# Sticky Action Bar

## Objective

Keep the primary actions visible when the page is longer than the screen.

A bar that pins the save, submit, or apply actions to the viewport once they would otherwise scroll away.

## 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 long forms and detail pages where the primary action sits below the fold, and pin those actions.
2. Let the bar sit inline in the normal flow until it is about to leave the viewport, then stick. Reserve its height so nothing shifts at the moment it changes state.
3. Keep the bar's DOM position fixed in source order so tab order does not change when it sticks. Moving the element on scroll breaks keyboard navigation.
4. Mirror the real state of the form in the bar: disabled while invalid, pending while saving, hidden when the user has no permission to act.
5. Do not stack a sticky action bar on top of an existing sticky header, mobile bottom navigation, or cookie banner. Pick one owner of the bottom edge.

## 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

- The bar must respect safe-area insets so it clears the iOS home indicator.
- An on-screen keyboard on mobile will fight a bottom-fixed bar — decide whether it rides above the keyboard or unsticks entirely.
- Page content must gain bottom padding equal to the bar's height, or the last field is unreachable.
- If permissions change mid-session, the bar's actions must update, not go stale.
- On short viewports where the bar would take a third of the screen, fall back to inline actions.
- Validation errors summarised in the bar must link to the offending field, not just say the form is invalid.

## 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

- [ ] The primary action is reachable at any scroll position on a page longer than the viewport.
- [ ] Nothing jumps when the bar transitions between inline and stuck.
- [ ] Tab order and accessible labels are identical in both states.
- [ ] No content is trapped under the bar, including on notched devices.
- [ ] Disabled, pending, and no-permission states in the bar match the form's real state.
- [ ] 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.