# Funnel Analytics

## Objective

Show where people move forward in a journey and where they drop out.

Definable multi-step funnels with conversion windows, identity stitching, and versioned definitions so historical results stay stable.

## 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. Let a user define a funnel as an ordered list of steps, each backed by an existing tracked event, with optional property filters per step.
2. Make ordering an explicit choice: strict ordered funnels require steps in sequence, unordered ones only require all steps to occur. State which mode a funnel is in on the chart.
3. Require a conversion window per funnel — a fixed span from first step to last. Without one, every funnel eventually converges on 100% and means nothing.
4. Stitch anonymous activity to the authenticated identity when a user signs in, so the top of the funnel is not permanently orphaned from the bottom.
5. Version funnel definitions. Editing steps must create a new version; historical results stay attached to the definition that produced them.
6. Do NOT define, rename, or capture events here — event capture, naming, and schema are owned by Event Tracking. Funnels read from it.

## 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

- Bots and internal traffic inflate the first step and deflate every rate below it. Exclude them by default and say so on the chart.
- Repeated events from one person must count once per funnel entry, not once per event.
- A user who completes step three without step two must be handled by the ordering rule, not quietly counted as converted.
- Users who enter near the end of the reporting range have not had a full conversion window yet; either exclude them or label the recent buckets as incomplete.
- Stitching must not merge two people who shared a device — bind to the sign-in, and never retroactively merge on a weak signal like IP address.
- A renamed or deprecated event must break the funnel loudly rather than silently reporting zero at that step.
- Small cohorts produce unstable percentages; show the underlying counts next to every rate.
- Deleted user data must disappear from funnel results too, which means recomputing or excluding rather than trusting a cached aggregate.

## 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

- [ ] A funnel is defined as ordered steps over existing tracked events with per-step filters.
- [ ] Ordered versus unordered mode and the conversion window are both explicit and shown on the chart.
- [ ] Anonymous sessions are stitched to identities on sign-in without merging distinct people.
- [ ] Bot and duplicate events are excluded, and the exclusion is disclosed.
- [ ] Editing a funnel creates a new version and leaves historical results intact.
- [ ] Absolute counts are shown alongside every conversion rate.
- [ ] Event capture and naming remain owned by Event Tracking.
- [ ] 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.