# Overlay Coordination Layer

## Objective

Give every menu, dialog, and drawer one set of rules for stacking, focus, and dismissal.

A single layer owning stacking order, focus handling, background inertness, scroll locking, and dismissal for all floating surfaces.

## 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. Inventory every floating surface the app already has — menus, popovers, tooltips, dialogs, drawers, backdrops, command palettes — and route each of them through the one layer rather than leaving each component to fend for itself.
2. Maintain an ordered stack of what is open. The topmost surface owns the Escape key and outside-click dismissal, and both must close exactly one level rather than everything at once.
3. Record the element that opened each surface, contain focus while it is open, and return focus to that element on close, including when the surface closes because a route changed.
4. Treat scroll locking and background inertness as counted resources held by the stack, released only when the last holder closes, so a nested dialog closing does not unlock the page beneath its parent.
5. Do not let components carry their own stacking constants. Assigning arbitrary depth values per component is exactly how a dropdown ends up behind the dialog that contains it, and the layer exists to remove that decision from every component.

## 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

- Nested overlays must work in both directions — a menu opened inside a dialog, and a dialog opened from a menu. Closing the inner one must leave the outer one open, focused, and still locking scroll.
- Escape must be coordinated by the stack. If every open surface listens for it independently, one keystroke closes the tooltip, the menu, and the dialog together and the user loses unsaved work.
- Background content must be marked inert while a surface is open and unmarked exactly on close. Leaving the flag set makes the whole page unreadable to a screen reader; setting it on an ancestor of the overlay hides the overlay itself.
- A component that throws or unmounts while open must not leave the page permanently scroll-locked or focus-trapped. Register cleanup with the layer so an error path still releases the lock.
- A route change, a sign-out, or a session expiry while a surface is open must close the stack cleanly rather than leaving an orphaned backdrop over the new page.
- On mobile, the on-screen keyboard changes the usable height under a drawer or sheet, and dismissal by swipe must not conflict with scrolling the surface's own content.
- Where these surfaces are mounted in the document is owned by the Portal Host Registry brief; this brief decides ordering, focus, and dismissal, and must not also create its own mount points.

## 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 floating surface in the app is opened and closed through the one layer.
- [ ] Stacking order is derived from the open stack, and no component declares its own depth value.
- [ ] Escape and an outside click each dismiss exactly one level of the stack.
- [ ] Focus is contained while a surface is open and returned to the opening element on close, including on route change.
- [ ] Scroll lock and background inertness are released only when the last open surface closes.
- [ ] An error or unmount while a surface is open leaves the page scrollable and focusable.
- [ ] Mounting is delegated to the portal host registry 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.