# Shadow Scale

## Objective

Replace ad hoc shadows with a small set tied to what each surface actually is.

A fixed ladder of shadow tokens, each named for an elevation role rather than a blur radius.

## 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. Collect every shadow currently declared anywhere in the app and group them by what they sit on — a resting card, a hovered card, a dropdown, a dialog, a toast. Most apps have a dozen near-identical values that collapse into four or five roles.
2. Name each step for its role, not its appearance. A token called menu tells the next person which surfaces may use it; a token called shadow-md-2 does not.
3. Use a single light source for the whole scale, so offset, blur, and spread grow together as elevation rises. Two shadows lit from different directions on the same screen read as a rendering bug.
4. Give the dark theme its own values rather than reusing the light ones. A soft black shadow on a near-black surface is invisible, so dark surfaces need a raised surface colour, a border, or both to carry the elevation.
5. Do not let shadow be the only thing separating a surface from its background. Windows high-contrast mode, forced-colours mode, and print all strip shadows entirely, and anything relying on one alone dissolves into the page.

## 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

- In dark themes a black shadow against a dark background conveys nothing. Elevation there has to come from a lighter surface tint or a border, with the shadow reduced to a faint halo at most.
- Any surface that uses a shadow to mark its boundary needs a fallback boundary — a background shift or a hairline border — because forced-colours mode and high-contrast settings remove shadows without warning.
- Shadows that disagree about light direction or softness look broken when they appear together. A dropdown lit from above beside a card lit from the left is immediately noticeable even to users who cannot say why.
- Print and forced-colours output should flatten the scale to nothing, or to a single hairline, rather than attempting to render a blurred shadow as a grey smear.
- Stacked elevations compound: a popover inside a dialog inside a sticky panel will render three shadows on top of each other unless nested surfaces are allowed to drop back to a flat treatment.
- Shadows on large full-width surfaces are expensive to repaint during scroll, so a sticky header with a heavy shadow will cost frames on lower-end devices.
- The elevation role a shadow claims must match the paint order it actually gets — that ordering belongs to Elevation Layer Map and Z-Index Registry, and a shadow token must never be used to fake a stacking fix.

## 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 app has one shadow scale of no more than six steps, each named for an elevation role.
- [ ] No component declares a raw shadow value outside the scale.
- [ ] Every shadow in the scale shares a single light direction and a consistent softness progression.
- [ ] Dark theme surfaces are distinguishable from their background without relying on the shadow.
- [ ] Forced-colours mode and print render every elevated surface with a visible boundary.
- [ ] Nested elevated surfaces do not stack three or more shadows on the same visual edge.
- [ ] 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.