# Transition Consistency

## Objective

Decide once what animates, for how long, and when motion is left out entirely.

A single set of rules governing which properties may be transitioned across the app, and which interactions deliberately have no motion at all.

## 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. Audit the app for every transition currently in place and record what it animates. Anything transitioning a property that forces the browser to recompute layout is the first thing to replace.
2. Restrict transitions to properties that can be composited — opacity and transforms — and express size and position changes that way rather than animating width, height, or offsets directly.
3. Write down the interactions that must never animate: value updates in a table the user is scanning, error messages appearing, and anything that would delay the confirmation of an action.
4. Make motion decorative rather than load-bearing. The state change lands immediately and the animation describes it afterwards, so a user on reduced motion or a slow device sees the same result at the same moment.
5. Do not implement this by adding a blanket transition to every element. A universal rule catches things it was never meant to, and produces elements that visibly drift when they should appear instantly.

## 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

- Transitions on properties that trigger layout or paint on every frame will stutter on modest hardware and on long lists; move them to transforms and opacity or drop them.
- Motion must never sit between the user and the result of their action — a button that waits for an animation to finish before committing feels broken regardless of how good the animation is.
- Rapid toggling must be handled explicitly. Clicking a disclosure five times quickly should end in the correct final state, with each transition picking up from wherever the last one reached.
- When the user has asked for reduced motion the app must honour it at the level of the whole system, and the layout must be identical with motion on and off.
- Durations and curves themselves are owned by Motion Token System; this entry decides which properties are eligible and where motion is omitted, and should reference those tokens rather than defining new values.
- Transitions inside a virtualised or paginated list must not fire when rows are recycled, or scrolling produces a wall of animation.
- An element that is animating must still be clickable and focusable throughout, not inert until the motion settles.

## 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

- [ ] Transitions are limited to compositable properties across the whole app.
- [ ] No interaction waits on an animation before its result is committed.
- [ ] Rapid repeated toggling always settles in the correct final state.
- [ ] Reduced-motion preferences are honoured system-wide, with identical layout in both modes.
- [ ] Timing values come from the shared motion tokens rather than being defined locally.
- [ ] Recycled rows in long lists do not replay entrance transitions.
- [ ] 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.