# Page Table of Contents

## Objective

Turn a long page into a scannable set of anchored sections.

A generated list of a page's headings, with anchor links and an indicator for the section currently in view.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Generate the table from the headings actually rendered on the page. Do not maintain a hand-written list beside the content — it will drift.
2. Derive stable anchor ids from heading text, and disambiguate repeated headings deterministically so a shared link keeps pointing at the same section.
3. Offset every jump by the height of any sticky header, so the target heading is not hidden underneath it.
4. Track the section in view and mark it, but debounce the switch — a highlight that flickers between two entries at a boundary is worse than none.
5. Do not render the table for pages with fewer than about three sections; it adds noise and no navigation value.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Headings inside collapsed sections must expand their section when linked to, not scroll to nothing.
- A deep-linked anchor on first load must land correctly even though layout shifts as images and fonts settle.
- Very long heading text must truncate in the table while keeping the full text as its accessible name.
- On mobile there is no room for a sidebar — collapse the table to a control at the top rather than dropping it.
- Content that loads after first paint must update the table, not leave it stale.
- Keyboard users must be able to tab into the table and land focus on the target heading after activating a link, not just scroll the page.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every rendered heading appears in the table, and nothing else does.
- [ ] Anchor ids are stable across reloads and unique when headings repeat.
- [ ] Jumps clear any sticky header.
- [ ] The active-section indicator does not flicker at boundaries.
- [ ] The table is usable on mobile and by keyboard, and moves focus to the target.
- [ ] Short pages render no table at all.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.