# Alignment Guides and Snapping

## Objective

Line elements up exactly while dragging, instead of landing a few pixels off.

Live guide lines and snapping that align a dragged element to the edges, centres, and spacing of its neighbours.

## 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. Snap to more than edges. Match left, right, top, bottom, horizontal centre, vertical centre, and equal spacing between three or more elements, and draw a guide for whichever match wins.
2. Hold a modifier key to suspend snapping entirely for the duration of a drag, so an author who genuinely wants an element two pixels off the grid can place it.
3. Compute the snap threshold in canvas coordinates, not screen pixels, so the pull feels the same at every zoom level. A fixed pixel threshold becomes unusable when zoomed out.
4. Restrict the candidate set to elements that are visible and near the dragged element rather than testing every node in the document, or the drag will stutter on a long page.
5. If the app already has drag-and-drop reordering, extend that drag layer rather than starting a second one. Do not run two independent drag implementations on the same canvas, because they will fight over pointer capture and produce dropped elements.

## 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

- Snapping with no escape hatch is a trap. Provide a modifier key that disables it mid-drag, and make the disabled state visible so the author knows why the guides vanished.
- Testing every element on the canvas against every drag frame will drop frames on a page of any size. Limit candidates to nearby, visible elements and recompute the set at the start of the drag rather than on each frame.
- Aligning only to edges leaves elements that look wrong. Centre alignment and equal spacing between siblings are what authors actually reach for, so match those too and label the spacing guide with its measurement.
- When several targets fall inside the threshold at once, guides will flicker between them frame by frame. Pick the closest match and require a clear margin before switching, so the guide holds still.
- A snap threshold measured in screen pixels pulls far too hard when zoomed out and barely at all when zoomed in. Scale the threshold by the current zoom so the feel is constant.
- Guides drawn only as thin coloured lines are invisible to some users and against some backgrounds. Give them sufficient contrast on both light and dark canvases and announce the resulting alignment for keyboard users.
- Keyboard nudging must be able to reach the same aligned positions as dragging, otherwise anyone who does not use a pointer cannot align anything.
- Snapping must not silently move an element after the drag ends. The position the author released at, adjusted by the shown guide, is the position that gets saved.

## 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

- [ ] Dragging shows guides for edge, centre, and equal-spacing alignment against nearby elements.
- [ ] A modifier key suspends snapping mid-drag and the interface shows it is suspended.
- [ ] The snap threshold scales with zoom so behaviour is consistent at any level.
- [ ] Guides stay stable when multiple targets are within range, with no flicker.
- [ ] Dragging stays smooth on a canvas with many elements.
- [ ] Guides are legible on light and dark canvases and keyboard nudging reaches the same positions.
- [ ] 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.