# Input Masks

## Objective

Guide people into the expected format without fighting what they type.

Formatted input fields that insert separators as the user types, while submitting a clean, normalized value.

## 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. Keep the caret where the user expects it after every insertion and deletion. Reformatting that jumps the caret to the end is the single most common failure of masked inputs.
2. Accept pasted values in any shape — with separators, without them, with spaces or dashes — and normalize on paste rather than rejecting.
3. Submit the normalized value to the server and format only for display. The server must never receive a display string it has to strip.
4. Deleting a separator must delete the adjacent character, not fight the user by re-inserting it.
5. Do not put the mask pattern in the field's accessible name, and do not let screen readers read every separator character. Announce the field's purpose and expected format once via the label and description.
6. Never use a mask as your only validation. The value must be validated server-side regardless of what the field allowed.

## 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

- Selecting the whole value and typing over it must reformat cleanly from scratch.
- Undo and redo must step through logical edits, not through each reformat.
- Autofill inserts a whole value at once, bypassing keystroke handlers — normalize on change, not only on keypress.
- Mobile keyboards behave differently; test composition and predictive input rather than assuming key events.
- A partially entered value must be preserved on validation failure, not cleared.
- The field must remain usable with JavaScript disabled or failed — a plain, unmasked input that still submits.

## 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

- [ ] The caret stays put during insertion, deletion, and mid-string editing.
- [ ] Pasted values are accepted with or without formatting.
- [ ] The server receives normalized values, never display strings.
- [ ] Screen readers announce the field's format once, not per separator.
- [ ] Autofill and mobile input produce correctly formatted values.
- [ ] Server-side validation exists independently of the mask.
- [ ] 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.