# Image Lightbox

## Objective

Open images at a useful size without sending users to a raw file URL.

An overlay viewer that shows an image at full size with zoom, keyboard control, and a clean way out.

## 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. Open the viewer as a modal layer with focus trapped inside it, and return focus to the thumbnail that opened it on close.
2. Support Escape, a visible close button, a click on the backdrop, and the browser back button. Back must close the viewer rather than leaving the page.
3. Show the low-resolution thumbnail immediately and swap in the full image when it loads, so the viewer is never a blank rectangle.
4. Support pinch and double-tap zoom on touch, and keyboard zoom and pan on desktop, with a reset to fit.
5. Do not load the original file at full resolution on a phone. Request a size appropriate to the viewport and device pixel ratio, or a large photo will exhaust memory and kill the tab.
6. The surrounding file list, ordering, and captions are owned by Attachment Gallery; this feature is the viewer only.

## 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

- A broken, deleted, or still-processing image must show a clear state inside the viewer, not an empty overlay or a browser error icon.
- A private image whose signed URL expires while the viewer is open must re-authorize or fail with an explanation.
- Background scrolling must be locked while open, and restored to the same position on close — including on iOS, where this is not automatic.
- Swipe to dismiss and swipe to pan conflict once zoomed; dismissal must only apply at the default zoom level.
- The image needs its alt text carried into the viewer; a decorative thumbnail becoming a focal image without a description is a regression.
- Very tall or very wide images must fit to the viewport on open rather than opening at native size.

## 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

- [ ] Focus is trapped while open and restored to the trigger on close.
- [ ] Escape, the close button, the backdrop, and browser back all close the viewer.
- [ ] A placeholder or thumbnail is visible while the full image loads.
- [ ] Zoom works by pinch, double-tap, and keyboard, with a reset to fit.
- [ ] Requested image size is bounded by viewport and device pixel ratio.
- [ ] Broken, private, and processing images each have their own state.
- [ ] Background scroll is locked and restored exactly.
- [ ] 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.