# Component Boundary Rules

## Objective

Decide which layers of the UI may import from which, and enforce it.

A stated dependency direction between primitives, patterns, and product components, checked automatically on every change.

## 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. Name the layers that already exist in the codebase — typically primitives, composed patterns, and product-specific components — and write the permitted direction of dependency between them. Do not invent a layering nobody is using; describe the one that is mostly there and then correct the exceptions.
2. State the direction as a rule with a reason. Primitives may not reach upward into product components because a primitive that knows about an invoice screen cannot be reused anywhere else, and cannot be tested without that screen's data.
3. Detect circular imports as part of the same check. Cycles between components produce initialisation-order failures that appear as an undefined component in an unrelated screen, long after the import that caused them.
4. Keep the boundary between code that renders on the server and code that requires the browser explicit, and make crossing it a deliberate, marked act rather than something discovered when a build fails.
5. Do not enforce this by review alone. Encode the rules in the automated checks that run on every change, and allow narrow exceptions only as individually justified entries with a comment saying why, so the exception list can be read and shrunk.

## 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

- Circular imports must be detected and rejected, because a cycle fails at module initialisation and surfaces as an undefined component somewhere unrelated to the file that introduced it.
- Exceptions must be possible but narrow. Each one belongs in a list, scoped to a specific file and a specific rule, with a written reason — a blanket exemption for a whole directory silently repeals the rule.
- The line between server-rendered and browser-only code needs its own boundary rule, since a component that reaches for the browser environment from a server-rendered layer fails at build or hydration rather than at import.
- Rules must be enforced in linting and in the automated checks on every change. A boundary policed by code review holds until the first busy week.
- Test files, stories, and playground examples legitimately import across layers and need their own rule set, or the exception list fills with entries that are not really violations.
- Type-only imports do not create a runtime dependency and can usually be permitted where a value import would not be, but the distinction has to be stated rather than left to the checker's defaults.
- The rules must be introduced with the existing violations recorded as a fixed baseline that can only shrink, otherwise the change is unmergeable and gets abandoned.

## 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 layers and the permitted dependency direction between them are written down with reasons.
- [ ] Violations fail the automated checks rather than depending on review.
- [ ] Circular imports between components are detected and rejected.
- [ ] Exceptions exist only as individually scoped entries carrying a stated reason.
- [ ] Server-rendered and browser-only code have an explicit, enforced boundary.
- [ ] Tests, stories, and examples are governed by their own rule set.
- [ ] Pre-existing violations are captured in a baseline that cannot grow.
- [ ] 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.