# Layout Shift Guard

## Objective

Stop content jumping as a page loads, and catch it when it starts happening again.

Reserved space for everything that arrives late, plus measurement that reports unexpected movement on the app's main screens.

## 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. Walk the app's main screens with the network throttled and note everything that moves after first paint. Images, avatars, embeds, banners, ad slots, and asynchronously loaded panels are the usual sources.
2. Give every image, video, iframe, and embed explicit dimensions or a fixed aspect ratio so its box exists before its content arrives, including the ones whose size comes from data.
3. Control the swap from fallback to web font so the two have compatible metrics, and accept a slightly imperfect fallback over a page that reflows once the real font lands.
4. Reserve space for anything conditional that appears above or beside existing content — validation messages, incident banners, trial notices, unsaved indicators — rather than letting it push the layout down when it arrives.
5. Do not count movement the user asked for. Expanding a row, opening a drawer, or loading the next page on a click is intended motion, and a guard that flags it will be turned off within a week.

## 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

- Media and embeds without declared dimensions collapse to nothing and then snap to full size; every one needs a reserved box, including those sized from record data.
- A fallback font with different metrics to the real one reflows every paragraph when the swap happens, so the two must be matched or the swap must be suppressed until the real font is ready.
- Banners, toasts, and validation messages are the most common late arrivals in an app that has otherwise been optimised, and each needs its space held open in advance.
- Movement caused directly by a user action must be excluded from measurement, or the signal is drowned in noise and stops being actionable.
- Sticky headers and action bars that change height on scroll will register as shifts unless their reserved height is fixed; this overlaps Sticky Action Bar, which owns the pinning behaviour while this entry owns the space reservation.
- A skeleton placeholder must match the real content's dimensions. A skeleton that is the wrong height causes the shift it was meant to prevent.
- Measurement must run on the devices and connections real users have, not only on a fast machine where everything arrives before first paint.

## 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 image, video, iframe, and embed on the main screens has reserved dimensions before its content loads.
- [ ] Font swapping produces no visible reflow of surrounding text.
- [ ] Conditional banners and validation messages appear in space already held for them.
- [ ] Layout movement is measured on the app's main screens and reported.
- [ ] Movement resulting from a direct user action is excluded from the measurement.
- [ ] Skeleton placeholders match the dimensions of the content that replaces them.
- [ ] 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.