# Product Variant Picker

## Objective

Let shoppers pick size and colour and see price, image, and stock update as they choose.

An option selector on the product page that resolves the chosen combination to a specific variant and updates price, imagery, and availability live.

## 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. Model options as named groups with ordered values, and map each valid combination to a concrete variant with its own price, stock level, and identifier. The picker resolves a selection to a variant; it does not guess.
2. Update price, availability, and the main image the moment a selection changes, without a page reload and without resetting the other groups the shopper has already chosen.
3. Reflect the selected variant in the URL so a shared or refreshed page opens on the same choice. Read that value on load and apply it before first paint so the page does not visibly switch options.
4. Drive the product's imagery through the existing Product Image Gallery rather than swapping a lone image element. When a variant has its own photos, the gallery switches to them; when it does not, it stays on the product's default set.
5. Do not let an invalid combination be submitted and rejected at add-to-cart. Compute which values are still reachable given the current selection and disable the rest, with a reason on the disabled value.

## 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

- Combinations that do not exist must be visibly unavailable in the picker, not discovered when add-to-cart fails. Recompute reachable values on every change, and when a shopper's earlier choice becomes impossible, tell them which one you cleared.
- The chosen variant belongs in the URL so it survives sharing, refresh, and the back button. Use the same identifier the server understands, and fall back to the default variant if the value is stale or unknown.
- Variants can differ in more than price. Weight, shipping class, and tax treatment all change the total, so the picker must surface a changed delivery cost rather than letting it appear as a surprise at checkout.
- A variant with no image of its own must fall back to the product's default imagery rather than rendering a broken or empty frame.
- Option groups must be keyboard navigable with arrow keys and announced as a named group with a current selection. Colour swatches need a text label as well as the colour; colour alone is not a label.
- A product with a single variant should not show a picker at all, and a product with hundreds of combinations needs the groups presented as selects rather than a grid of swatches.
- Price and stock must be re-checked server-side when the item is added. The picker's view of availability is a convenience, not authority.
- Selecting an out-of-stock but real combination should still be allowed if the app offers back-in-stock notification, so the shopper can register interest rather than hit a dead end.

## 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

- [ ] Choosing options updates price, availability, and imagery without a page reload.
- [ ] Combinations that do not exist are disabled with a reason, and add-to-cart never fails on an invalid selection.
- [ ] The selected variant is encoded in the URL and restored on load before first paint.
- [ ] Variants without their own images fall back to the product default with no broken frame.
- [ ] Option groups are operable by keyboard and announced with their current selection.
- [ ] Variant-specific price, weight, and shipping differences are reflected before checkout.
- [ ] 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.