/design-engineering

name: design-engineering
description: Apply interface craft when building or reviewing UI - motion, easing, timing, springs, component feel, and visual foundations. Use when building a component, animation, transition, hover or press state, modal, drawer, toast, or when polishing an interface so it feels right. Says "make this feel better", "add an animation", "polish the UI", "review this component".

design-engineering

In a world where most software works well enough, feel is the differentiator. Most of what makes an interface feel right is detail the user never consciously notices - the aggregate of small correct choices. Build for that aggregate.

Motion decision framework

Answer these in order before writing any animation.

1. Should it animate at all?

The deciding factor is frequency. The more often a user sees it, the less it should move.

| How often the user sees it | Decision | |----------------------------|----------| | 100+ times/day (shortcuts, command palette) | No animation, ever | | Tens of times/day (hover, list nav) | Remove or drastically reduce | | Occasional (modals, drawers, toasts) | Standard animation | | Rare or first-time (onboarding, celebration) | Room for delight |

Never animate a keyboard-initiated action. It repeats all day; animation makes it feel slow and disconnected from the keypress.

2. What is the purpose?

Every animation needs an answer to "why does this move?" Valid ones: spatial consistency (enters and exits the same edge), state change, feedback (a press that confirms the interface heard you), or preventing a jarring appear/disappear. "It looks cool" is not a purpose for anything the user sees often.

3. What easing?

• Entering or exiting -> `ease-out` (fast start, feels responsive).
• Moving or morphing on screen -> `ease-in-out`.
• Hover or color change -> `ease`.
• Constant motion (marquee, progress) -> `linear`.

Never use `ease-in` for UI. It delays the first moment of movement - exactly when the user is watching - so a 300ms `ease-in` dropdown feels slower than a 300ms `ease-out` one. The built-in curves are weak; use stronger custom ones:

--ease-out: cubic-bezier(0.23, 1, 0.32, 1);       /* UI interactions */
--ease-in-out: cubic-bezier(0.77, 0, 0.175, 1);   /* on-screen movement */
--ease-drawer: cubic-bezier(0.32, 0.72, 0, 1);    /* iOS-like drawer */

4. How fast?

| Element | Duration | |---------|----------| | Button press feedback | 100-160ms | | Tooltip, small popover | 125-200ms | | Dropdown, select | 150-250ms | | Modal, drawer | 200-500ms |

Keep frequent, interactive UI under 300ms; the modal and drawer range above (up to 500ms) is the deliberate exception for occasional, first-time surfaces. Perceived speed is real: a faster spinner makes a load feel faster at the same load time, and an instant tooltip after the first one makes a whole toolbar feel quick.

Springs

Springs settle on physics, not a fixed duration, so they feel alive and, unlike CSS keyframes, keep their velocity when interrupted. Reach for them on drag and gesture interactions the user can reverse mid-motion, and on decorative, mouse-tracked motion. Keep bounce subtle (0.1 to 0.3) and usually absent - save it for drag-to-dismiss and playful moments, not functional UI.

Component feel

• **Press feedback.** Any pressable element gets `transform: scale(0.97)` on

`:active` with a ~160ms `ease-out` transition. Subtle (0.95 to 0.98); it makes the interface feel like it is listening.

• **Never animate from `scale(0)`.** Nothing real appears from nothing. Start at

`scale(0.9)` or higher plus opacity so the entrance has shape.

• **Scale from the trigger.** Popovers and dropdowns scale from their origin, not

their center. Modals stay centered.

• **Respect `prefers-reduced-motion`.** Drop or shorten non-essential motion.

Visual foundations

• One spacing scale, one type scale. Consistency reads as intent; ad-hoc values

read as noise.

• Restrained color: a small palette with clear roles beats many accents.
• Make every interactive state explicit - hover, active, focus-visible, disabled.

A control with no states feels dead.

Review format

When reviewing UI code, output a single markdown table, one row per issue, with `Before | After | Why`. Do not use separate "Before:" / "After:" lines.

| Before | After | Why | |--------|-------|-----| | `transition: all 300ms` | `transition: transform 200ms ease-out` | Name exact properties; avoid `all` | | `transform: scale(0)` | `scale(0.95); opacity: 0` | Nothing appears from nothing | | `ease-in` on a dropdown | strong `ease-out` | `ease-in` reads as sluggish | | no `:active` state | `scale(0.97)` on `:active` | Presses must feel responsive |