From 69f0bb549f026285af28f7a82ea988a34dabab26 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 13 Oct 2026 12:07:23 -0400
Subject: [PATCH] restore resetting effects to CLEAN in is_dirty while time
 travelling

Effects left MAYBE_DIRTY are forced to re-run when a paused branch is resumed.
---
 .../svelte/src/internal/client/runtime.js     |  6 ++-
 .../async-effect-resume-no-rerun/Child.svelte |  9 +++++
 .../async-effect-resume-no-rerun/_config.js   | 37 +++++++++++++++++++
 .../async-effect-resume-no-rerun/main.svelte  | 29 +++++++++++++++
 4 files changed, 79 insertions(+), 2 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 62d26c0391..07f8773915 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -185,8 +185,10 @@ export function is_dirty(reaction) {
 		if (
 			(flags & CONNECTED) !== 0 &&
 			// During time traveling we don't want to reset the status so that
-			// traversal of the graph in the other batches still happens
-			batch_values === null
+			// traversal of the graph in the other batches still happens. Effects
+			// can be reset because block/async effects execute right away and others
+			// are deferred and re-dirtied as needed.
+			(batch_values === null || (flags & DERIVED) === 0)
 		) {
 			set_signal_status(reaction, CLEAN);
 		}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte
new file mode 100644
index 0000000000..6f48eaf129
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte
@@ -0,0 +1,9 @@
+<script>
+	let { big } = $props();
+
+	$effect(() => {
+		console.log(`effect ${big}`);
+	});
+</script>
+
+<p>child</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js
new file mode 100644
index 0000000000..4fd60a98ce
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js
@@ -0,0 +1,37 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs, raf }) {
+		await tick();
+		const [x, s, toggle, shift] = target.querySelectorAll('button');
+		assert.deepEqual(logs, ['effect false']);
+
+		// keep an earlier batch pending, so that the next one commits while time travelling
+		x.click();
+		await tick();
+
+		// `big` stays `false`, so the effect must not re-run
+		s.click();
+		await tick();
+		assert.deepEqual(logs, ['effect false']);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>x</button><button>s</button><button>toggle</button><button>shift</button><p>1</p><div><p>child</p></div>'
+		);
+
+		// outro, then reintro before the outro finished: the branch is resumed
+		toggle.click();
+		await tick();
+		raf.tick(50);
+		toggle.click();
+		await tick();
+		raf.tick(200);
+
+		assert.deepEqual(logs, ['effect false']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte
new file mode 100644
index 0000000000..30d68f614b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte
@@ -0,0 +1,29 @@
+<script>
+	import { fade } from 'svelte/transition';
+	import Child from './Child.svelte';
+
+	let x = $state(0);
+	let s = $state(0);
+	let show = $state(true);
+	const big = $derived(s > 100);
+
+	const queued = [];
+
+	function push(v) {
+		if (v === 0) return v;
+		return new Promise((resolve) => queued.push(() => resolve(v)));
+	}
+</script>
+
+<button onclick={() => x++}>x</button>
+<button onclick={() => s++}>s</button>
+<button onclick={() => (show = !show)}>toggle</button>
+<button onclick={() => queued.shift()?.()}>shift</button>
+
+<p>{await push(x)}</p>
+
+{#if show}
+	<div transition:fade={{ duration: 100 }}>
+		<Child {big} />
+	</div>
+{/if}