Are the ai2 Underline collapsibles free?

Yes. The whole styled layer is free and MIT licensed, like the component core.

Can I change the size?

Yes. Every variation takes a size prop of sm, md, lg or xl, aligned to the base Button height scale.

Underline collapsibles

Five minimal collapsibles whose trigger grows an animated underline as it opens: sweep, center, grow, double and dashed. Each toggles a token content panel with an accessible trigger and is sized.

CollapsibleUnderline

Installation

The styled layer is free and installs like any other ai2 component.

Run the following command

npx shadcn@latest add @ai2/collapsible-underline

Dependencies, the @ai2/tokens theme and the component file are installed together.

Install dependencies

npm install motion lucide-react

Copy the source

components/ui/collapsible-underline.tsx

"use client"

import * as React from "react"
import { ChevronDown } from "lucide-react"
import { AnimatePresence, motion, useReducedMotion } from "motion/react"

import { cn } from "@/lib/utils"

/* Underline collapsible family: 5 minimal collapsibles with an animated underline
   that glides beneath the trigger while open. Each export is a complete
   collapsible: a single trigger + a single expanding body, a button with
   aria-expanded, and a framer height auto animation. Works controlled
   (open/onOpenChange) and uncontrolled (defaultOpen). The difference is on the
   UNDERLINE: how it appears changes (Sweep, Center, Grow, Double, Dashed). Color
   comes ONLY from tokens. Alpha via color-mix. No transform under reduced-motion.
   Size = padding + text scale. */

export type StyledSize = "sm" | "md" | "lg" | "xl"

interface StyledCollapsibleProps {
  className?: string
  size?: StyledSize
  title?: React.ReactNode
  children?: React.ReactNode
  open?: boolean
  defaultOpen?: boolean
  onOpenChange?: (o: boolean) => void
}

const headerPad: Record<StyledSize, string> = {
  sm: "px-1 py-2.5 text-sm",
  md: "px-1 py-3 text-sm",
  lg: "px-1 py-4 text-base",
  xl: "px-1 py-5 text-lg",
}

const bodyPad: Record<StyledSize, string> = {
  sm: "px-1 pb-2.5 pt-2 text-sm",
  md: "px-1 pb-3 pt-2 text-sm",
  lg: "px-1 pb-4 pt-3 text-base",
  xl: "px-1 pb-5 pt-3 text-base",
}

const iconSize: Record<StyledSize, string> = {
  sm: "size-4",
  md: "size-4",
  lg: "size-5",
  xl: "size-5",
}

const headerBase =
  "relative flex w-full items-center justify-between gap-3 text-left font-medium outline-none transition-colors focus-visible:ring-[3px] focus-visible:ring-ring/50"

const defaultTitle = "What is included?"

const defaultChildren =
  "Every plan ships the full component library, semantic tokens, and lifetime updates for the tier you own. Add any component with a single CLI command and keep the files fully editable."

/* Controlled and uncontrolled open state in a single hook. */
function useControllableOpen({
  open,
  defaultOpen,
  onOpenChange,
}: {
  open?: boolean
  defaultOpen?: boolean
  onOpenChange?: (o: boolean) => void
}) {
  const isControlled = open !== undefined
  const [internal, setInternal] = React.useState<boolean>(defaultOpen ?? false)
  const current = isControlled ? (open as boolean) : internal
  const toggle = React.useCallback(() => {
    const next = !current
    if (!isControlled) setInternal(next)
    onOpenChange?.(next)
  }, [current, isControlled, onOpenChange])
  return { isOpen: current, toggle }
}

function Chevron({ isOpen, size }: { isOpen: boolean; size: StyledSize }) {
  const reduce = useReducedMotion()
  return (
    <motion.span
      aria-hidden
      className="shrink-0 text-muted-foreground"
      animate={reduce ? undefined : { rotate: isOpen ? 180 : 0 }}
      transition={reduce ? { duration: 0 } : { type: "spring" as const, stiffness: 300, damping: 24 }}
    >
      <ChevronDown className={iconSize[size]} />
    </motion.span>
  )
}

function Panel({
  isOpen,
  size,
  children,
}: {
  isOpen: boolean
  size: StyledSize
  children: React.ReactNode
}) {
  const reduce = useReducedMotion()
  return (
    <AnimatePresence initial={false}>
      {isOpen && (
        <motion.div
          key="panel"
          initial={reduce ? false : { height: 0, opacity: 0 }}
          animate={{ height: "auto", opacity: 1 }}
          exit={{ height: 0, opacity: 0 }}
          transition={
            reduce ? { duration: 0 } : { duration: 0.25, ease: [0.4, 0, 0.2, 1] }
          }
          className="overflow-hidden"
        >
          <div className={cn(bodyPad[size], "text-muted-foreground")}>{children}</div>
        </motion.div>
      )}
    </AnimatePresence>
  )
}

/* Shared shell: a minimal trigger with a static track underneath and a primary line sweeping in when open. lineClass and origin carry each variant's animation technique. */
function UnderlineShell({
  props,
  origin,
  lineClass,
  secondLine,
  grow,
}: {
  props: StyledCollapsibleProps
  origin: string
  lineClass: string
  secondLine?: boolean
  grow?: boolean
}) {
  const { className, size = "md", title = defaultTitle, children = defaultChildren } = props
  const { isOpen, toggle } = useControllableOpen(props)
  const reduce = useReducedMotion()
  const target = reduce
    ? { scaleX: isOpen ? 1 : 0, opacity: isOpen ? 1 : 0 }
    : grow
      ? { scaleX: isOpen ? 1 : 0, scaleY: isOpen ? 1 : 0.4, opacity: isOpen ? 1 : 0 }
      : { scaleX: isOpen ? 1 : 0, opacity: isOpen ? 1 : 0 }
  return (
    <div data-slot="styled-collapsible" className={cn("flex flex-col", className)}>
      <button
        type="button"
        aria-expanded={isOpen}
        onClick={toggle}
        className={cn(headerBase, headerPad[size])}
      >
        {title}
        <Chevron isOpen={isOpen} size={size} />
        <span
          aria-hidden
          className="pointer-events-none absolute inset-x-0 bottom-0 h-px bg-[color-mix(in_oklab,var(--color-foreground)_14%,transparent)]"
        />
        <motion.span
          aria-hidden
          className={cn("pointer-events-none absolute inset-x-0 bottom-0", origin, lineClass)}
          initial={false}
          animate={target}
          transition={reduce ? { duration: 0 } : { type: "spring" as const, stiffness: 240, damping: 28 }}
        />
        {secondLine ? (
          <motion.span
            aria-hidden
            className={cn(
              "pointer-events-none absolute inset-x-0 bottom-1 h-px",
              origin,
              "bg-[color-mix(in_oklab,var(--color-primary)_45%,transparent)]"
            )}
            initial={false}
            animate={{ scaleX: isOpen ? 1 : 0, opacity: isOpen ? 1 : 0 }}
            transition={reduce ? { duration: 0 } : { type: "spring" as const, stiffness: 240, damping: 28 }}
          />
        ) : null}
      </button>
      <Panel isOpen={isOpen} size={size}>
        {children}
      </Panel>
    </div>
  )
}

/* Sweep: a primary line sweeping in from the left. */
export function SweepCollapsible(props: StyledCollapsibleProps) {
  return (
    <UnderlineShell props={props} origin="origin-left" lineClass="h-0.5 bg-primary" />
  )
}

/* Center: a line opening outwards from the centre. */
export function CenterCollapsible(props: StyledCollapsibleProps) {
  return (
    <UnderlineShell props={props} origin="origin-center" lineClass="h-0.5 bg-primary" />
  )
}

/* Grow: a line thickening as it opens from the left. */
export function GrowCollapsible(props: StyledCollapsibleProps) {
  return (
    <UnderlineShell props={props} origin="origin-left" lineClass="h-1 rounded-full bg-primary" grow />
  )
}

/* Double: a two-layer double line. */
export function DoubleCollapsible(props: StyledCollapsibleProps) {
  return (
    <UnderlineShell props={props} origin="origin-left" lineClass="h-0.5 bg-primary" secondLine />
  )
}

/* Dashed: a dashed primary line. */
export function DashedCollapsible(props: StyledCollapsibleProps) {
  return (
    <UnderlineShell
      props={props}
      origin="origin-left"
      lineClass="h-0 border-b-2 border-dashed border-primary"
    />
  )
}

Manual installs skip the @ai2/tokens theme, so add the token CSS from the theming guide or the tone colors will be missing.

Variations

5 takes on the same idea. Each is its own export, and every one accepts a size prop (sm, md, lg, xl) aligned to the base Button scale.

Sweep

A primary line sweeps left to right when open.

Center

The line opens outward from the center.

Grow

The line thickens as it sweeps in from the left.

Double

Two stacked lines sweep in together.

Dashed

A dashed primary line sweeps in on open.

ai2 Underline collapsibles: 5 styled variations on the token system

The ai2 Underline collapsibles are a set of 5 decorative button variations from the styled layer of the @ai2 design system, built around minimal disclosures with an animated sweeping underline. They are free and MIT licensed, and every color comes from a semantic token, so they theme with the rest of ai2 in light and dark.

Motion runs on framer-motion: framer-motion springs the underline in and animates the panel height. The styled layer is opt-in, so the dependency only lands if you use it; the base components stay lean. Under reduced motion, the underline and panel change state instantly with no transforms.

What is in the ai2 Underline collapsibles?

5 exports in one file: Sweep, Center, Grow, Double and Dashed. Each renders a native button and takes a size prop (sm, md, lg, xl) aligned to the base Button. They are separate from the base Button on purpose: the base keeps its clean variant, tone and size axes, while the styled layer carries the effects.

You own the file. Copy the one category file and you have all 5 variations, with no runtime dependency on ai2 itself.

Why use it

On-system by construction: Every color resolves to an ai2 semantic token, so the buttons follow your theme in light and dark with no extra work.
Effect without the sprawl: The decorations live in a dedicated styled file, so the base Button keeps its clean, predictable API.
Accessible and honest: Each renders a real button element, keeps a visible focus ring, and respects prefers-reduced-motion.

Features

Token-driven color: No hardcoded hex or oklch; the look recolors with your theme tokens.
framer-motion: framer-motion springs the underline in and animates the panel height.
Reduced-motion aware: Under prefers-reduced-motion, the underline and panel change state instantly with no transforms.
Size aligned to the base: Every variation takes sm, md, lg and xl matching the base Button height scale, so styled and base buttons line up in a row.

Production tips

Use it for emphasis, not everywhere: Styled buttons draw the eye. Reserve them for the one action you want people to take on a screen, and use the base Button for the rest.
Keep labels as verbs: The decoration adds weight, so a clear action label keeps the button scannable.
Pick one variation per surface: The variations share a family; using two different ones in the same view competes for attention.

Works with the rest of ai2

The Underline collapsibles sit alongside the base Button and the rest of the @ai2 registry. They share the same token file, so a styled action next to a base button or a badge stays visually consistent in both modes.