# Code Editor Field

## Objective

Edit code, JSON, and config with syntax highlighting instead of a bare textarea.

A form field backed by a real code editor: syntax highlighting, a language mode per field, and inline validation that tolerates work in progress.

## 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. Load the editor library lazily, only on views that use it, and render a usable plain textarea until it arrives. It must never block first paint.
2. Set the language mode per field from what the field actually holds, and validate against that mode.
3. Report syntax errors as you go, but never block typing or reset the content. Half-written JSON is a normal intermediate state; only submission should be gated.
4. Preserve the content exactly: tabs versus spaces, trailing whitespace, line endings, and the final newline. An editor that reformats on save destroys diffs.
5. Offer an explicit escape hatch to the plain textarea, and use it automatically when the fancy editor cannot initialise.

## 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

- Screen readers and browser autofill do not cope with syntax-highlighted layers — the fallback textarea must be a real, labelled control.
- Pasting a very large file must not freeze the tab; cap the size, disable highlighting above a threshold, and say so.
- Tab must indent inside the editor but must not trap keyboard users — give them a documented way out.
- Content saved with CRLF line endings must come back the same way unless the app deliberately normalises them.
- A failed save must keep the buffer intact; never clear the editor on error.
- Highlighting themes must follow the app's light and dark themes, not ship their own colours.

## 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 editor library is lazily loaded and never blocks initial render.
- [ ] Each field highlights and validates using its declared language.
- [ ] Invalid intermediate content can be typed freely; only submit is blocked.
- [ ] Whitespace, tabs, and line endings survive a save-and-reload round trip.
- [ ] A labelled plain-textarea fallback exists and is used when the editor fails.
- [ ] Editor colours match the app's theme in both light and dark.
- [ ] 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.