fix(layout): resolve LayoutGroup id at init and stop swallowing context errors

[JonathonRP]

The problem

packages/motion-start/src/motion/index.svelte.ts assembled configAndProps like this:

const configAndProps = $derived.by(() => {
	const propsState = $state({
		...motionConfig,
		...motionProps,
		layoutId: useLayoutId(() => motionProps),
	});
	return propsState;
});

useLayoutId calls useLayoutGroupContext(), which reaches into Svelte's context system. getContext/hasContext are component-initialisation APIs — they throw lifecycle_outside_component whenever component_context is null. Putting one inside a value that recomputes on every props change means the context lookup is re-entered long after initialisation, from whatever call site happens to pull on the derived.

That was made invisible by a blanket catch in packages/motion-start/src/context/LayoutGroupContext.svelte.ts:

function useLayoutGroupContext() {
	try {
		return getLayoutGroupContext();
	} catch {
		return null;
	}
}

The catch existed for a legitimate reason — motion elements are routinely rendered with no LayoutGroup ancestor, and createContext's getter throws missing_context in that case. But it caught everything, so a lifecycle violation was indistinguishable from "no provider" and degraded to layoutGroupId === undefined. The observable symptom would be layoutId losing its LayoutGroup prefix after a prop change, which silently breaks shared-layout animations between grouped elements.

Honest scoping note

I wrote the regression test first and it passed against the unfixed code. Digging into svelte@5.56.8, update_reaction restores the reaction's captured component context before recomputing (set_component_context(reaction.ctx) in internal/client/runtime.js), and $derived signals capture ctx: component_context at creation (internal/client/reactivity/deriveds.js). Because renderMotionComponent runs inside MotionScope's initialisation, the derived captures a non-null ctx and getContext still resolves on recompute. So on this Svelte version the prefix does not actually drop today.

What remains real, and is what this PR fixes:

• The call is still an initialisation-only API invoked from a recomputing derived. It only works by virtue of a Svelte implementation detail about ctx restoration, and it breaks the moment the value is pulled from outside a reaction (a tick().then(), a rAF callback, a microtask render) or the derived is created without a component context.
• The blanket catch genuinely does erase that failure mode. That is the part that made the hazard undiagnosable, and it is removed.
• The redundant per-recompute context lookup is now done once.

I've left the reproduction test in as a regression guard rather than deleting it, and added a direct test that the context helper throws outside initialisation.

The fix

• Resolve once at init. const layoutGroupId = useLayoutGroupContext()?.id; now sits in the component body, outside any $derived. The derived reads the already-resolved value.
• useLayoutId → getLayoutId. The old use* hook was the footgun; it's replaced by a pure getLayoutId(props, layoutGroupId) that is safe to call from a derived. Nothing else in the repo referenced useLayoutId.
• No more blanket catch. useLayoutGroupContext now uses an explicit hasContext check for the documented "no provider" case and lets everything else throw. I checked all four call sites first:
  • LayoutGroup.svelte — useLayoutGroupContext() || { id: oldId }, legitimately runs with no provider at the root of a group. Covered by hasContext.
  • MeasureLayout.svelte — ?? { forceRender: () => {} }, same.
  • motion/index.svelte.ts — now init-only.
  • LayoutGroupConsumer.svelte (test fixture) — now has explicit coverage for the no-provider case.
• Dead code removed. createLayoutGroupContext and LayoutGroupContextType (the rune-store implementation) had no importers anywhere in the repo — only doc/plan files mention them. Deleted, along with their re-export from the src/context barrel. The barrel isn't in package.json#exports and isn't imported anywhere, so this isn't a public API break.

I kept the .svelte.ts extension on LayoutGroupContext.svelte.ts even though it no longer contains runes, because src/index.ts re-exports the LayoutGroupContext type from that path.

Tests

• layout-group-motion.svelte.spec.ts (new): mounts motion.div with layoutId="box" inside <LayoutGroup id="group">, changes a style prop, and asserts both visualElement.getProps().layoutId and projection.options.layoutId are still group-box. It asserts the props recompute actually happened (style.width === '200px') so it can't pass vacuously.
• layout-group.svelte.spec.ts: added coverage that useLayoutGroupContext() returns null with no provider and throws when called outside component initialisation.

Verification

• bun run test:run — 620 passed, 1 skipped. The single failure (package-imports.spec.ts "fully specifies relative JavaScript imports") is a 5s-timeout flake on this machine and reproduces identically on a clean git stashed tree; it's unrelated to this change.
• bun run check (svelte-check) — 0 errors, 0 warnings.
• biome lint on the touched files — no new diagnostics (remaining ones are pre-existing noExplicitAny/CRLF noise shared with main).

Follow-up, not done here

use-visual-element.svelte.ts has the same shape — $derived(useSwitchLayoutGroupContext()) — and SwitchLayoutGroupContext.ts / DeprecatedLayoutGroupContext.ts both still use blanket try/catch. Left alone deliberately to keep this PR scoped; worth folding into the planned context refactor.

[changeset-bot]

🦋 Changeset detected

Latest commit: 57478b4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
motion-start	Patch

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📥 Commits

Reviewing files that changed from the base of the PR and between 0adcef2 and 57478b4.

📒 Files selected for processing (7)

• .changeset/slimy-hoops-repair.md
• packages/motion-start/src/components/LayoutGroup/__tests__/LayoutGroupMotionFixture.svelte
• packages/motion-start/src/components/LayoutGroup/__tests__/layout-group-motion.svelte.spec.ts
• packages/motion-start/src/components/LayoutGroup/__tests__/layout-group.svelte.spec.ts
• packages/motion-start/src/context/LayoutGroupContext.svelte.ts
• packages/motion-start/src/context/index.svelte.ts
• packages/motion-start/src/motion/index.svelte.ts

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commit: 57478b4