animate

Files (3)

• RECIPES.md
• SKILL.md
• VOCABULARY.md

SKILL.md (first lines)

# Building Animations

A construction skill. It does ONE thing: turn a request for motion into an implementation that would survive a strict review. It does not audit a codebase (that's `improve-animations`), critique a diff (that's `review-animations`), or hunt for places that could animate (that's `find-animation-opportunities`). React Native is out of scope.

## Operating Posture

You are a senior design engineer building the animation yourself. The bar is the one `review-animations` enforces. Write it so it passes that review the first time.

Two failure modes, and the first is worse:

1. **Animating something that shouldn't animate.** The gate below exists to produce zero lines of code sometimes. That's a success, not a dodge.
2. **Animating the right thing with the wrong ingredients** — `ease-in` on an entrance, `scale(0)`, keyframes on a toast, a duration that makes a dropdown feel sluggish.

Never present motion options as a menu. Make the call, state the reasoning in one line, write the code.

## Hard Rules

1. **Run the sequence in order.** Steps 1 and 2 gate everything. Don't reach for a curve before you know whether it animates at all.
2. **No approximated values.** Every curve, duration, and spring config comes from the tables below. Never invent `cubic-bezier(0.4, 0, 0.2, 1)` because it looks familiar.
3. **Extend the codebase's tokens, don't fork them.** If `--ease-out` or a duration scale already exists, use it. Adding a parallel system is a defect.
4. **Reduced motion and hover gating ship with the animation**, not as a follow-up.
5. **Cheapest tool that works.** Don't install a motion library for a fade.

## The Build Sequence

### 1. Should this animate at all?

| Frequency | Decision |
| --- | --- |
| 100+ times/day (keyboard shortcuts, command palette toggle) | **No animation. Ever.** Stop here. |
| Tens of times/day (hover effects, list navigation) | Near-imperceptible only — fast and subtle, or nothing |
| Occasional (modals, drawers, toasts) | Standard animation |
| Rare / first-time (onboarding, success, celebration) | The delight budget lives here |

**Keyboard-initiated actions are a disqualifier, not a judgment call.** Raycast has no open/close animation — that is correct for something opened hundreds of times a day.

If the request fails this gate, say so plainly and don't write the animation. Offer the non-motion alternative (instant state change, a static affordance) instead.

### 2. What is the purpose?

Name it in one of these words before continuing:

- **Feedback** — confirming the interface heard the user
- **Spatial consistency** — showing where something came from or went
- **State indication** — making a state change legible
- **Preventing a jarring change** — bridging content that would otherwise teleport
- **Explanation** — demonstrating how something works (marketing/onboarding only)
- **Delight** — allowed *only* at the rare/first-time tier

Can't name it? Don't build it. "It looks cool" on a frequently-seen element is a reason to stop.

Also check **function**: data the user is reading or acting on should not move for style. A decorative mouse-tracking effect belongs on a marketing page, not on a graph in a banking app.

### 3. Pick the tool — cheapest that works

Walk down; stop at the first that fits.

| Need | Tool |
| --- | --- |
| Hover, press, color, a state toggle you control with a class or attribute | **CSS transition** |
| Entry animation on mount, no JS state | **CSS `@starting-style`** |
| Predetermined motion that must stay smooth while the page is busy loading | **CSS animation** (runs off the main thread) |
| Programmatic control with CSS performance, no library | **WAAPI** (`element.animate()`) |
| Springs, layout animations, exit animations, gesture-driven values | **Motion** (`motion.dev`) |

CSS animations beat JS under load — they run off the main thread, while `requestAnimationFrame`-based animation drops frames while the browser loads, scripts, or paints. Use CSS for predetermined motion, JS for dynamic and interruptible motion.

If the task needs a *component* rather than an animation — a toast, a drawer, a command menu, a dropdown — stop and invoke `pick-ui-library`. Hand-rolling those is how you end up with a `<div>` dropdown and no focus management.

### 4. Pick the properties

- **`transform` and `opacity` only.** They skip layout and paint and run on the GPU. `width`/`height`/`margin`/`padding`/`top`/`left` trigger all three. (`clip-path` is the sanctioned fourth — see RECIPES.md. `height` is tolerated only for accordions, where there's no transform equivalent.)
- **Never `scale(0)`.** Start from `scale(0.9–0.97)` + `opacity: 0`. Nothing in the real world appears from nothing.
- **`transform-origin` at the trigger** for popovers, dropdowns, menus, tooltips — `var(--transform-origin)` in Base UI. **Modals are exempt**; they're not anchored to a trigger, so they stay centered.
- **Percentages in `translate()`** are relative to the element's own size — `translateY(100%)` moves by its own height whatever the content. Prefer over hardcoded pixels.
- **In Motion, use the full transform string.** `x`/`y`/`scale` shorthands are not hardware-accelerated and drop frames under load:

```jsx
<motion.div animate={{ x: 100 }} />                          // drops frames under load
<motion.div animate={{ transform: "translateX(100px)" }} />  // hardware accelerated
```

- **Never drive a child's transform from a CSS variable on the parent** — it recalculates styles for every child. Set `transform` on the element directly.

### 5. Easing and duration — or a spring

**Easing**, in decision order:

| Situation | Easing |
| --- | --- |
| Entering or exiting | `ease-out` |
| Moving / morphing on screen | `ease-in-out` |
| Hover / color change | `ease` |
| Constant motion (marquee, progress) | `linear` |
| Default | `ease-out` |

**Never `ease-in` on UI.** It starts slow, delaying the exact moment the user is watching. `ease-out` at 200ms *feels* faster than `ease-in` at 200ms.

Built-in CSS easings are too weak. Use these:

```css
--ease-out: cubic-bezier(0.23, 1, 0.32, 1);        /* strong ease-out for UI */
--ease-in-out: cubic-bezier(0.77, 0, 0.175, 1);    /* strong ease-in-out for on-screen movement */
--ease-drawer: cubic-bezier(0.32, 0.72, 0, 1);     /* iOS-like drawer curve (Ionic) */
```

Need a curve that isn't here? Take it from [easing.dev](https://easing.dev/) or [easings.co](https://easings.co/). Don't hand-roll one.

**Duration:**

| Element | Duration |
| --- | --- |
| Button press feedback | 100–160ms |
| Tooltips, small popovers | 125–200ms |
| Dropdowns, selects | 150–250ms |
| Modals, drawers | 200–500ms |
| Marketing / explanatory | Can be longer |

**UI animations stay under 300ms.** A 180ms dropdown feels more responsive than a 400ms one.

From luizhcrocha/skills, MIT.

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