feat(split): responsive + percentage snap points, snapFrom, onSnap callback

[gfazioli]

Summary

Builds on top of #37 (merged) with the four enhancements we agreed on:

Enhancement	Prop / API	Closes
Percentages + responsive sets	snapPoints now accepts Array<number | string> and ResponsiveValue<...>	#39
Snap relative to the "after" pane	snapFrom?: 'before' | 'after' (default 'before')	#40
onSnap callback	onSnap?: (point: number | null) => void on Split.Resizer	#41
Visual hook	data-snapping attribute on the resizer root while in a snap zone	—

Plus:

• normalizeSnapPoints now rejects negative values and malformed percentage strings, and trims whitespace from valid ones.
• useResponsiveValue treats arrays as plain values (previously it would have tried to match an array as a breakpoint map).
• New documentation section + dedicated demo (snapAdvanced).

onSnap callback semantics

• Fired once when the resizer enters a snap zone, with the resolved pixel value of the snap point (expressed in the snapFrom reference — so for snapFrom="after" you get back the target size of the right/bottom pane, matching what the user passed into snapPoints).
• Fired once when it leaves, with null.
• Not fired on every pointer tick.

API shape decisions

• onSnap lives only on Split.Resizer (not cascaded via context). Rationale: with multiple resizers, a single callback on Split would receive events from all of them without a way to distinguish. Keeps v1 simple. A cascaded-with-meta variant can come later.
• snapFrom is a single prop (not per-point). User preference: keep the simple case simple; the mixed case is rare enough to defer.

Test plan

• yarn test — 27/27 jest tests (17 snap helpers + 10 Split integration), 0 oxlint warnings, typecheck + format clean.
• yarn clean && yarn build && yarn docgen && yarn docs:build — all OK.
• Visual check at http://localhost:9281 (responsive sets, percentage resolution, onSnap reactivity, data-snapping attribute).

Follow-up

After merge, release as minor (4.0.7 → 4.1.0). Changelog will credit @woutervanerp for the original snap points feature (already merged via #37) and cover these enhancements.

[Copilot]

Pull request overview

Extends Split/Resizer snap functionality to support percentage and responsive snap point sets, add snapFrom to snap relative to the after pane, and introduce an onSnap callback + data-snapping attribute for UI feedback.

Changes:

• Expand snapPoints to accept number | string (percentages) and responsive breakpoint maps; add snapFrom support.
• Add onSnap(point | null) eventing and data-snapping state while in a snap zone.
• Update tests and docs with new behaviors and an “advanced snapping” demo.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
package/src/hooks/use-responsive-value.ts	Treat arrays as scalar values so responsive resolution doesn’t mis-handle snapPoints arrays.
package/src/Split.tsx	Plumbs snapFrom through Split → context so resizers can inherit it.
package/src/Split.test.tsx	Adds integration tests for percentage snapping, snapFrom="after", onSnap, and data-snapping.
package/src/Resizer/snap.ts	Adds snap point parsing/validation, percentage resolution, snapFrom, and reporting of snappedPoint.
package/src/Resizer/snap.test.ts	Adds/updates helper tests for new snap point types and snappedPoint reporting.
package/src/Resizer/SplitResizer.tsx	Resolves responsive snap sets, tracks snapping state, emits onSnap, and exposes data-snapping.
docs/docs.mdx	Documents percentage/responsive snap sets, snapFrom, onSnap, and data-snapping; adds advanced demo section.
docs/demos/index.ts	Exports the new snapAdvanced demo.
docs/demos/Split.demo.snapAdvanced.tsx	Adds a new demo showcasing percentages, snapFrom="after", responsive sets, and onSnap.