# Z-Index Registry

## Objective

Keep every stacking value in one file so nobody has to guess a bigger number.

A single source of named z-index tokens, with reserved ranges per layer and a check that blocks raw values.

## 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. Find every z-index in the codebase, including inline styles and values buried in third-party overrides, and list them with the component that owns each. The count is usually higher than anyone expects and several will be arbitrary large numbers.
2. Reserve a range for each layer rather than assigning single values, so a new dropdown can slot between existing ones without renumbering everything above it. Leave real gaps between ranges.
3. Name each token after the layer it belongs to and derive the ordering from Elevation Layer Map. That feature decides what covers what; this one holds the numbers and stops them being invented locally.
4. Add a check that fails when a raw numeric z-index appears outside the registry, and run it in the same place the rest of the style rules are enforced. Without it the registry decays within a release or two.
5. Do not fix a stacking bug by raising a number. If an overlay renders behind something despite a higher value, the cause is a local stacking context and raising it further only hides the problem until the next collision.

## 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

- Ranges must be reserved per layer with room to grow, because a scheme of consecutive integers forces a rename of every higher token the first time something new needs to sit in the middle.
- A component whose value is correct but which still renders underneath is trapped in an ancestor's stacking context created by a transform, filter, or opacity. The check should surface that case rather than leaving developers to escalate numbers.
- Genuine exceptions exist — third-party widgets, embedded players, payment iframes that insist on their own values — and each needs a named token with a comment recording why, rather than an unexplained outlier.
- The lint has to cover inline styles and dynamically computed values, not only static stylesheets, or the values simply move to where the check cannot see them.
- Negative z-index places an element behind its own parent's background and is almost never what was intended; treat it as a case requiring explicit justification.
- Third-party scripts injected at runtime can set values above anything the registry defines, so the top of the range needs headroom and a documented response.
- Elements in the browser's native top layer are outside the registry's control entirely, and the registry should say so rather than implying it governs them.

## 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

- [ ] Every z-index in the app resolves to a named token in one registry file.
- [ ] Layers occupy reserved numeric ranges with gaps left for insertion.
- [ ] A check fails the build on any raw z-index outside the registry, including inline styles.
- [ ] The registry's order matches the ordering defined by Elevation Layer Map.
- [ ] Each exception carries a named token and a recorded reason.
- [ ] No stacking bug in the app is resolved by an arbitrarily large number.
- [ ] 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.