# Tag Input

## Objective

A field where typing a word and pressing Enter turns it into a removable chip.

The input control for attaching tags to a record: type to filter, Enter to commit, Backspace to remove, chips for what is already there.

## 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. Build the control only. The tag catalog itself — scoping, renaming, deleting, filtering lists by tag — is owned by Labels and Tags; extend that rather than building a second one.
2. Normalise on commit, not on every keystroke: trim whitespace, collapse runs of spaces, apply the app's case rule. Rewriting the value while someone is mid-word makes the field feel broken.
3. Support two modes: create-new-allowed and existing-only. In existing-only mode the field must say so rather than silently swallowing an unmatched entry.
4. Make the whole thing keyboard-drivable: arrow keys through suggestions, Enter to commit, Backspace on an empty input to target the last chip, Escape to close the suggestion list.
5. Do not re-specify listbox roles and announcements — Accessible Multi-Select owns those semantics. Reuse them.

## 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

- Punctuation people mean to keep, like C++ or .NET, must survive normalisation.
- A tag that normalises to one already attached is a duplicate — reject it quietly and flash the existing chip rather than adding a second.
- Backspace must delete the last chip in one press only after the input is empty, and it should select before it destroys.
- Pasting a comma- or newline-separated list should create several tags at once, not one tag containing commas.
- A very long tag must not push the input off the row; wrap or truncate the chip, never the field.
- Removing a chip must return focus somewhere sensible, not to the top of the document.

## 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

- [ ] Tags can be created, selected, and removed without touching the mouse.
- [ ] Normalisation happens on commit and never rewrites the value mid-typing.
- [ ] Duplicates after normalisation cannot be added.
- [ ] Existing-only mode refuses unknown values with a visible message.
- [ ] Pasting a delimited list produces separate tags.
- [ ] The catalog itself is read from Labels and Tags, not duplicated here.
- [ ] 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.