# Collapsible Sections

## Objective

Let users fold away the parts of a crowded page they do not need right now.

Sections with a heading that expands and collapses, so a long page can be reduced to the parts in use.

## 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. Find the pages carrying several distinct groups of content and give each group a real heading plus a disclosure control.
2. Make the whole heading row the trigger, using a native button with the correct expanded state exposed, and render the collapsed content in a way that is genuinely removed from tab order and screen reader flow.
3. Choose sensible defaults per section rather than collapsing everything — a page that opens fully collapsed is a page with no content.
4. When a validation error, a search match, or a deep link targets content inside a collapsed section, expand that section and scroll to the target.
5. Do not collapse required actions, blocking warnings, or anything the user must act on. Hiding a problem behind a chevron does not solve it.

## 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

- Persisted expansion state must be discarded when the section no longer exists or the user's permissions have changed.
- Browser find-in-page cannot search collapsed content — accept this and make section headings descriptive enough to find.
- Collapsing a section containing focus must move focus to the section heading, not lose it.
- The animation must respect prefers-reduced-motion, and must not be applied to sections restored as collapsed on first paint.
- A section whose content is empty should say so rather than expanding into nothing.
- Sections that are expanded on load must not shift layout after data arrives — reserve space or keep them collapsed until loaded.

## 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 section has an accessible trigger reporting its expanded state.
- [ ] Collapsed content is out of tab order and hidden from assistive technology.
- [ ] Validation errors, deep links, and search hits auto-expand their section.
- [ ] Warnings and required actions are never collapsible.
- [ ] Focus is handled deliberately when a section closes.
- [ ] Animation respects prefers-reduced-motion and does not run on initial paint.
- [ ] 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.