Interaction physics

Interaction Physics

Give every user action a physical response: consistent durations, meaningful easing, and feedback that arrives instantly. The prime directive: every animation must do a job — confirm an action, guide attention, or explain a state change. Motion that does none of these gets deleted, not tuned. Deliver two artifacts together: a project-wide animation spec (so future motion stays consistent) and the implemented code for the interactions in scope.

When to use / when not to

Use for: hover/press/focus feedback, enter/exit transitions (modals, toasts, pages, list items), form feedback animation, animation audits, choosing durations and easings, prefers-reduced-motion support.

Hand off instead when the real need is:

• Which loading indicator to show and when → skills/frontend-design/loading-states (this skill animates it; that skill chooses it)
• Optimistic UI, perceived latency, Core Web Vitals → skills/frontend-design/performance-optimization
• Rendering pipeline, layout thrashing, frame budgets → skills/frontend-design/design-engineer-mindset
• Non-motion visual polish (spacing, contrast, hierarchy) → skills/frontend-design/visual-hierarchy-refactoring

Step 0 — Inspect the codebase, then ask only what's left

1. Grep for existing motion: transition, animation, @keyframes, framer-motion, motion., react-spring, gsap, Tailwind transition-*/animate-* classes. Record every duration and easing in use — inconsistency here (e.g. 150ms, 200ms, 250ms, 300ms all for hovers) is usually the first finding.
2. Check for prefers-reduced-motion handling anywhere. Absence is an automatic fix item.
3. Identify the animation vehicle: if the project already uses a motion library, implement with it; otherwise default to CSS transitions/keyframes. Never add a motion library for effects CSS can do.
4. Find dead interactions: buttons without hover/active states, modals/toasts that appear with no transition, form errors that pop in.

Ask the user (one batch, only if not inferable): desired personality (calm/professional vs playful — infer from the product's domain if obvious) and scope (whole app pass vs specific flows). If the user said "the app feels janky", don't interrogate — audit, state assumptions, and proceed.

Workflow

1. Write the animation spec first

Fix the vocabulary before touching code. Standard duration classes:

Class	Duration	Easing	Applies to
Micro feedback	150–250ms	ease-out	hover, press, focus, toggle, color/state change
Structural transition	300–500ms	ease-out in, ease-in out	modal, drawer, page, accordion, toast
Attention	500–1000ms	ease-in-out	alerts, pulses, one-shot emphasis
Continuous	~1s loop	linear	spinners, indeterminate progress

Decision rules:

• Easing by direction: entering elements decelerate (ease-out); exiting elements accelerate (ease-in); on-screen moves use ease-in-out; only mechanical loops use linear. Bouncy curves like cubic-bezier(0.34, 1.56, 0.64, 1) are reserved for deliberate playfulness — at most one or two places in an app.
• Duration by size and distance: small elements moving short distances sit at the bottom of the range (hover tint: 150–200ms); large surfaces or long travel at the top (full-screen drawer: 400–500ms). A tooltip and a modal must not share a duration.
• Exits faster than entrances (~75% of the enter duration) — users have already decided to leave.
• Collapse whatever inconsistent values you found in Step 0 into at most 3–4 named tokens (e.g. --duration-fast: 200ms; --duration-base: 300ms; --duration-slow: 450ms), defined as CSS variables or theme tokens.

2. Specify each microinteraction with the four-part anatomy

For every interaction in scope, state: Trigger (user action / system event) → Rules (what changes, duration, easing) → Feedback (what the user perceives) → Loops & modes (does it repeat, can it be interrupted). Interruptibility rule: state-driven transitions (CSS transition or a motion library) beat fire-and-forget keyframes for anything a user can toggle rapidly — a half-open menu must reverse smoothly, not restart.

3. Implement under the performance and accessibility constraints

• Animate only transform and opacity (compositor-friendly). Never animate width, height, top, left, or box-shadow directly; fake shadow animation by crossfading a pseudo-element's opacity. Animating background-color is acceptable for small controls only.
• Ship the reduced-motion block in the same change — see the baseline in references/patterns.md. Decorative motion disappears; informational motion (progress) may remain as opacity-only.
• Feedback never rides on motion alone: pair every animated state change with a color/text/icon change so it survives reduced-motion.
• Keyboard parity: every :hover effect gets a :focus-visible equivalent.
• No flashing faster than 3 times per second, ever (seizure risk — this is a WCAG requirement, not a style preference).

Canonical micro-feedback example (full gallery — buttons, form validation, toasts, modals, page transitions — in references/patterns.md):

button {
  transition: background-color 200ms ease-out, transform 200ms ease-out;
}
button:hover:not(:disabled)  { background-color: var(--color-primary-dark); }
button:active:not(:disabled) { transform: scale(0.98); }
button:focus-visible         { outline: 2px solid var(--color-focus); outline-offset: 2px; }

4. Verify

Run the app if possible (or reason through the render path): confirm transitions fire, exits animate (a common miss — elements unmount before the exit animation), rapid toggling doesn't stutter or restart, and the reduced-motion media query actually suppresses movement.

Required output format

Deliver both artifacts:

1. The code — implemented transitions/animations, using the project's existing motion vehicle and tokens.

2. Animation Spec (markdown, becomes the project's motion reference):

## Motion tokens
| Token | Value | Easing | Used for |
| --duration-fast | 200ms | ease-out | hover, press, focus, toggles |
| --duration-base | 300ms | ease-out/ease-in | toasts, dropdowns, accordions |
| --duration-slow | 450ms | ease-out/ease-in | modals, drawers, page transitions |

## Interaction inventory
| Interaction | Trigger | Motion | Duration/easing | Status |
| Button hover | pointer/focus | bg tint + scale on press | fast / ease-out | implemented |
| Modal open | click | fade + 20px rise | slow / ease-out | implemented |
| Toast exit | timeout/dismiss | slide right + fade | base×0.75 / ease-in | implemented |

## Audit findings (for audit engagements)
| Issue | Location | Fix |
| 5 different hover durations | [files] | collapsed to --duration-fast |
| No reduced-motion support | global | baseline block added |

## Deliberately not animated
[List, with one-line reasons — restraint is part of the spec]

Quality bar (check before delivering)

• Every animation in delivered code maps to a purpose (confirm / guide / explain) — name it if challenged
• All durations come from the spec's tokens; no magic numbers left in touched files
• Enter = ease-out, exit = ease-in, loops = linear — no exceptions without a stated reason
• Only transform/opacity animated (small-control color changes excepted)
• prefers-reduced-motion block present and effective
• Every :hover state has a :focus-visible twin
• Animated state changes also change color/text/icon (motion is not the only signal)
• Nothing flashes > 3×/second; no infinite decorative loops

Hard don'ts: no scroll-hijacking; no animation on keystroke in text inputs; no adding a motion library when CSS suffices; no animating layout properties.

Integration

• skills/frontend-design/component-architecture → provides the component states (hover/active/loading/disabled) this skill animates; add tokens there if a token system exists.
• skills/frontend-design/loading-states → decides which indicator appears; this skill supplies its shimmer/spin/progress motion.
• skills/frontend-design/performance-optimization ← consumes the spec's constraint that only compositor properties animate; hands back findings if animations still drop frames.
• skills/frontend-design/accessibility-excellence ← audits reduced-motion and flash-rate compliance of what you ship.

References

• references/patterns.md — full implementation gallery: complete button state set, form validation, skeleton/spinner/progress motion, toast enter/exit, page and modal transitions, the reduced-motion baseline block. Read it when writing the actual CSS.