ix: ensure transient writes to tracked parent effects works as expected

Author: trueadm

.changeset/brown-rockets-shake.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure transient writes to tracked parent effects works as expected

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

@@ -460,6 +460,13 @@ export function update_reaction(reaction) {
 		// the same version
 		if (previous_reaction !== null) {
 			read_version++;
+			if (untracked_writes !== null) {
+				if (previous_untracked_writes === null) {
+					previous_untracked_writes = untracked_writes;
+				} else {
+					previous_untracked_writes.push(.../** @type {Source[]} */ (untracked_writes));
+				}
+			}
 		}
 
 		return result;

packages/svelte/tests/runtime-runes/samples/untracked-write-pre/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, logs }) {
+		assert.deepEqual(logs, ["Outer", 'Inner', "Outer", 'Inner']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/untracked-write-pre/main.svelte

@@ -0,0 +1,14 @@
+<script>
+
+	let value = $state(0);
+
+	$effect.pre(() => {
+			console.log("Outer");
+			value;
+
+			$effect.pre(() => {
+					console.log("Inner");
+					value = 10;
+			});
+	});
+</script>