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