[utpalsinghdev]

Summary

Contents could remain several sections behind after a large scroll. Switching to another document with the same heading count could leave it tracking the previous document's heading elements.

Replace the intersection-based tracking with current heading positions at the 80px reading line. Scroll updates are coalesced into one animation frame, and content mutations and resizes refresh the active section.

Update the focused tests for large jumps, reverse scrolling, same-count document replacement, and empty content. Remove the unused intersection-observer test helper.

Closes #2231.

Verification

• Reproduced both reported failures in Mac Brave at 1440×900 using native wheel input.
• Verified the fix in Mac Brave across slow and large scrolls, reverse scrolling, heading clicks, equal-heading-count file switches, and top/bottom positions. All nine Contents checks passed.
• Focused tests: 13 passed.

DOM_TESTS=1 bun test \
  packages/ui/hooks/useScrollViewport.test.ts \
  packages/ui/hooks/useActiveSection.test.tsx \
  packages/ui/hooks/useActiveSection.positions.test.tsx \
  packages/editor/App.bundle.test.tsx

bun x tsc --noEmit -p packages/ui/tsconfig.json
bun run --cwd apps/hook vite build
git diff --check