Show and hide content with smooth CSS-driven animations powered by panel size CSS custom properties.

Collapsible

A disclosure widget that toggles visibility of a content panel. It animates open and close transitions using --collapsible-panel-height and --collapsible-panel-width CSS custom properties, supports hidden="until-found" for browser find-in-page, and exposes open state via events and a controller.

import { Collapsible } from "@areia/slots";

document.querySelector("#root")!.innerHTML = `
  <div data-slot="collapsible" class="w-72">
    <button data-slot="collapsible-trigger" class="win95-button">
      Toggle
    </button>
    <div data-slot="collapsible-content" class="win95-inset mt-2 p-3">
      Content revealed when the trigger is activated.
    </div>
  </div>
`;

const root = document.querySelector(
  '[data-slot="collapsible"]',
)!;
const controller = Collapsible.createCollapsible(root, {
  defaultOpen: false,
});

Import

import { Collapsible } from "@areia/slots";

Usage

import { Collapsible } from "@areia/slots";

// Auto-bind a single root
const root = document.querySelector(
  '[data-slot="collapsible"]',
)!;
const controller = Collapsible.createCollapsible(root, {
  defaultOpen: false,
  onOpenChange: (open) => console.log("open changed:", open),
});

// Auto-bind all unbound roots in scope
const controllers = Collapsible.create();

Expected Markup

The collapsible expects a trigger button and a content panel inside the root.

<div data-slot="collapsible">
  <button data-slot="collapsible-trigger">Toggle</button>
  <div data-slot="collapsible-content">
    Content revealed when the trigger is activated.
  </div>
</div>

Data Slots

Slot	Required	Description
`collapsible`	Yes	Root container.
`collapsible-trigger`	Yes	Button that toggles the content panel.
`collapsible-content`	Yes	Panel whose visibility is controlled.

CSS Custom Properties

The controller sets the following CSS custom properties on the content element to enable smooth open/close animations. During transitions the values are set to the content’s pixel dimensions, and reset to auto once the transition settles.

Property	Description
`--collapsible-panel-height`	Pixels during open/close transition, `auto` when settled.
`--collapsible-panel-width`	Pixels during open/close transition, `auto` when settled.

Use these variables to animate height, width, block-size, or inline-size via CSS transitions:

[data-slot="collapsible-content"] {
  overflow: hidden;
  height: var(--collapsible-panel-height);
  width: var(--collapsible-panel-width);
  transition:
    height 200ms ease-out,
    width 200ms ease-out;
}

Generated Attributes

Attribute	Element(s)	Description
`data-state`	Root, content	`"open"` or `"closed"`.
`aria-expanded`	Trigger	`"true"` when open, `"false"` when closed.
`aria-controls`	Trigger	Points to the content element `id`.
`aria-labelledby`	Content	Points to the trigger element `id`.
`role`	Content	Always set to `"region"`.
`hidden`	Content	Set to `true` when closed (or `"until-found"` when `hiddenUntilFound` is enabled).

Events

Outbound

Event	Detail	Description
`collapsible:change`	`{ open: boolean }`	Fired on the root when the open state changes.

Inbound

Event	Detail	Description
`collapsible:set`	`{ open: boolean }`	Dispatch on the root to programmatically set the open state.

API Reference

Options

Option	Type	Default	Description
`defaultOpen`	`boolean`	`false`	Initial open state.
`hiddenUntilFound`	`boolean`	`false`	Use `hidden="until-found"` instead of `hidden` when closed, so browser find-in-page can reveal content.
`onOpenChange`	`(open: boolean) => void`	—	Called when the open state changes. Not called on initial render.

Controller

Member	Type	Description
`open()`	`() => void`	Open the collapsible.
`close()`	`() => void`	Close the collapsible.
`toggle()`	`() => void`	Toggle the collapsible.
`isOpen`	`boolean` (getter)	Current open state.
`destroy()`	`() => void`	Remove all listeners, observers, and clean up.

Controller

The controller is returned by Collapsible.createCollapsible() and provides imperative control over the collapsible’s state.

const controller = Collapsible.createCollapsible(root);

// Open programmatically
controller.open();

// Close programmatically
controller.close();

// Toggle
controller.toggle();

// Read current state
console.log(controller.isOpen); // true | false

// Clean up
controller.destroy();