# Resizable Panels

## Objective

Let users resize sidebars, inspectors, and work areas to fit the task.

Drag handles between panels, with minimum and maximum sizes, keyboard control, and sizes that persist.

## 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. Identify the panel boundaries worth making adjustable — usually a sidebar, a list, and a detail area. Not every divider needs a handle.
2. Give every panel a minimum and maximum size expressed relative to the container, and clamp to them during the drag rather than after it.
3. Make each handle focusable and operable with arrow keys, with a visible focus ring and an accessible name saying what it resizes.
4. Persist each size against the user and the specific layout it belongs to, and validate a restored size against the current container before applying it.
5. Do not resize on every mouse move if the panels contain expensive content. Move the handle during the drag and commit the layout on release.

## 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 saved size from a wide monitor must not swallow the whole layout on a laptop — clamp restored values.
- If the layout changes in a later release, stale saved sizes must be discarded rather than applied to panels that no longer exist.
- Dragging must not select text or trigger drag-and-drop inside the panels.
- Below the responsive breakpoint, panels should collapse to a stacked or single-pane layout and the handles should disappear entirely.
- Double-clicking a handle should reset that boundary to its default, and there must be a way to reset the whole layout.
- A panel dragged to its minimum must still show something meaningful, or collapse fully with an obvious way to bring it back.

## 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

- [ ] Every handle enforces a minimum and maximum size during the drag.
- [ ] Handles can be focused and moved with the keyboard, with a visible focus state.
- [ ] Sizes persist per user and are re-validated against the viewport on restore.
- [ ] The layout collapses to a single-column arrangement on small screens with no orphaned handles.
- [ ] There is a reset for an individual boundary and for the whole layout.
- [ ] Dragging never selects text or leaves the pointer capture stuck.
- [ ] 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.