fix: improve effect over-fire on store subscription init (#10535)

Author: trueadm

.changeset/cool-roses-trade.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve effect over-fire on store subscription init

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

@@ -1210,8 +1210,11 @@ export function set_signal_value(signal, value) {
 		//
 		// $effect(() => x++)
 		//
+		// We additionally want to skip this logic for when ignore_mutation_validation is
+		// true, as stores write to source signal on initialization.
 		if (
 			is_runes(null) &&
+			!ignore_mutation_validation &&
 			current_effect !== null &&
 			current_effect.c === null &&
 			(current_effect.f & CLEAN) !== 0

packages/svelte/tests/runtime-runes/samples/store-subscribe-effect-init/_config.js

@@ -0,0 +1,22 @@
+import { test } from '../../test';
+import { log } from './log.js';
+
+export default test({
+	html: `<button>increment</button>`,
+
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		assert.deepEqual(log, [1]);
+
+		await btn?.click();
+		assert.deepEqual(log, [1, 2]);
+
+		await btn?.click();
+		assert.deepEqual(log, [1, 2, 3]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/store-subscribe-effect-init/log.js

@@ -0,0 +1,2 @@
+/** @type {any[]} */
+export const log = [];

packages/svelte/tests/runtime-runes/samples/store-subscribe-effect-init/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	import { untrack } from 'svelte';
+	import { writable } from 'svelte/store';
+	import { log } from './log';
+
+	const storeCount = writable(0)
+	let count = $state(0);
+
+	$effect(() => {
+		$storeCount;
+		untrack(() => {
+			count++;
+			log.push(count);
+		});
+	});
+</script>
+
+<button on:click={() => $storeCount++}>increment</button>