UI Animation

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

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.
• 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. Force a reflow (void document.body.offsetHeight) after the flip and remove the override on the next frame, or use next-themes disableTransitionOnChange.

Easing defaults

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 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:

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(0.9-0.96) not scale(0) (small popovers at the low end, full dialogs at the high end: a large surface already travels far in absolute pixels), and 30-50ms staggers (total under 300ms, most important element leading). Full rules and code in references/component-patterns.md and 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.
• Motion that janks only sometimes (on open, during navigation, while data lands) is usually a long task sharing the tick, not a costly animation. Don't start an animation and expensive work in the same tick: start the motion, let a frame land, then do the work, or defer it to transitionend.
• See references/performance-deep-dive.md for WAAPI, compositing layers, long tasks during animation, 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).
• 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:

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: animate? purpose? easing? speed?
2. Pick duration from the easing defaults table above. If the value is contested or the component is hard to reach, dial it live in the DevTools bezier editor rather than guessing, then bake the result into source (references/live-tuning.md).
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: 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.

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.