# Baseline Grid Overlay

## Objective

Check vertical rhythm against a real baseline grid instead of judging it by eye.

A configurable baseline and column grid drawn over the interface for typography and vertical-rhythm review.

## 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 default baseline interval from the app's existing type scale and line heights rather than picking a round number, so the grid describes the system the app already claims to follow.
2. Let the reviewer change the interval, offset, colour, and opacity from the tool itself, and remember the last settings, because a grid that has to be reconfigured on every reload does not get used twice.
3. Anchor the grid to the document origin and keep it registered to the content as the page scrolls, so a heading four screens down can be checked against the same rhythm as the first one.
4. Offer a column grid alongside the baseline, matched to the app's real container widths and gutters, and let each be toggled independently.
5. Do not treat this as a linter. This feature owns drawing the reference; deciding that an element is off-grid and reporting it as a defect belongs to UI Lint Rules, and measuring the actual gap between two boxes belongs to Spacing Overlay.

## 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

- Browser zoom and fractional device pixel ratios make hairlines drift or disappear; recompute line positions from the real rendered scale rather than assuming one CSS pixel equals one device pixel.
- If the app supports more than one density or compact mode, the baseline interval changes with it — the overlay must follow the active mode rather than always drawing the comfortable-mode grid.
- The grid must stay fixed to the content, not to the viewport, or every scroll appears to shift the rhythm and the review is worthless.
- Some elements are deliberately off-grid for optical reasons — icon alignment, cap-height corrections, badges. Provide a way to mark and remember those exceptions so they stop being re-reported at every review.
- Baselines sit at the text baseline, not the top of the line box; aligning the grid to line-box tops will make correctly set type look wrong.
- The overlay must sit above the interface but below modals and toasts, or it becomes impossible to review a dialog against the grid.
- The grid must be absent from production builds, and its toggle must not be reachable by a signed-in customer who guesses a URL parameter.

## 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 baseline grid can be toggled on any screen, with configurable interval, offset, and opacity.
- [ ] Grid settings persist between reloads.
- [ ] Lines remain crisp and correctly spaced at every zoom level and device pixel ratio.
- [ ] The grid follows the active density mode's baseline interval.
- [ ] The grid stays registered to the content while scrolling rather than to the viewport.
- [ ] Deliberate optical exceptions can be marked and are not re-flagged.
- [ ] The overlay is unavailable in production builds.
- [ ] 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.