Merge 55cf7e9ded into 020242d6be

.changeset/calm-forks-read.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: evaluate uninitialized derived values outside their fork batch

packages/svelte/src/internal/client/runtime.js

@@ -681,7 +681,7 @@ export function get(signal) {
 
 		var is_new = (derived.f & REACTION_RAN) === 0;
 
-		if (is_dirty(derived)) {
+		if ((derived.v === UNINITIALIZED && !batch_values?.has(derived)) || is_dirty(derived)) {
 			if (should_connect) {
 				// set the flag before `update_derived`, so that the derived
 				// is added as a reaction to its dependencies

packages/svelte/tests/runtime-runes/samples/fork-derived-uninitialized-attribute/Child.svelte

@@ -0,0 +1,6 @@
+<script>
+	const props = $props();
+	const klass = (value) => `box ${value}`;
+</script>
+
+<button class={klass(props.label)} disabled={props.disabled}>x</button>

packages/svelte/tests/runtime-runes/samples/fork-derived-uninitialized-attribute/_config.js

@@ -0,0 +1,14 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	test({ assert, target }) {
+		const [fork, update] = target.querySelectorAll('button');
+
+		flushSync(() => fork.click());
+		flushSync(() => update.click());
+
+		assert.htmlEqual(target.innerHTML, '<button>fork</button> <button>update</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-uninitialized-attribute/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let target = $state(false);
+	let disabled = $state(false);
+</script>
+
+<button onclick={() => fork(() => (target = true))}>fork</button>
+<button onclick={() => (disabled = true)}>update</button>
+
+{#if target}
+	<Child label="a" {disabled} />
+{/if}