---
id: T01
parent: S05
milestone: M016
provides: []
requires: []
affects: []
key_files: ["frontend/src/components/ReadingHeader.tsx", "frontend/src/components/TableOfContents.tsx", "frontend/src/pages/TechniquePage.tsx", "frontend/src/App.css"]
key_decisions: ["Hide section name on mobile (<600px) rather than truncating both, for readability at 375px", "pointer-events:none when hidden so invisible bar doesn't block clicks"]
patterns_established: []
drill_down_paths: []
observability_surfaces: []
duration: ""
verification_result: "Build passes (tsc + vite, 0 errors). reading-header class found in App.css and ReadingHeader.tsx. activeId referenced 5 times in TechniquePage.tsx."
completed_at: 2026-04-03T06:01:11.303Z
blocker_discovered: false
---

# T01: Lifted scroll-spy state from TableOfContents to TechniquePage, created ReadingHeader sticky bar with slide-in animation triggered by H1 leaving viewport

> Lifted scroll-spy state from TableOfContents to TechniquePage, created ReadingHeader sticky bar with slide-in animation triggered by H1 leaving viewport

## What Happened
---
id: T01
parent: S05
milestone: M016
key_files:
  - frontend/src/components/ReadingHeader.tsx
  - frontend/src/components/TableOfContents.tsx
  - frontend/src/pages/TechniquePage.tsx
  - frontend/src/App.css
key_decisions:
  - Hide section name on mobile (<600px) rather than truncating both, for readability at 375px
  - pointer-events:none when hidden so invisible bar doesn't block clicks
duration: ""
verification_result: passed
completed_at: 2026-04-03T06:01:11.304Z
blocker_discovered: false
---

# T01: Lifted scroll-spy state from TableOfContents to TechniquePage, created ReadingHeader sticky bar with slide-in animation triggered by H1 leaving viewport

**Lifted scroll-spy state from TableOfContents to TechniquePage, created ReadingHeader sticky bar with slide-in animation triggered by H1 leaving viewport**

## What Happened

Extracted the IntersectionObserver and activeId state from TableOfContents into parent TechniquePage. TableOfContents now receives activeId as a prop. Created ReadingHeader component — a thin fixed bar showing article title + current section heading, controlled by a visible prop driven by a separate IntersectionObserver on the H1 element. Added CSS with transform translateY transition for slide-in/out, z-index 150, and mobile truncation. Reading header only renders on v2 format pages with sections.

## Verification

Build passes (tsc + vite, 0 errors). reading-header class found in App.css and ReadingHeader.tsx. activeId referenced 5 times in TechniquePage.tsx.

## Verification Evidence

| # | Command | Exit Code | Verdict | Duration |
|---|---------|-----------|---------|----------|
| 1 | `cd frontend && npm run build 2>&1 | tail -5` | 0 | ✅ pass | 1000ms |
| 2 | `grep -l 'reading-header' src/App.css src/components/ReadingHeader.tsx` | 0 | ✅ pass | 50ms |
| 3 | `grep -c 'activeId' src/pages/TechniquePage.tsx` | 0 | ✅ pass (5 matches) | 50ms |


## Deviations

None.

## Known Issues

None.

## Files Created/Modified

- `frontend/src/components/ReadingHeader.tsx`
- `frontend/src/components/TableOfContents.tsx`
- `frontend/src/pages/TechniquePage.tsx`
- `frontend/src/App.css`


## Deviations
None.

## Known Issues
None.
