# Async Boundary Component

## Objective

Give every asynchronous region the same set of states instead of hand-rolling each one.

A shared wrapper that renders pending, error, empty, refreshing, and ready for any region backed by a request.

## 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. Find the regions of the app that depend on a request — lists, detail panels, widgets, charts, dropdown options — and route each one through the shared boundary rather than a local set of flags.
2. Model the states explicitly and exhaustively so that pending, error, empty, refreshing, and ready are the only possibilities, and the caller cannot render a sixth thing by accident.
3. Distinguish the first load, which has nothing to show, from a refetch, which already has content. The first deserves a skeleton shaped like the eventual result; the second deserves a quiet indicator over what is already on screen.
4. Make the empty state a first-class slot the caller fills, because an empty search result, an empty inbox, and a filtered-to-nothing table say different things and need different next actions.
5. The mechanics of the state graph itself — legal transitions, cancellation, timeouts — belong to UI State Machine Pattern; this boundary owns what those states look like and where they are placed. Take the machine from there rather than defining a second one here.

## 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 background refresh must keep the previous content visible. Dropping back to a skeleton because a poll fired makes a stable screen flash for no reason, and it throws away the user's scroll position.
- The initial load and a later refetch are not the same event and must not share one flag. Collapsing them is what produces a full-page spinner on every keystroke of a filter.
- Every error state needs a retry that re-runs only that region's request, and every in-flight request needs to be cancellable when the user navigates away or changes the parameters underneath it.
- Boundaries nest, and a parent skeleton wrapped around three child skeletons reads as a broken page. Decide which level owns the visible pending treatment and have the inner ones render nothing while the outer one is pending.
- A request that resolves in twenty milliseconds should not flash a loader at all — hold the pending treatment behind a short delay, and once it is shown, keep it long enough not to strobe.
- A response that arrives after its parameters have changed must be discarded, or a slow earlier request will overwrite the results of a newer one.
- The transition into the ready state must not shift the page. Reserve the space the content will occupy so the skeleton and the result are the same 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

- [ ] Every request-backed region in the app renders through the shared boundary.
- [ ] Pending, error, empty, refreshing, and ready are the complete and only set of rendered states.
- [ ] A background refresh leaves previous content and scroll position in place.
- [ ] Errors offer a retry scoped to that region, and in-flight requests are cancelled on navigation or parameter change.
- [ ] Nested boundaries produce a single pending treatment rather than stacked skeletons.
- [ ] Fast responses complete without any loader appearing, and no loader flashes for less than its minimum duration.
- [ ] The state machine is taken from UI State Machine Pattern rather than 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.