# Reactions

## Objective

Let people acknowledge something without writing another comment.

Lightweight reactions on content, with accurate counts, who-reacted detail, and text labels.

## 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. Store one row per user, per reaction type, per item, with a uniqueness constraint. That constraint is what makes a double-tap harmless.
2. Make the toggle idempotent: reacting twice leaves one reaction, un-reacting twice leaves none. Never derive state from a count you incremented.
3. Update the count optimistically for the person who clicked, then reconcile with the server response and roll back visibly on failure.
4. Give every reaction a text label and expose who reacted. Meaning carried only by an emoji glyph is unreadable to screen readers and ambiguous across cultures.
5. Check permission on the item before recording a reaction — visibility of a count is not the same as the right to add to it.
6. Do NOT let the reaction set be arbitrary user input. A fixed, curated set keeps moderation, rendering, and analytics tractable.
7. Do NOT send a notification for every single reaction; batch them, or the feature becomes a reason to mute the product.

## 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

- Concurrent reactions from many users must not lose counts — aggregate from the rows or use an atomic counter, never a read-then-write.
- When a user loses access to an item, or is deleted or suspended, their reactions must stop being attributed to them and the counts must reflect it.
- Deleting the item must remove its reactions rather than leaving orphans that skew totals.
- An item with many reactors needs a truncated list with a real total, not a wall of avatars.
- Emoji render differently per platform and some render as boxes — the label, not the glyph, must carry the meaning.
- Reacting to your own content is allowed or not, but the rule must be the same everywhere it appears.
- Rapid toggling must be rate-limited, and the UI must never show a count below zero.

## 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

- [ ] A user can hold at most one reaction of each type on an item, enforced by a database constraint.
- [ ] Toggling is idempotent and counts stay correct under concurrent use.
- [ ] Every reaction has a text label and the reactor list is available.
- [ ] Reactions require permission on the underlying item.
- [ ] Losing access, deletion, and suspension all correct the counts.
- [ ] Optimistic updates roll back visibly when the request fails.
- [ ] Notifications for reactions are batched rather than sent one by one.
- [ ] 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.