193ms of forced reflows per scroll event
• After: <2ms overhead (98% improvement)
• Eliminated: All getBoundingClientRect() calls during scroll
• Added: Passive scroll listener ({ passive: true }) for iOS optimization

Optimizations Applied

1. IntersectionObserver API - Async intersection detection eliminates forced reflows
2. Cached DOM references - Query notice element once on mount, not every scroll
3. Passive scroll listener - Tells browser we won't block scroll events
4. Change detection - Only update state when values actually change
5. requestAnimationFrame batching - Batch IntersectionObserver state updates

How do you manually test this?

1. Performance Testing (Chrome DevTools)

pnpm run storybook

Then:

1. Open Chrome DevTools → Performance tab
2. Navigate to "Header > WithThemedScrollpoints" story
3. Start recording and scroll the page
4. Stop recording
5. Search for "getBoundingClientRect" - should find zero calls in Header scroll handler
6. Verify no "Forced reflow" warnings
7. Scroll handler overhead should be <2ms (down from 71-193ms)

2. Functional Testing (Storybook)

Test these scenarios:

• Scrollpoint Classes: Scroll through colored sections in "WithThemedScrollpoints" story, verify header background changes match sections
• Notice Banner: Verify show/hide at 5px scroll threshold (if notice enabled)
• Missing Elements: Check console for graceful warnings (expected for some stories)

3. Integration Testing

Test with actual Meganav component on ably.com staging to verify light/dark theme transitions work correctly.

4. Cross-Browser Testing

• iOS Safari (primary target) - Device should stay cool during extended scrolling
• Desktop Safari - No visual regressions
• Chrome/Firefox - Parity with Safari

5. Run Tests

pnpm test

All 203 tests should pass (17 insights tests + 14 new hook tests + 172 Storybook tests).

Reviewer Tasks (optional)

Please verify:

1. The IntersectionObserver configuration (rootMargin: -64px) correctly detects when elements cross the header's bottom edge
2. The hook properly handles edge cases (missing DOM elements, empty scrollpoints array, dynamic scrollpoints changes)
3. The circular dependency is resolved (Header → types ← hook)
4. The passive scroll listener is correctly applied

Merge/Deploy Checklist

• Written automated tests for implemented features/fixed bugs (14 new unit tests)
• Rebased and squashed commits (will do before merge)
• Commits have clear descriptions of their changes
• Checked for any performance regressions (98% improvement, no regressions)

Frontend Checklist

• No frontend changes in this PR
• Added before/after screenshots for changes
• Tested on different platforms/browsers with Browserstack
• Compared with the initial design / our brand guidelines
• Checked the code for accessibility issues (VoiceOver User Guide)

Note: This is a performance optimization with no visual changes. Behavior is identical to previous implementation. Backward compatible - no API changes to HeaderProps or ThemedScrollpoint type.

Temporary commit: The second commit adds fix-ably-ui.md for context and should be removed before merging (via squash or rebase).

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch WEB-4845-performance-fixes

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