# Scroll Position Restoration

## Objective

Bring people back to where they were, not to the top of the list.

Restore scroll position when a user returns to a list they navigated away from.

## 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. Record the scroll position when leaving a scrollable view and restore it when returning to the same view.
2. Key the saved position on the full view state: route, filters, search, sort, and page. A position from a different filter is worse than the top of the list.
3. Restore only on a return, meaning back navigation or a return from a detail view. A fresh visit starts at the top.
4. Wait for the content that determines page height before restoring, or the restore will land in the wrong place.
5. Do not fight the browser's own restoration. Take it over explicitly for the views you manage instead of running both.

## 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

- Images and embeds without reserved dimensions change the page height after restore. Reserve space or restore after they settle.
- In a virtualized list the original node no longer exists. Restore by item identity and offset, not pixel value.
- An infinite list must restore the loaded pages too, or the target position does not exist yet.
- If the item the user was looking at has since been deleted, land near where it was rather than jumping.
- Saved positions must be discarded when the view state changes, and must not grow without bound.
- The restore jump must not be animated; scrolling the user through a thousand rows is worse than arriving.

## 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

- [ ] Returning to a list from a detail view lands on the same item.
- [ ] Positions are keyed by route, filters, search, sort, and pagination.
- [ ] A fresh navigation to the view starts at the top.
- [ ] Restoration happens after height-affecting content has loaded, with no visible second jump.
- [ ] Virtualized and infinite lists restore correctly.
- [ ] Stored positions are bounded and expire.
- [ ] 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.