# Comments and Mentions

## Objective

Keep discussion on the record instead of in chat and email.

Threaded comments attached to a record, with @-mentions that notify the person mentioned.

## 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. Attach comments to a record polymorphically so the same component works across object types rather than being rebuilt per model.
2. Store mentions as structured references to user IDs, not as raw text parsed at render time. Display names change; IDs do not.
3. Restrict the mention picker to users who can already see the record, and reject mentions of anyone else on the server.
4. Notify each mentioned user once per comment. Track what has been sent so an edit or a delivery retry cannot fire a second notification.
5. Do NOT allow raw HTML in comment bodies. Sanitize to a small allowlist and render links with rel=noopener — comments are the most-pasted surface in any app.

## 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

- Editing a comment to add a new mention should notify only the newly added person, never everyone again.
- A deleted user's past mentions must still render as inert text, not as a broken link or a blank.
- Deleting a comment with replies must not orphan them — either soft-delete with a tombstone or cascade deliberately.
- Mentioning someone must never grant them access. If they cannot open the record, the notification is a dead link.
- Comments arriving in realtime must insert in a stable order, and must not scroll the reader away from what they were reading.
- Show edited and deleted state. A silently rewritten comment is a trust problem in any thread people rely on.

## 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

- [ ] Comments attach to any supported record type through one implementation.
- [ ] Mentions are stored as user references and survive display-name changes.
- [ ] Only users with access to the record can be mentioned, enforced server-side.
- [ ] Each mention notifies exactly once, including across edits and retries.
- [ ] Comment bodies are sanitized to an allowlist and links carry rel=noopener.
- [ ] Edited and deleted comments are visibly marked and ordering stays stable under realtime updates.
- [ ] 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.