Splitter

Accessible, unstyled resizable panel groups following the WAI-ARIA window splitter pattern.


Package	`@vizejs/ui/splitter`
Maturity	stable
Own the source	`vize lib pull splitter`
Requires	collection, context, controllable-state, id
Aliases	resizable panels, split view, split pane, window splitter, panel group
Covers	WAI-ARIA window splitter pattern, react-resizable-panels, Reka UI Splitter, Ark UI Splitter

Usage

import { SplitterGroup, SplitterHandle, SplitterPanel } from "@vizejs/ui/splitter";

Or copy the source into your project with vize lib pull splitter (see Source Distribution).

API

`SplitterGroup`

Source: src/families/layout/splitter/splitter-group.vue

Props

Prop	Type	Default	Description
`id`	`string \| null`	`undefined`	Consumer-owned group id. `null` and `undefined` select a deterministic fallback.
`layout`	`SplitterLayout`	`undefined`	Controlled panel sizes in percent (`v-model:layout`). `undefined` selects uncontrolled behavior, which also lets `useSplitterPersistence` load a stored layout after mount.
`defaultLayout`	`SplitterLayout`	`undefined`	Initial sizes by panel index. Panels without an entry use their `defaultSize`.
`orientation`	`SplitterOrientation`	`"horizontal"`	Layout axis. `"horizontal"` places panels side by side with vertical separators.
`dir`	`SplitterDirection`	`undefined`	Reading direction used to map horizontal arrow keys and pointer movement. `undefined` inherits `DirectionProvider`/`LocaleProvider`, then `"ltr"`.
`disabled`	`boolean`	`false`	Disable pointer and keyboard resizing while keeping the current layout.
`keyboardStep`	`number`	`10`	Percent moved by one arrow key press on a handle.

Events

Event	Payload	Description
`update:layout`	`[layout: SplitterLayout]`	Fired with the next layout whenever sizes change.
`resize`	`[layout: SplitterLayout, reason: SplitterResizeReason]`	Fired after a distinct layout change with the input that caused it.
`resizeStart`	`[handleIndex: number]`	Fired when a pointer drag starts on a handle.
`resizeEnd`	`[layout: SplitterLayout]`	Fired when a pointer drag ends.

Slots

Slot	Slot props	Description
`default`	`SplitterGroupSlotState`	SplitterPanel and SplitterHandle children. Receives the resolved layout and drag state.

Exposed

Member	Type	Description
`element`	`HTMLDivElement \| null`	Rendered group element.
`id`	`string`	Group base id.
`layout`	`SplitterLayout`	Resolved layout.
`setLayout`	`(layout: SplitterLayout) => boolean`	Request a new layout. Invalid lengths are rejected and sizes are clamped to constraints.
`reset`	`() => boolean`	Restore `defaultLayout` or the panel defaults.

`SplitterHandle`

Source: src/families/layout/splitter/splitter-handle.vue

Props

Prop	Type	Default	Description
`id`	`string \| null`	`undefined`	Consumer-owned separator id.
`disabled`	`boolean`	`false`	Disable this handle while keeping the rest of the group resizable.
`order`	`number`	`undefined`	Deterministic order for conditionally rendered handles.
`ariaLabel`	`string`	`undefined`	Accessible name, for example "Resize sidebar".
`ariaLabelledby`	`string`	`undefined`	Space-separated ids that label the separator.

Slots

Slot	Slot props	Description
`default`	`SplitterHandleSlotState`	Optional grip content. Receives the value, orientation, and drag state.

Exposed

Member	Type	Description
`element`	`HTMLDivElement \| null`	Rendered separator element.
`value`	`number`	Size in percent of the panel before the handle.
`focus`	`(options?: FocusOptions) => void`	Move focus to the separator.

`SplitterPanel`

Source: src/families/layout/splitter/splitter-panel.vue

Props

Prop	Type	Default	Description
`id`	`string \| null`	`undefined`	Consumer-owned panel id, also used by `aria-controls` on the adjacent handle.
`defaultSize`	`number`	`undefined`	Initial size in percent. Give every panel a size for exact server rendering.
`minSize`	`number`	`0`	Smallest expanded size in percent.
`maxSize`	`number`	`100`	Largest size in percent.
`collapsible`	`boolean`	`false`	Allow the panel to snap to `collapsedSize` when dragged past half of `minSize`.
`collapsedSize`	`number`	`0`	Size in percent while collapsed.
`order`	`number`	`undefined`	Deterministic order for conditionally rendered panels.

Events

Event	Payload	Description
`collapse`	`[]`	Fired when the panel snaps to its collapsed size.
`expand`	`[]`	Fired when a collapsed panel grows back to at least its minimum size.
`resize`	`[size: number, previous: number]`	Fired with the panel's new size whenever it changes.

Slots

Slot	Slot props	Description
`default`	`SplitterPanelSlotState`	Panel content. Receives the current size and collapsed state.

Exposed

Member	Type	Description
`element`	`HTMLDivElement \| null`	Rendered panel element.
`id`	`string`	Panel id.
`size`	`number`	Current size in percent.
`collapsed`	`boolean`	Whether the panel is collapsed.
`collapse`	`() => boolean`	Collapse a collapsible panel, remembering its size.
`expand`	`() => boolean`	Restore a collapsed panel to its remembered or minimum size.
`resize`	`(size: number) => boolean`	Resize the panel toward `size` percent through its adjacent handle.

Behavior

Normative state x input -> outcome table for splitter-group.vue, splitter-panel.vue, and splitter-handle.vue (@vizejs/ui/splitter), plus the useSplitterPersistence hook and the pure layout helpers. Every row is proven by the named test in splitter.test.ts, splitter-layout.test.ts, or splitter-ssr.test.ts; compile-only guarantees live in splitter.types.test-d.ts.

The handle follows the WAI-ARIA APG window splitter pattern: a focusable role="separator" whose aria-valuenow is the size of the panel before it (its primary pane, named by aria-controls) and whose aria-orientation is perpendicular to the group axis. Sizes are percentages of the group and the only inline styles are the flex declarations that carry them.

ID	State	Input	Outcome	Evidence
S1	default	render	flex group, `flex-grow` sizes, separator value/min/max/controls, perpendicular orientation	`renders the APG window splitter contract with flex-carried sizes`
S2	focused handle	ArrowLeft/Right, Home, End	resize by `keyboardStep` inside min/max; Home/End jump to the primary pane's bounds	`arrow keys resize by the keyboard step within constraints; Home and End jump`
S3	vertical group / RTL	ArrowUp/Down, ArrowLeft	vertical groups use block arrows; RTL flips inline arrows	`vertical groups use ArrowUp/ArrowDown and RTL flips horizontal arrows`
S4	collapsible primary pane	Enter, Home, ArrowRight	Enter collapses and restores the remembered size; collapsed panes grow straight to `minSize`	`collapsible panels snap closed past half their minimum and Enter toggles them`
S5	pointer drag	pointerdown, move, up	sizes follow the pointer from the drag origin, clamp to constraints, emit start/end	`pointer drags resize from the drag origin and report start and end`
S6	disabled group	keys, pointer	no resize, separator leaves the tab order and reports `aria-disabled`	`disabled groups and handles ignore keyboard and pointer input`
S7	controlled `layout`	resize, invalid layout	update events fire while the parent owns sizes; wrong panel counts fall back to defaults	`controlled layouts wait for the parent and invalid layouts fall back to defaults`
S8	exposed instances	setLayout, collapse, expand, resize, reset	typed imperative layout control with validation	`exposes group and panel controls for programmatic layout changes`
S9	nested groups	keys on the inner handle	each group owns its own panels and handles	`nested groups resize independently`
S10	`useSplitterPersistence`	mount, resize, malformed storage	defaults render first, the stored layout loads after mount, changes are written back	`useSplitterPersistence loads after mount and writes later layouts`
S11	missing provider	setup	panels and handles fail closed with the shared context diagnostic	`compound parts require a matching group provider`
S12	layout helpers	validate, default, clamp, resize	remainder sharing, clamping, cascading resize, and midpoint collapse snapping	`splitter-layout.test.ts`
S13	SSR and hydration	isolated render/mount	byte-identical exact sizes, controlled layouts before registration, hydration without warnings	`splitter-ssr.test.ts`

Notes

For exact server rendering give every panel a defaultSize, or pass defaultLayout/layout to the group; unsized panels share the remainder once all panels have registered.
Resizing only moves the boundary between the handle's neighbours, cascading through further panels when a neighbour reaches its minimum.