fix: ensure reactive graph is fully traversed in the marking phase for non-runes mode (#13059)

* fix: ensure signal status is set in legacy mode

fixes #13051

* add (failing) test

* alternative fix

* add changeset

* alternative fix

* add comment

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
Co-authored-by: Dominic Gannaway <dg@domgan.com>

.changeset/angry-weeks-design.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure reactive graph is fully traversed in the marking phase for non-runes mode

packages/svelte/src/internal/client/reactivity/effects.js

@@ -33,7 +33,8 @@ import {
 	UNOWNED,
 	CLEAN,
 	INSPECT_EFFECT,
-	HEAD_EFFECT
+	HEAD_EFFECT,
+	MAYBE_DIRTY
 } from '../constants.js';
 import { set } from './sources.js';
 import * as e from '../errors.js';
@@ -281,6 +282,12 @@ export function legacy_pre_effect_reset() {
 		for (var token of context.l.r1) {
 			var effect = token.effect;
 
+			// If the effect is CLEAN, then make it MAYBE_DIRTY. This ensures we traverse through
+			// the effects dependencies and correctly ensure each dependency is up-to-date.
+			if ((effect.f & CLEAN) !== 0) {
+				set_signal_status(effect, MAYBE_DIRTY);
+			}
+
 			if (check_dirtiness(effect)) {
 				update_effect(effect);
 			}

packages/svelte/tests/runtime-legacy/samples/store-updated-in-reactive-statement/Child.svelte

@@ -0,0 +1,15 @@
+<script>
+	import { writable } from 'svelte/store';
+	import { store } from './state.js';
+
+	export let value;
+
+	const copy = writable(value);
+
+	$: {
+		copy.set(value);
+		store.set({ value });
+	}
+</script>
+
+<p>{$copy}</p>

packages/svelte/tests/runtime-legacy/samples/store-updated-in-reactive-statement/_config.js

@@ -0,0 +1,18 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+import { store } from './state.js';
+
+export default test({
+	html: '<p>0</p><button>1</button>',
+
+	before_test() {
+		store.set({ value: 0 });
+	},
+
+	async test({ assert, target }) {
+		const button = target.querySelector('button');
+		flushSync(() => button?.click());
+
+		assert.htmlEqual(target.innerHTML, '<p>1</p><button>1</button>');
+	}
+});

packages/svelte/tests/runtime-legacy/samples/store-updated-in-reactive-statement/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	import { store } from './state.js';
+	import Child from './Child.svelte';
+</script>
+
+<Child value={$store.value} />
+
+<button on:click={() => store.set({ value: 1 })}>1</button>

packages/svelte/tests/runtime-legacy/samples/store-updated-in-reactive-statement/state.js

@@ -0,0 +1,3 @@
+import { writable } from 'svelte/store';
+
+export const store = writable({ value: 0 });