From 27c90dfa8322c9f225e22a730a776c4f4cbd2c5e Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Sat, 19 Jul 2025 08:23:27 +0200
Subject: [PATCH] fix: don't reexecute derived with no dependencies on teardown
 (#16438)

The prior logic was wrong because it reexecuted when something was clean, but we want to when it's not. The remaining fix was to also check the reactions: If an effect is destroyed and it was the last reaction of a derived then the derived is set to `MAYBE_DIRTY`. We therefore also need to check if the derived still has anyone listening to it, and only then reexecute it.

Fixes #16363
---
 .changeset/clever-toys-decide.md              |  5 ++++
 .../svelte/src/internal/client/runtime.js     |  8 +++--
 .../props-derived-teardown/Teardown.svelte    | 11 +++++++
 .../samples/props-derived-teardown/_config.js | 30 +++++++++++++++++++
 .../props-derived-teardown/main.svelte        | 25 ++++++++++++++++
 5 files changed, 77 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/clever-toys-decide.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/props-derived-teardown/Teardown.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/props-derived-teardown/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/props-derived-teardown/main.svelte

diff --git a/.changeset/clever-toys-decide.md b/.changeset/clever-toys-decide.md
new file mode 100644
index 0000000000..57eb2b0058
--- /dev/null
+++ b/.changeset/clever-toys-decide.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't reexecute derived with no dependencies on teardown
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 306b9b9dd9..3d3c89975c 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -653,8 +653,12 @@ export function get(signal) {
 
 			var value = derived.v;
 
-			// if the derived is dirty, or depends on the values that just changed, re-execute
-			if ((derived.f & CLEAN) !== 0 || depends_on_old_values(derived)) {
+			// if the derived is dirty and has reactions, or depends on the values that just changed, re-execute
+			// (a derived can be maybe_dirty due to the effect destroy removing its last reaction)
+			if (
+				((derived.f & CLEAN) === 0 && derived.reactions !== null) ||
+				depends_on_old_values(derived)
+			) {
 				value = execute_derived(derived);
 			}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/Teardown.svelte b/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/Teardown.svelte
new file mode 100644
index 0000000000..f71890d31e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/Teardown.svelte
@@ -0,0 +1,11 @@
+<script>
+	import { onDestroy } from 'svelte'
+	
+	const { callback } = $props()
+	
+	onDestroy(() => {
+		callback()
+	})
+</script>
+
+<div>teardown</div>
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/_config.js b/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/_config.js
new file mode 100644
index 0000000000..c4e7d1307b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/_config.js
@@ -0,0 +1,30 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>click</button>
+				<div>teardown</div>
+				<div>1</div>
+				<div>2</div>
+				<div>3</div>
+			`
+		);
+		const [increment] = target.querySelectorAll('button');
+
+		increment.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>click</button>
+				<div>1</div>
+				<div>3</div>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/main.svelte b/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/main.svelte
new file mode 100644
index 0000000000..ff9e19b2c6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/props-derived-teardown/main.svelte
@@ -0,0 +1,25 @@
+<script>
+	import { SvelteSet } from 'svelte/reactivity'
+	import Teardown from './Teardown.svelte'
+
+	class Test {
+		originalIds = $state.raw([1, 2, 3])
+		ids = $derived(new SvelteSet(this.originalIds))
+	}
+
+	let show = $state(true)
+	const test = new Test()
+
+	function callback() {
+		test.ids.delete(2)
+	}
+</script>
+
+
+<button onclick={() => (show = !show)}>click</button>
+{#if show}
+	<Teardown {callback} />
+{/if}
+{#each test.ids as id}
+	<div>{id}</div>
+{/each}