# Split View

## Objective

Let users compare or edit two records side by side without opening another tab.

Two independent panes, each holding its own record, its own URL, and its own loading and error state.

## 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. Represent both panes in the URL so a split view can be linked, bookmarked, and restored. One pane's record must not be inferable from the other's.
2. Give each pane its own fetch, its own loading state, and its own error state. A failure in one pane must leave the other usable.
3. Scope actions to the pane they were triggered in. Saving, deleting, or navigating in one pane must never silently swap the other pane's record.
4. Below the responsive breakpoint, show one pane at a time with an explicit control to switch, and keep the inactive pane's state rather than discarding it.
5. Drag handles, minimum and maximum widths, keyboard resizing, and size persistence are owned by Resizable Panels; extend that rather than building a second resize implementation.

## 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

- Opening the same record in both panes must either be prevented or kept genuinely in sync — two stale copies of one record is the worst outcome.
- Deleting the record in one pane must leave that pane in a clear removed state, not a blank frame or a redirect that closes the split.
- A restored split view must handle a record that is now deleted or no longer permitted, showing that pane's own error rather than failing the whole view.
- Unsaved edits in a pane must survive the other pane navigating, and must be flagged before the split is closed.
- Keyboard focus must move between panes deliberately, and each pane needs a landmark or label so screen reader users know which is which.
- A global search or command action must have a defined target pane — the focused one — rather than picking one at random.

## 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

- [ ] Both panes are encoded in the URL and the view restores from it.
- [ ] Each pane loads, errors, and empties independently.
- [ ] An action in one pane never changes the other pane's record.
- [ ] Narrow screens present a single pane with an explicit switch that preserves both panes' state.
- [ ] Restoring a split view with a deleted or forbidden record degrades that pane only.
- [ ] Resize behaviour is delegated to Resizable Panels, not reimplemented.
- [ ] 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.