# Enter and Exit Animation Pattern

## Objective

Animate overlays, menus, and panels in and out without flicker or orphaned elements.

A shared lifecycle for anything that appears and disappears, keeping the element present until its exit finishes and returning focus at the correct moment.

## 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 everything in the app that appears and disappears — dialogs, drawers, menus, popovers, toasts, tooltips, expanding rows — and give them all one lifecycle rather than each rolling its own.
2. Hold the element in the document through its exit and remove it only once the exit has actually completed or been interrupted. Removing it at the moment state changes means the exit never plays.
3. Move focus into the surface once its entrance is far enough along to be visible, and return it to the element that opened the surface only after the exit is done, so the page does not scroll to a target that is still fading.
4. Make every sequence interruptible. Reopening something mid-exit should reverse from its current position, and closing during an entrance should not leave the element stranded halfway.
5. Do not tie removal to a fixed timer that happens to match the duration. Timers drift from the real animation, and any change to the motion tokens silently breaks the element's removal.

## 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

- An element removed the instant its state flips will never show its exit; it must stay mounted, and inert to interaction, until the exit resolves.
- Focus handling is the part that most often goes wrong — moving it in too early strands the user on an invisible surface, and returning it too late lets a keystroke land on the page behind.
- Interrupted sequences must reverse from where they are. An element caught between entering and leaving must not jump to either extreme or end up stuck with no state at all.
- Under reduced motion the surface appears and disappears immediately, and the mount and unmount timing must still be correct — the lifecycle does not disappear just because the animation does.
- An element still mid-exit must not be reachable by tab, must not be announced to screen readers, and must not block clicks on what is behind it.
- If the surface is removed for a reason unrelated to the user — a route change, a session expiry, a record being deleted — it must be torn down cleanly rather than left animating against a page that no longer exists.
- Nested surfaces, such as a popover inside a dialog, need a defined order so the inner one finishes leaving before the outer one starts.

## 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 appearing and disappearing surface in the app uses the same lifecycle.
- [ ] Exit animations play fully; no element is removed before its exit completes.
- [ ] Focus enters after the entrance is visible and returns only after the exit finishes.
- [ ] Interrupting an entrance or exit reverses from the current position with no jump or stranded element.
- [ ] Reduced motion removes the animation while leaving mount and unmount timing correct.
- [ ] An exiting element is not focusable, not announced, and does not intercept clicks.
- [ ] Route changes and forced closures tear surfaces down without leaving orphaned nodes.
- [ ] 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.