# Deep-Linkable Modals

## Objective

Make dialogs and detail panels linkable without losing the page behind them.

Modals and detail panels that live at their own URL, so they can be shared, bookmarked, and closed with the back button.

## 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. Pick the overlays worth linking — record detail, edit forms, share dialogs — and give each one a real URL.
2. When that URL is opened directly, with no page underneath, render the content as a full page instead of an overlay floating on nothing.
3. Make the browser back button close the modal and leave the user on the page they were on, not two steps back.
4. Restore focus to the element that opened the modal when it closes, and to a sensible landmark when the modal was opened from a cold URL.
5. Do NOT push a new history entry for every internal state change inside the modal — tab switches and accordion toggles should not each require a back press to escape.

## 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

- Opening a modal URL directly must render a standalone page, not a blank backdrop.
- The page behind the modal must not scroll, and must return to its previous scroll position on close.
- If the linked record was deleted or the viewer lacks permission, show that inside the overlay and keep the base route intact.
- Forward navigation must reopen the modal, not leave the URL and the UI disagreeing.
- A modal opened from a cold URL has no trigger element to return focus to — define where focus goes.
- Closing with unsaved changes must warn before discarding, including on back.

## 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 deep-linkable overlay has a shareable URL that works when pasted into a fresh tab.
- [ ] Direct loads render as a full page; in-app opens render as an overlay.
- [ ] Back closes the modal and stays on the underlying page.
- [ ] Focus is trapped while open and restored to the trigger on close.
- [ ] Deleted or forbidden content fails inside the overlay without breaking the base route.
- [ ] Background scroll is locked and restored.
- [ ] 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.