ui-animation

Summary

---
name: ui-animation
description: Builds, reviews, and measures UI motion, including springs, gestures, scroll effects, and curve fitting from recordings. Use when asked to "add animation", "match this easing", "reverse engineer this motion", or find animation opportunities. For action semantics use product-design; for visual layout use ui-design.
---

# UI Animation

- **IS:** designing, implementing, reviewing, debugging UI motion (springs, gestures, drag, easing, CSS transitions, keyframes, Motion), sweeping an interface for the moments that would genuinely benefit from motion, measuring motion from a recording (extract frames, track, fit curves) to emit code plus a handoff spec, and naming a described motion effect (reverse-lookup vocabulary).
- **IS NOT:** choosing overall visual direction, palettes, or typography (use `ui-design` Direction mode), auditing a whole page's UI quality (use `ui-design` Audit mode), or named text-effect specs (use the external `animate-text` skill where installed).

## Routing boundary

`product-design` owns action semantics, scope, reversibility, and contested state choices. `ui-design` builds and styles those states. `ui-animation` owns timing, gestures, and measured motion. A routine missing loading or error state stays with the UI build; a gesture replacing a control needs a product decision and an accessible alternative before its physics.


## Reference files

| File | Read when |
| --- | --- |
| [references/discovery-workflow.md](references/discovery-workflow.md) | Finding worthwhile opportunities for motion in an existing interface |
| [references/decision-framework.md](references/decision-framework.md) | Default: deciding whether/why to animate, picking easing character; also the seam list for a Discovery sweep |
| [references/spring-animations.md](references/spring-animations.md) | Spring physics, Motion `useSpring`, configuring spring params, Apple damping/response values, asymmetric open/close character, interruption mechanics |
| [references/component-patterns.md](references/component-patterns.md) | Buttons, popovers, tooltips, drawers, modals, toasts with animation |
| [references/clip-path-techniques.md](references/clip-path-techniques.md) | clip-path for reveals, tabs, hold-to-delete, comparison sliders |
| [references/gesture-drag.md](references/gesture-drag.md) | Drag, swipe-to-dismiss, momentum, pointer capture, velocity handoff, momentum projection, rotary/knob drag, detents, carousel `touch-action` |
| [references/scroll-animations.md](references/scroll-animations.md) | Scroll-triggered reveals, scrubbed/scroll-driven animation (`animation-timeline`, `useScroll`), parallax, sticky scrollytelling, and when a scroll animation shouldn't exist |
| [references/performance-deep-dive.md](references/performance-deep-dive.md) | Jank, CSS vs JS, WAAPI, CSS variables trap, Framer Motion caveats |
| [references/debugging-symptoms.md](references/debugging-symptoms.md) | An animation feels off and the cause isn't named: symptom-indexed tables for sluggish, robotic, cheap, jumpy, and misfiring motion |
| [references/svg-animation.md](references/svg-animation.md) | Animating vector art: line drawing (`stroke-dashoffset`), SVG transform-origin traps, path morphing, shakes, ambient life |
| [references/review-format.md](references/review-format.md) | Reviewing animation code: ten standards (each with flag-on-sight triggers), Before/After/Why table, Block/Approve verdict |
| [references/contextual-animations.md](references/contextual-animations.md) | Contextual icon swaps, word-level stagger entrances, peripheral de-emphasis, fixed-offset exits |
| [references/transition-recipes.md](references/transition-recipes.md) | Installing a CSS transition: container morph, card resize, badge, dropdown, modal, panel, page slide, icon swap, number pop-in, odometer roll, text swap, success, avatar hover, error shake |
| [references/measurement-guide.md](references/measurement-guide.md) | Reverse-engineer: what to measure, eye vs script, reading `metrics.json`, choosing an ROI |
| [references/curve-fitting.md](references/curve-fitting.md) | Reverse-engineer: reading `fit_curves.py` output, spring vs bezier, judging fit error, asymmetric open/close |
| [references/code-output.md](references/code-output.md) | Reverse-engineer: emitting code for CSS, Motion/Framer Motion, SwiftUI, React Native, UIKit |
| [references/choreography.md](references/choreography.md) | Reverse-engineer: multi-element/multi-phase motion: staggers, blur-before-move, per-edge settling |
| [references/vocabulary.md](references/vocabulary.md) | Naming a motion effect the user describes vaguely ("what's it called when...") |

## Core rules

- Animate for feedback, orientation, continuity, or deliberate delight. If it's just "it looks cool" and the user sees it often, don't.
- Keep keyboard focus and repeated navigation immediate. A state transition may animate if focus and task completion do not wait for it.
- Prefer CSS transitions for interruptible UI: keyframes restart from zero on interruption, transitions retarget. Use keyframes only for predetermined sequences.
- Implementation priority: CSS transitions > WAAPI > CSS keyframes > JS (`requestAnimationFrame`); under load CSS stays smooth while JS drops frames.
- Asymmetric timing: occasional interactions can enter slightly slower, exit fast. High-frequency ephemeral UI (hover highlights, popovers, panel toggles) inverts this: enter instantly (0ms), exit with a brief fade (100-150ms) so the action feels immediate.
- Tappable controls press on `:active` at 0ms and set `touch-action: manipulation`.
- Use `@starting-style` for DOM entry; fall back to a `data-mounted` attribute where unsupported.
- A small `filter: blur(2px)` hides rough crossfades between swapped content.

## Motion design principles

- **Continuity over teleportation.** Elements visible in both states transition in place; expand from where elements sit rather than fading in a new instance. Never duplicate a persistent element or hard-cut between views that share components; hard cuts lose spatial context.
- **Directional motion matches position.** Tab and carousel transitions animate in the direction matching spatial layout (left-to-right forward, right-to-left back).
- **Emerge from the trigger.** Overlays, trays, and panels animate outward from the element that opened them; generic centre-screen entrances break spatial orientation. Better still where the shapes allow: let the trigger *become* the surface (see the container-morph recipe).
- **Confirm in place, not in a corner.** An action's result belongs on the control that caused it: the button becomes "Copied", holds, and reverts. A toast in the far corner makes the user's eye leave the thing they just touched to find out whether it worked. Reserve corner toasts for results with no on-screen origin (a background job finishing, an incoming message).
- **Animate paired states together.** If open animates, close animates. If hover has motion, focus and pressed states get equivalent feedback. Do not polish only one half of a repeated interaction.
- **Delight scales inversely with frequency.** Rarer interactions get more personality; high-frequency actions must be invisible.
- **Motion enhances perceived speed.** Smooth transitions feel faster than hard cuts, even at identical load times.

## What to animate

- Movement: `transform` and `opacity` only; they skip layout and paint.
- State feedback: `color`, `background-color`, and `opacity` are acceptable.
- Never animate layout properties (`width`, `height`, `top`, `left`); they trigger layout recalc every frame. (Exception: a deliberate container tween, see the card-resize and container-morph recipes.)
- Never use `transition: all`; it animates unintended properties and silently adopts future ones. List them explicitly.
- Avoid `filter` animation for core interactions; if unavoidable keep blur ≤ 20px (heavy blur is expensive, especially in Safari).
- SVG: apply transforms on a `<g>` wrapper with `transform-box: fill-box; transform-origin: center`; without it they rotate/scale around the canvas origin. Line drawing, path morphing, and the Motion SVG origin override live in [references/svg-animation.md](references/svg-animation.md).
- `transform: scale()` also scales children (icons, text, borders scale proportionally), unlike `width`/`height`: a feature for press feedback, but account for it when an inner element must stay fixed-size.
- Disable transitions during theme switches (`[data-theme-switching] * { transition: none !important }`), or every themed property animates at once.

## Easing defaults

| Element                       | Duration     | Easing                           |
| ----------------------------- | ------------ | -------------------------------- |
| Button press feedback         | 100-160ms    | `cubic-bezier(0.22, 1, 0.36, 1)` |
| Tooltips, small popovers      | 125-200ms    | `ease-out` or enter curve        |
| Dropdowns, selects            | 150-250ms    | `cubic-bezier(0.22, 1, 0.36, 1)` |
| Modals, drawers               | 200-350ms    | `cubic-bezier(0.22, 1, 0.36, 1)` |
| Move/slide on screen          | 200-300ms    | `cubic-bezier(0.25, 1, 0.5, 1)`  |
| Page transitions              | 250-400ms    | enter or move curve              |
| Hover (colour/opacity)        | 200ms        | `ease`                           |
| Hover (transform/scale)       | 100-150ms    | enter curve                      |
| Illustrative/marketing        | Up to 1000ms | Spring or custom                 |

Keep routine UI under 300ms; scale duration with distance (a full-screen slide can exceed 300ms, a 6px tooltip shift stays under 150ms).

**Named curves**

- **Enter:** `cubic-bezier(0.22, 1, 0.36, 1)` for entrances and transform-based hover
- **Move:** `cubic-bezier(0.25, 1, 0.5, 1)` for slides, drawers, panels
- **Drawer (iOS-like):** `cubic-bezier(0.32, 0.72, 0, 1)` (extremely steep start; the reason its 500ms doesn't read as slow)
- **Expo out:** `cubic-bezier(0.19, 1, 0.22, 1)` for dramatic reveals, card hovers, text reveals
- **Press:** `cubic-bezier(0.25, 0.46, 0.45, 0.94)` for button press feedback
- **On-screen move:** `cubic-bezier(0.645, 0.045, 0.355, 1)` for back-and-forth movement that stays on screen

Avoid `ease-in` for UI: it starts slow, so the element lags the user's action and feels sluggish. Prefer custom curves from [easing.dev](https://easing.dev/) over built-in `ease`/`ease-out`, whose gentle acceleration reads soft, not decisive.

## Transition decision rules

Match the UI element first, then pick the recipe from [references/transition-recipes.md](references/transition-recipes.md):

| UI pattern | Recipe |
|---|---|
| Trigger + floating dot/count | Notification badge |
| Trigger grows into the surface it opens | Container morph |
| Trigger + anchored surface | Menu dropdown |
| Centred surface on top of page | Modal dialog |
| Panel sliding into existing container | Panel reveal |
| List ↔ detail or wizard steps | Page side-by-side slides |
| Element dimension changes | Card resize |
| Text updating in place | Text state swap |
| Two icons in same slot | Icon swap |
| Number arriving on its own | Number pop-in |
| Number the user is driving | Odometer digit roll |
| Confirmation / success moment | Success celebration |
| Hovering item in horizontal stack | Avatar group hover |
| Form validation error | Error state shake |

Prefer lower-overhead transitions (CSS-only) unless the design requires JS orchestration.

## Spatial and sequencing

- Popover `transform-origin` at the trigger (modals stay `center`), dialog/menu entrances from `scale(0.85-0.9)` not `scale(0)`, and 30-50ms staggers (total under 300ms, most important element leading). Full rules and code in [references/component-patterns.md](references/component-patterns.md) and [references/contextual-animations.md](references/contextual-animations.md).
- **Paired elements rule:** elements that animate together (modal + overlay, tooltip + arrow, FAB + label) must share easing and duration. Mismatched timing is the usual cause of "something feels off".

## Accessibility

- Gate hover (motion and paint) behind `@media (hover: hover) and (pointer: fine)`, or touch devices replay hover on tap. Inspect the generated CSS before adding a gate; Tailwind v4 already wraps `hover:` in `@media (hover: hover)`.
- During direct manipulation, keep the element locked to the pointer with no easing; add easing only after release.

## Performance

- Pause looping animations off-screen with `IntersectionObserver`; they burn GPU even when invisible.
- Toggle `will-change` only during heavy motion and only for `transform`/`opacity`; remove it after. Each promotion costs compositor memory; permanent promotion across many elements is worse than none.
- Do not animate drag via CSS variables on a container; every update recalculates styles for all children. Set `transform` directly on the moving element.
- Motion `x`/`y` values are the default for axis movement and drag (they bypass React re-renders). Use a full `transform` string when one owner must combine multiple transform functions, interop with non-Motion code, or survive a busy main thread: the shorthands run on `requestAnimationFrame` and drop frames when motion coincides with navigation, data loading, or hydration; CSS/WAAPI stay smooth there.
- See [references/performance-deep-dive.md](references/performance-deep-dive.md) for WAAPI, compositing layers, and the CSS vs JS comparison table.

## Anti-patterns

High-signal failures not covered above:

- Animating on mount without a user trigger: unexpected motion disorients; the user did nothing to cause it.
- Hard stops on drag boundaries feel broken; apply friction/damping so movement diminishes past it (see gesture-drag reference).
- Animating both a container and staggering its children: pick one entrance per container. If the panel slides in, its content should already be visible on arrival.
- Tooltip animation after the first is open: subsequent tooltips in the group open instantly, or the toolbar feels laggy.
- Scroll-revealing product UI, above-the-fold content, or every section of a page: scroll reveals belong to a few chosen moments on marketing surfaces, run once, and never re-animate on scroll-up (see [references/scroll-animations.md](references/scroll-animations.md)).
- Easing or duration on scrubbed (scroll-driven) motion: scroll position is the clock, so any curve or duration makes it lag the scrollbar. `linear` and no duration is correct there, and only there.
- Installing `framer-motion` for new work: the package is now `motion` and React imports come from `motion/react`. The old package still resolves, so a mixed codebase compiles while shipping two copies of the library.

## Workflow

Copy and track:

```text
Animation progress:
- [ ] Step 1: Decide whether the interaction should animate
- [ ] Step 2: Choose purpose, easing, and duration
- [ ] Step 3: Pick the implementation style
- [ ] Step 4: Load the relevant component or technique reference
- [ ] Step 5: Validate timing, interruption, and device behavior
```

1. Answer the four questions in [references/decision-framework.md](references/decision-framework.md): animate? purpose? easing? speed?
2. Pick duration from the easing defaults table above.
3. Choose implementation: CSS transition > WAAPI > spring > keyframe > JS.
4. Load the reference for your component or technique.
5. When reviewing, apply the strict posture in [references/review-format.md](references/review-format.md): measure against the ten standards, output the Before/After/Why table, then a tiered verdict ending in a Block/Approve decision.

## Validation

Produce evidence for each check (DevTools observations, not "looks fine"):

- Grep the diff for layout property transitions (`width`, `height`, `top`, `left`) and `transition: all`.
- Retoggle components rapidly; confirm transitions retarget instead of restarting from zero.
- Slow to 10% in the DevTools Animations panel to catch timing and `transform-origin` issues invisible at full speed.
- Confirm `will-change` is toggled around animations, not permanently set, and looping animations pause off-screen.
- Test touch interactions on real devices; simulators under-report gesture and hover-on-tap issues.
- Honor `prefers-reduced-motion`: replace spatial travel and looping effects with immediate state changes or restrained fades, then exercise the same task in that mode.

## Discovery workflow

For "where should this animate", load `references/discovery-workflow.md` and `references/decision-framework.md`. Report opportunities supported by purpose and usage frequency. Implement a suggestion only when implementation is in scope.

## Reverse-engineer workflow

Use this branch to measure an existing animation from a screen recording, then emit code and a handoff spec that reproduce it. The scripts under `scripts/` are the canonical, deterministic path; run them rather than reconstructing their logic.

Resolve every `scripts/` command below relative to the installed skill directory, not the application working directory.

**Dependencies:** `ffmpeg` for frame extraction (`brew install ffmpeg`); Python with `pip install opencv-python numpy scipy` for tracking and curve fitting. Degrades gracefully: with only ffmpeg you can extract frames and reason visually; tracking and fitting need the Python packages.

```text
Reverse-engineer progress:
- [ ] Step 1: Extract frames + contact sheet (per direction if open differs from close)
- [ ] Step 2: Vision pass: identify element, effects, phases
- [ ] Step 3: Decide precision (eye-only vs scripted)
- [ ] Step 4: Track motion and fit curves (if escalating)
- [ ] Step 5: Annotate choreography (delays, asymmetry)
- [ ] Step 6: Emit code for the target(s)
- [ ] Step 7: Validate against the recording
```

1. **Extract.** Run `python3 scripts/extract_frames.py <video> <outdir>`. Trim to just the transition with `--start`/`--duration`; if the interaction has both an open and a close, trim two windows and run the pipeline once per direction (they are almost never mirror images). Match `--fps` to the source (probe with `ffprobe`), never sampling above the source rate. Open `contact_sheet.png` first.
2. **Vision pass.** Name the element(s) that move, every effect (translate, scale often anisotropic, opacity, blur, corner radius, shadow, color), and the phases, noting which property leads and lags. Use the checklist in `references/measurement-guide.md`.
3. **Decide precision.** Simple fade or linear slide: read timing off the contact sheet, skip to step 5. Elastic, springy, or multi-property motion: escalate to step 4 (eyeballing a spring is unreliable).
4. **Track and fit.** Run `python3 scripts/track_motion.py <outdir>` for `metrics.json` (pass `--bbox X,Y,W,H` to isolate one element), then `python3 scripts/fit_curves.py <outdir>/metrics.json` for spring params, cubic-bezier, and per-property fit error. Pass the same `--fps` you extracted with. Read `references/curve-fitting.md` to pick the model; high error on both means multi-phase motion (split and fit each segment).
5. **Annotate.** Load `references/choreography.md`. Build the timing-offset table (when each property starts and settles); lead/lag gaps and over-stretch carry more feel than any single curve.
6. **Emit.** Substitute fitted parameters into the templates in `references/code-output.md` for the target. Keep movement on `transform`/`opacity`. Emit two transitions when open and close differ, plus the consolidated handoff spec so it can be implemented without the video.
7. **Validate.** Re-derive: play the emitted animation, screen-record it, run it back through `extract_frames.py`, and compare contact sheets side by side. Slow to 0.1x to confirm phase order and over-stretch survive. Confirm the code only animates `transform`, `opacity`, and `filter`.

**Reverse-engineer gotchas:**

- `fit_curves.py` defaults to `--fps 30`: extract at 60 but fit at the default and every `duration_ms` doubles while fitted stiffness drops to a quarter. Always pass the extraction fps to the fit.
- Sampling above the source rate duplicates frames: a 24 fps GIF extracted at 60 inflates fit error with plateaued runs in `metrics.json`. Probe and match the source rate.
- Screen recordings drop frames and iOS/QuickTime captures are variable-frame-rate; consecutive identical rows are duplicated frames, not a pause. Re-record at a steadier rate if plateaus dominate.
- Measure open and close as separate clips and report two curves; never fit one and reuse it reversed (see `references/choreography.md`). Treat a fit `error` above 0.08 as suspect.

Maintenance only: when changing Discovery routing or the gate, run the scenarios in `evaluations/` as a regression rubric. They never load during a user task.

## Related skills

- `product-design`: which states exist, what an action affects, and whether it is reversible. Route here first when a gesture replaces a control, since swipe-to-delete and hold-to-confirm change what the user can do before they change how it moves.
- `ui-design` Direction mode: visual direction, palettes, typography; settle the visual system before tuning motion.
- `ui-design` Audit mode: page/feature-level UI quality audit. Motion craft and fixes belong here.
- Optional external `animate-text` skill where installed: curated named text effects (typewriter, line reveal, stagger builds) with exact JSON specs.

Maintenance only: `evals/evals.json` contains regression scenarios for changes to this skill; it does not load during a user task.

Security audits