# Motion system — Deck Hearth

Reference for the motion-token surface and the `prefers-reduced-motion`
contract. Motion tokens are defined in `styles/globals.css` alongside
the [`design-tokens`](DESIGN_TOKENS.md) and consumed as
`var(--motion-duration-…)` / `var(--motion-ease-…)`.

## Why bother

Motion shapes how an app *feels* more than any other surface. Loose
animations (random durations, jittery easings, no reduced-motion
respect) read as amateur. The 4-tier taxonomy here exists so every
new animation lands in one of four buckets — never an arbitrary
duration — and every animation is reduced-motion safe by default.

## Duration taxonomy

Four discrete durations + an instant escape hatch. Pick the bucket
that matches the *meaning*, not the visual feel.

| Token | Value | Use | Examples |
| --- | --- | --- | --- |
| `--motion-duration-instant` | `0ms` | Theme/route transitions where any duration is wrong; hover state for keyboard-only users | Color flips on `data-theme` change |
| `--motion-duration-quick` | `150ms` | Hover, focus, micro-feedback. Snappy enough that users don't perceive it as animation | Button hover scale, focus-ring fade-in |
| `--motion-duration-default` | `250ms` | Default for state transitions. The token most CSS `transition:` blocks should reach for | Card hover, link color, glass-surface opacity |
| `--motion-duration-slow` | `400ms` | Entering/exiting layout surfaces. Slow enough to read; fast enough not to feel sluggish | Modal scale/opacity, sidebar drawer slide, popover open |
| `--motion-duration-deliberate` | `600ms` | Hero moments, celebration, onboarding. Use sparingly — feels heavy if overused | Empty-state illustrations, scanner success confetti |

**Rule of thumb:** if you can't justify why a duration is *not* `default`,
use `default`.

## Easing taxonomy

Three curves. Two ease-outs (one calm, one playful) plus linear for
progress indicators.

| Token | Curve | Use |
| --- | --- | --- |
| `--motion-ease-out` | `cubic-bezier(0.16, 1, 0.3, 1)` | Default for entrances + state changes. Decelerates softly — reads as "settling." |
| `--motion-ease-spring` | `cubic-bezier(0.34, 1.56, 0.64, 1)` | Slight overshoot for delight (button press, modal open). Use only when the motion is the *point* — sparingly. |
| `--motion-ease-linear` | `linear` | Progress indicators (spinners, loading bars). Any non-linear curve here implies state change, which is wrong for a progress affordance. |

## Composition recipes

| Recipe | CSS |
| --- | --- |
| Default hover | `transition: all var(--motion-duration-quick) var(--motion-ease-out);` |
| Default state change | `transition: var(--motion-duration-default) var(--motion-ease-out);` |
| Modal enter | `transition: opacity var(--motion-duration-slow) var(--motion-ease-out), transform var(--motion-duration-slow) var(--motion-ease-spring);` |
| Sidebar slide | `transition: transform var(--motion-duration-slow) var(--motion-ease-out);` |
| Loading spinner | `animation: spin var(--motion-duration-deliberate) var(--motion-ease-linear) infinite;` |

## The `prefers-reduced-motion` contract

WCAG 2.2 SC 2.3.3 Level AAA: provide a mechanism for users to disable
non-essential motion. Operating systems already expose this preference;
`@media (prefers-reduced-motion: reduce)` reads it and we honor it.

The site-wide rule in `styles/globals.css` collapses every animation
+ transition to `0.01ms` when the user has reduced motion enabled.
End-states are preserved (vs `animation: none` which can flicker).

### When motion is *essential*

Some motion is essential to communicate state — a loading spinner
indicating in-flight work, an actively-scanning camera reticle. For
those, add the `.motion-essential` class to the animating element
(or to a parent — children inherit via `.motion-essential *`):

```html
<div class="motion-essential">
  <svg class="animate-spin">…</svg>
</div>
```

Use this **only** when stopping the animation would hide meaningful
state. A purely decorative bounce or shimmer is NOT essential — leave
it to collapse with reduced motion.

### Current `motion-essential` consumers

None yet — when sub-convoy #5 (`liquid-glass-card-surfaces`) lands the
scanner reticle as a glass-aware motion, it will be the first
documented consumer.

## Audit of existing animations

These keyframes pre-date the motion taxonomy and continue to play
under the per-class rules in `styles/globals.css`:

| Animation | Duration (legacy) | Status |
| --- | --- | --- |
| `mythic-sparkle` | 4s infinite | Decorative, collapses under reduced motion. Reconciled with glass in #5. |
| `rare-shimmer` | 3s infinite | Decorative, collapses under reduced motion. |
| `uncommon-twinkle` | 2.5s infinite | Decorative, collapses under reduced motion. |
| `enchanted-rainbow` | 3s infinite | Decorative, collapses under reduced motion. |
| `float` (logo) | 4s infinite | Decorative, collapses under reduced motion. |
| `fire-glow` (page background) | 12s infinite | Scheduled for deletion by `cleanup-legacy-design-css` (#8). Replaced by localized `ember-float` on landing hero only. |
| `ember-float` | 10s linear infinite | Kept for landing hero accent. Collapses under reduced motion. |

## Adding a new animation

1. Pick a duration token — never a literal `ms` value.
2. Pick an easing token — never a literal cubic-bezier.
3. Test with the OS reduced-motion preference toggled ON. The
   animation should collapse without breaking the layout.
4. If the animation is essential, wrap in `.motion-essential` and
   document the why in a code comment.

## Related convoys

- `.convoys/motion-system-pass.md` — this convoy.
- `.convoys/liquid-glass-design-tokens.md` — token surface motion lives in.
- `.convoys/cleanup-legacy-design-css.md` — deletes the legacy
  page-level `fire-glow` background animation.