# Table Column Chooser

## Objective

Let users choose which columns matter instead of forcing one table layout on everyone.

A control for showing, hiding, and reordering table columns, driven by a column definition rather than markup.

## 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. Derive the chooser from a single column definition that also drives the table itself. Two lists will drift apart.
2. Filter the offered columns by permission before rendering. A column the user cannot read must not appear in the chooser at all, not even greyed out — its presence leaks the shape of the data.
3. Mark identity and action columns as required so they cannot be hidden. A table with no way to tell rows apart or act on them is broken, not customised.
4. Provide a reset to the default column set, always reachable, including when the user has hidden almost everything.
5. Storage, scoping to device or account or workspace, and applying the saved choice before first paint are owned by Persistent View Preferences; use that mechanism rather than inventing a second one.

## 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

- A saved choice referencing a renamed or deleted column must be migrated or dropped silently, never crash the table.
- A newly added column needs a defined default: visible for everyone, or hidden until chosen. Decide, and state it.
- Hiding a column that is currently the sort key must either clear the sort or keep sorting by it with an explanation.
- Hiding a column must not remove it from exports unless the user asked for that — say which behaviour applies.
- The chooser must be usable with a keyboard, and reordering must have a keyboard path, not drag only.
- With many columns the chooser needs its own search and grouping, or it becomes a wall of checkboxes.

## 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 chooser and the table are generated from one column definition.
- [ ] Columns the user cannot view never appear in the chooser.
- [ ] Required identity and action columns cannot be hidden.
- [ ] Saved choices survive column renames and removals without error.
- [ ] A reset to defaults is always available.
- [ ] Persistence is delegated to Persistent View Preferences.
- [ ] 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.