Disclosure

Splitter

Two panes with a separator between them that resizes them, dragged or moved with the keyboard.

Element: <slab-splitter>
Built on: [role=separator]
As Markdown: splitter.md

Example

Here's a styled version, <slab-styled-splitter>. It's not in the package — it's an example of what you can build with the headless element. Copy it and make it yours at the bottom of this page. Switch to "As it ships" to see the same markup with the headless element and no CSS at all. That's your starting point.

Styled example

As it ships

Mixes

Footing, slab, wall.

Slab

4,000 psi, a 4-inch slump, broom finish. Cure for seven days before it takes a load.

HTML

<slab-styled-splitter label="Mixes" min="20" max="80" value="35" style="block-size: 14rem">
  <div slot="primary">
    <strong>Mixes</strong>
    <p>Footing, slab, wall.</p>
  </div>
  <div slot="secondary">
    <strong>Slab</strong>
    <p>4,000 psi, a 4-inch slump, broom finish. Cure for seven days before it takes a load.</p>
  </div>
</slab-styled-splitter>

Mixes

Footing, slab, wall.

Slab

4,000 psi, a 4-inch slump, broom finish. Cure for seven days before it takes a load.

HTML

<slab-splitter label="Mixes" min="20" max="80" value="35" style="block-size: 14rem">
  <div slot="primary">
    <strong>Mixes</strong>
    <p>Footing, slab, wall.</p>
  </div>
  <div slot="secondary">
    <strong>Slab</strong>
    <p>4,000 psi, a 4-inch slump, broom finish. Cure for seven days before it takes a load.</p>
  </div>
</slab-splitter>

Usage

Install

pnpm add slab-elements lit

HTML

<script type="module">
  import 'slab-elements/splitter.js';
</script>

<slab-splitter label="Sidebar" min="15" max="50" value="25">
  <nav slot="primary">…</nav>
  <main slot="secondary">…</main>
</slab-splitter>

The separator is named by label, which should name the primary pane, since that’s the one value sizes. Give the splitter a size, such as a block-size, or it’s as tall as its tallest pane.

Keyboard

Key	What it does
Left, Right	Moves the separator by `step`, the way the arrow points, so Left shrinks the primary pane unless it’s on the right. Up and Down when `orientation` is “horizontal”.
Home, End	Moves the separator to `min` or `max`.
Enter	Collapses the primary pane to `min`, or restores it to where it was.

Behavior

A window splitter with no visual styles: two panes and a [role=separator] between them that's dragged, or moved with the keyboard, to resize them.

value is how much of the splitter the primary pane takes up, as a percentage, and the secondary pane takes the rest. The separator keeps its own size, whatever your styles make it, and the panes share what's left. min and max bound it, so a sidebar can't be squeezed to nothing or take over.

The separator is named by label, which should name the primary pane, and reports value as its aria-valuenow. Left and Right move it, or Up and Down when orientation is "horizontal", by step. Home and End move it to min and max. Enter collapses the primary pane to min, and restores it to where it was before.

Laying out the panes and sizing them is what a splitter does, so that's functional and done here: the host is a grid of the two panes and the separator. Give the host a size, or it'll be as tall as its tallest pane. Style the rest through its parts and through :state(collapsed), :state(disabled) and :state(dragging).

API

`<slab-splitter>`

Attributes
Name	Type	Default	Description
`label` Required	`string`		Names the separator. Name it after the primary pane, as the separator resizes it.
`disabled`	`boolean`	`false`
`max`	`number`	`100`	The most of the splitter the primary pane can take up, as a percentage.
`min`	`number`	`0`	The least of the splitter the primary pane can take up, as a percentage. Enter collapses the primary pane to it.
`orientation`	`'horizontal' \| 'vertical'`	`'vertical'`	The separator's. A vertical one sits between panes side by side, and a horizontal one between panes stacked on top of each other.
`step`	`number`	`5`	How far each arrow key moves the separator, as a percentage.
`value`	`number`	`50`	How much of the splitter the primary pane takes up, as a percentage. It's always between `min` and `max`.
`version`	`string`

Methods
Signature	Description
`focus(options?: FocusOptions): void`

Events
Name	Type	Description
`input`	`Event`
`change`	`Event`

Slots
Name	Type	Description
`primary`	`Element`
`secondary`	`Element`

CSS parts
Name	Description
`container`	The panes and the separator
`primary`	The primary pane
`secondary`	The secondary pane
`separator`	The separator between the panes

Copy and own it

<slab-styled-splitter> isn't in the package. It's an example — copy it into your project and change whatever you want. Under the hood, it's just the headless element, subclassed, with styles set. That's it. Keyboard, focus, ARIA, forms, and events all come from the headless element, and the styles only reach in through ::part() and :state(), same as yours would.

To use it, copy these files, swap the tag in @customElement() for one of your own, and import it. tokens.styles.ts holds the colors and fonts every styled example shares, so you only need it once.

TypeScript

These files are TypeScript, and @customElement() is one of Lit's decorators (TypeScript's experimental flavor). You'll need two options in your tsconfig, per Lit's docs: experimentalDecorators to compile them, and useDefineForClassFields turned off so class fields don't stomp on the accessors Lit sets up.

tsconfig.json

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "useDefineForClassFields": false
  }
}

Using plain JavaScript? Drop the types and register the class with customElements.define() instead of the decorator.

splitter.ts

import { customElement } from 'lit/decorators.js';
import Splitter from 'slab-elements/splitter.js';
import styles from './splitter.styles.js';
import tokens from './tokens.styles.js';

declare global {
  interface HTMLElementTagNameMap {
    'slab-styled-splitter': StyledSplitter;
  }
}

/**
 * Splitter, styled: two panes in a raised box, split by an ink bar that turns
 * ember while it's hovered, focused or dragged.
 */
@customElement('slab-styled-splitter')
export default class StyledSplitter extends Splitter {
  static override styles = [tokens, styles];
}

splitter.styles.ts

import { css } from 'lit';

/*
  The separator is an ink bar with a grip at its middle, drawn with a
  pseudo-element so it needs no markup. Its hit area is wider than the bar,
  so it's easy to grab.
*/
export default css`
  :host {
    background-color: var(--paper);
    border: var(--border);
    box-shadow: var(--shadow);
    color: var(--ink);
    font-family: var(--font-body);
  }

  [part~='primary'],
  [part~='secondary'] {
    padding: 1rem;
  }

  [part~='separator'] {
    align-items: center;
    background-color: var(--ink);
    cursor: col-resize;
    display: flex;
    inline-size: 0.5rem;
    justify-content: center;
    transition: background-color 100ms;

    &::before {
      background-color: var(--paper);
      block-size: 2rem;
      content: '';
      inline-size: 2px;
    }

    &:hover {
      background-color: var(--ember);
    }

    &:focus-visible {
      background-color: var(--ember);
      outline: 3px solid var(--ember);
      outline-offset: 2px;
    }
  }

  :host([orientation='horizontal']) [part~='separator'] {
    block-size: 0.5rem;
    cursor: row-resize;
    inline-size: auto;

    &::before {
      block-size: 2px;
      inline-size: 2rem;
    }
  }

  :host(:state(dragging)) [part~='separator'] {
    background-color: var(--ember);
  }

  :host(:state(disabled)) [part~='separator'] {
    background-color: var(--ash);
    cursor: not-allowed;
  }

  @media (prefers-reduced-motion: reduce) {
    [part~='separator'] {
      transition: none;
    }
  }
`;

tokens.styles.ts

import { css } from 'lit';

/*
  The styled components' palette and type, taken from the logo: ink for the
  courses, ember for the base, paper behind them.

  Each is a short alias for a "--slab-styled-*" property, with the default as
  its fallback. Retheming is "--slab-styled-ember" and the rest on ":root",
  which every shadow root inherits. Were the defaults declared under those
  names here instead, each host's own declaration would shadow the page's.

  Each default is "light-dark()", so the components follow the page's
  "color-scheme": "dark" for dark, "light" for light, and "light dark" to
  follow the operating system. Ink and paper trade places in the dark, so
  ink is always the text and paper always what's behind it. Contrast is given
  as light, then dark.
*/
export default css`
  :host {
    /* Secondary text. 6.8:1 and 6.9:1 on paper. */
    --ash: var(--slab-styled-ash, light-dark(#5e5a55, #a8a19a));
    --border: 2px solid var(--ink);

    /* Behind disabled and read-only controls, and hovered rows. */
    --chalk: var(--slab-styled-chalk, light-dark(#f4f1ec, #2b2926));

    /*
      The logo's orange, brighter in the dark. 4.25:1 and 5.2:1 on paper is
      enough for borders, fills and large text, but not always for small text,
      which takes "--ember-deep".
    */
    --ember: var(--slab-styled-ember, light-dark(#d9492a, #ec5f3b));

    /*
      Orange for small text on paper, and for paper text on it. 5.7:1 and
      7.4:1.
    */
    --ember-deep: var(--slab-styled-ember-deep, light-dark(#b3381a, #ff8566));
    --font-body: var(
      --slab-styled-font-body,
      'IBM Plex Sans',
      system-ui,
      sans-serif
    );
    --font-display: var(
      --slab-styled-font-display,
      'Zilla Slab',
      rockwell,
      'Roboto Slab',
      serif
    );
    --font-mono: var(
      --slab-styled-font-mono,
      'IBM Plex Mono',
      ui-monospace,
      monospace
    );
    --ink: var(--slab-styled-ink, light-dark(#1c1c1c, #f4f1ec));
    --paper: var(--slab-styled-paper, light-dark(#fff, #1a1918));

    /* The hard, offset shadow a raised surface casts. */
    --shadow: 4px 4px 0 var(--ink);
  }
`;