From d3a01bd5a7d09384e29a599834d9b554a7a973ee Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 20 Jul 2025 17:19:56 -0400
Subject: [PATCH] fix: always mark reactions of deriveds (#16457)

* tweak

* fix

* test

* changeset
---
 .changeset/angry-hornets-hug.md               |  5 ++
 .../internal/client/reactivity/deriveds.js    |  4 +-
 .../src/internal/client/reactivity/sources.js | 19 +++----
 .../async-time-travelling-derived/_config.js  | 56 +++++++++++++++++++
 .../async-time-travelling-derived/main.svelte | 26 +++++++++
 5 files changed, 98 insertions(+), 12 deletions(-)
 create mode 100644 .changeset/angry-hornets-hug.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/main.svelte

diff --git a/.changeset/angry-hornets-hug.md b/.changeset/angry-hornets-hug.md
new file mode 100644
index 0000000000..ffe59db100
--- /dev/null
+++ b/.changeset/angry-hornets-hug.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: always mark reactions of deriveds
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index fa6a9e02a1..7f730e365e 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -337,7 +337,9 @@ export function update_derived(derived) {
 
 	// don't mark derived clean if we're reading it inside a
 	// cleanup function, or it will cache a stale value
-	if (is_destroying_effect) return;
+	if (is_destroying_effect) {
+		return;
+	}
 
 	if (batch_deriveds !== null) {
 		batch_deriveds.set(derived, derived.v);
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 9b534d2d71..f6b14f3360 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -314,9 +314,6 @@ function mark_reactions(signal, status) {
 		var reaction = reactions[i];
 		var flags = reaction.f;
 
-		// Skip any effects that are already dirty
-		if ((flags & DIRTY) !== 0) continue;
-
 		// In legacy mode, skip the current effect to prevent infinite loops
 		if (!runes && reaction === active_effect) continue;
 
@@ -326,15 +323,15 @@ function mark_reactions(signal, status) {
 			continue;
 		}
 
-		set_signal_status(reaction, status);
+		// don't set a DIRTY reaction to MAYBE_DIRTY
+		if ((flags & DIRTY) === 0) {
+			set_signal_status(reaction, status);
+		}
 
-		// If the signal a) was previously clean or b) is an unowned derived, then mark it
-		if ((flags & (CLEAN | UNOWNED)) !== 0) {
-			if ((flags & DERIVED) !== 0) {
-				mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
-			} else {
-				schedule_effect(/** @type {Effect} */ (reaction));
-			}
+		if ((flags & DERIVED) !== 0) {
+			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
+		} else if ((flags & DIRTY) === 0) {
+			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}
 }
diff --git a/packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/_config.js b/packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/_config.js
new file mode 100644
index 0000000000..06437d2e31
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/_config.js
@@ -0,0 +1,56 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [a, b, update] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>a</button>
+				<button>b</button>
+				<button>0</button>
+				<h1>a</h1>
+			`
+		);
+
+		b.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>a</button>
+				<button>b</button>
+				<button>0</button>
+				<h1>b</h1>
+			`
+		);
+
+		update.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>a</button>
+				<button>b</button>
+				<button>1</button>
+				<h1>b</h1>
+			`
+		);
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>a</button>
+				<button>b</button>
+				<button>1</button>
+				<h1>a</h1>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/main.svelte
new file mode 100644
index 0000000000..5fca286a78
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-time-travelling-derived/main.svelte
@@ -0,0 +1,26 @@
+<script>
+	let object = $state(null);
+	let count = $state(0);
+
+	const condition = $derived(object === null);
+</script>
+
+<svelte:boundary>
+	<button onclick={() => (object = null)}>a</button>
+	<button onclick={() => (object = {})}>b</button>
+
+	<button onclick={async () => {
+		count++;
+		await Promise.resolve();
+		object = {};
+	}}>{await count}</button>
+
+	{#if condition}
+		<h1>a</h1>
+	{:else}
+		<h1>b</h1>
+	{/if}
+
+	{#snippet pending()}{/snippet}
+</svelte:boundary>
+