# Before and After Slider

## Objective

Let people drag a handle to compare two versions of the same image.

A pair of images stacked in one frame with a draggable divider that reveals more of one and less of the other.

## 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. Fix the frame using the app's existing aspect-ratio handling and render both images inside it, so the layout never depends on when either image finishes loading.
2. Make the handle a real focusable control that moves with arrow keys in small steps and jumps to the ends with home and end, with the current position exposed as a value rather than only as a visual.
3. Hold the after image hidden until both images have decoded, then reveal them together. A comparison that shows one image alone for a moment reads as a broken layout.
4. Clamp the divider to the frame bounds and keep tracking the pointer after it leaves the frame, releasing cleanly when the button is released anywhere on the page.
5. Do not require the caller to supply pre-composited images. Accept two independent image references and let the component do the clipping, so the same pair can be reused elsewhere in the app.

## 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

- Two images of different dimensions will not line up, and the comparison becomes meaningless. Either reject the mismatch at the point the pair is configured or letterbox both to a shared frame using one consistent rule.
- A handle that only responds to dragging excludes keyboard users entirely. Arrow keys must move the divider in fine steps, with larger jumps available, and the control must be reachable by tab.
- Dragging past the edge of the frame must clamp the divider at zero or one hundred percent rather than producing a negative offset or letting the images detach from the frame.
- If the second image loads after the first, it will flash into place under the divider. Wait for both to decode before revealing, and show a placeholder at the reserved size until then.
- A vertical finger movement that starts on the handle must let the page scroll instead of being captured as a drag. Only take over the gesture once horizontal movement dominates.
- Both images need their own alt text, and the labels for which side is before and which is after must be visible rather than assumed from left and right.
- On a narrow screen the handle must stay large enough to grab with a thumb, which usually means a wider hit area than the visible divider line.

## 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 frame is sized before either image loads and never shifts.
- [ ] Mismatched dimensions are rejected or letterboxed by one consistent rule.
- [ ] The handle is keyboard operable and exposes its position accessibly.
- [ ] Dragging beyond the frame clamps cleanly and releases correctly.
- [ ] Neither image is visible alone before both have loaded.
- [ ] Vertical page scrolling still works when a touch gesture starts on the handle.
- [ ] Both images carry alt text and the sides are labelled.
- [ ] 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.