Carousels — Hero Expressive extension

components specs/components/carousels-expressive.kmd

Material 3 Expressive extension to base Hero carousel (specs/components/ carousels.kmd). Adds shape morphing between selected/peek items (square ↔ pill ↔ squircle) and gesture inertia driven by spring physics (motion.kmd R9.1). Connected to shape-library.kmd morph contract.

When this spec applies

Primary triggers

Add Hero Expressive variant to a carousel surface

All triggers

Apply Expressive look on Hero carousel surfaces
Implement hero feed (Vista, Reel) with Expressive variant

Spec — Carousels Hero Expressive

Strict extension to carousels.kmd §Hero layout. Adds shape morph + spring-driven inertia. Base spec remains valid; Expressive opt-in per surface OR per preset.

Princípios

Strict extension — base Hero layout from carousels.kmd unchanged.
Shape morph items — selected item uses larger radius, peek items smaller.
Spring inertia — gesture release follows spring decay, NOT linear.
Width tokens — large item 64-90% viewport; peek 10-32%.

R1 — Anatomy (extension)

Base Hero per carousels.kmd. Expressive adds:

   ┌──┐ ┌──────────────────┐ ┌──┐
   │ ◐│ │      ◯           │ │◐ │
   │  │ │   ⬢⬡⬢            │ │  │   ← peek items (Cookie-7-ish) + selected (Pill-ish)
   │ ◐│ │      ◯           │ │◐ │
   └──┘ └──────────────────┘ └──┘

During swipe, shapes morph in real-time (corner radius interpolates).

R2 — Shape tokens

Position	Default shape (per `shape-library.kmd`)	Corner radius
Selected (center large)	`Pill` or `shape-corner-extra-large`	32dp+
Peek (sides)	`shape-corner-large`	16-24dp
Mid-transition	interpolated linear	varies

During swipe gesture:

Position 0% (center): selected shape full radius.
Position 50% (mid-swipe): item halfway out, radius interpolates toward peek.
Position 100% (off-screen): peek shape OR fully gone.

R3 — Spring inertia

Release gesture (swipe-release):

Velocity captured; preserved into spring motion-spatial-default.
Spring decay: snap to nearest item center (large OR peek).
Bounce: subtle overshoot (max ±5dp) by default; configurable per preset.

Forbidden anti-pattern: linear decay OR friction-based (felt "slidey").

R4 — Width tokens

Token	Range	Use
`hero-item-large`	64-90% viewport	Selected center
`hero-item-peek`	10-32% viewport	Sides
`hero-gap`	8-16dp	Between items

Compact width-class: large 90% + peek 5% (almost only large). Medium/Expanded: large 64-70% + peek 18% each side.

R5 — Per-preset variation

Preset	Hero Expressive style
`material3`	Subtle (radius delta 16dp; minimal bounce)
`material_expressive`	Full (radius delta 32dp; bounce ±5dp)
`material2`	Disabled — falls back to baseline carousels
`terminal_classic`	n/a (no carousels in terminal)
`brutalist`	Sharp corners only; no morph; no bounce (instant snap)
`cyberpunk_neon`	Default + glow on selected
`minimalist_mono`	No radius variation; only width difference

R6 — Surface bindings

Surface	API
Flutter	Extension of `KoderCarousel` (per `carousels.kmd`); `KoderCarousel(variant: hero, expressive: true)`
Web	`<koder-carousel variant="hero" expressive>`
Compose	`KoderCarousel(variant = Hero, expressive = true)`
SwiftUI	idem
CLI / TUI	n/a

R7 — Acessibilidade

Items: role="listitem" per base spec.
Selected: aria-current="true".
Swipe alternative: arrow keys advance one item at a time.
Reduced-motion: no shape morph; no bounce; snap direct (instant transition).
Touch target: each item ≥ 48dp tap area.

R8 — Reduced-motion

Shape morph: disabled (static shapes per position).
Spring inertia: replaced by linear snap (no overshoot, no bounce).
Item swap: instant.

R9 — i18n

Inherits from carousels.kmd. No new keys.

T-suite

T1 Mount Hero Expressive: selected item visible center with Pill shape; peek items sides with shape-corner-large.
T2 Swipe gesture: radius interpolates continuously during drag.
T3 Release with velocity: spring decay; snaps to nearest center.
T4 Bounce: subtle overshoot visible (~5dp) before settle in material_expressive.
T5 Width tokens: Compact width-class → large 90%; Medium → 64% + 18% peeks.
T6 Reduced-motion: no morph + no bounce; instant snap.
T7 Keyboard: arrows advance one item.
T8 Selected aria-current: assistive tech announces correctly.
T9 Material2 preset: falls back to base carousels (no extension).

Cross-link

Base: carousels.kmd §Hero
Shape morph: shape-library.kmd R2
Motion: motion.kmd R9.1
Layout: app-layout/window-size-classes.kmd (width tokens)
Refs: M3 Carousel guidelines (Expressive Hero)

References

specs/components/carousels.kmd
specs/themes/motion.kmd
specs/themes/shape-library.kmd