fix: skip effects inside dynamic component that is about to be destroyed

When a dynamic component was updated to a different instance and its props were updated at the same time, effects inside the component were still called with the already-changed props. The fix is to mark the branch as skipped to never got to those effects. Fixes #16387
sveltejs · dummdidumm · Aug 11, 2025 · Aug 11, 2025 · Aug 11, 2025 · Aug 11, 2025
commit 7a153cefe49181006952519eb0634de833ad900e
diff --git a/.changeset/neat-files-do.md b/.changeset/neat-files-do.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: skip effects inside dynamic component that is about to be destroyed
diff --git a/packages/svelte/src/internal/client/dom/blocks/svelte-component.js b/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
@@ -62,8 +62,10 @@ export function component(node, get_component, render_fn) {
 			if (defer) {
 				offscreen_fragment = document.createDocumentFragment();
 				offscreen_fragment.append((target = create_text()));
+				if (effect) {
+					/** @type {Batch} */ (current_batch).skipped_effects.add(effect);
+				}
 			}
-
 			pending_effect = branch(() => render_fn(target, component));
 		}
 

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -11,7 +11,8 @@ import {
 	RENDER_EFFECT,
 	ROOT_EFFECT,
 	USER_EFFECT,
-	MAYBE_DIRTY
+	MAYBE_DIRTY,
+	EFFECT_RAN
 } from '#client/constants';
 import { async_mode_flag } from '../../flags/index.js';
 import { deferred, define_property } from '../../shared/utils.js';
@@ -599,6 +600,7 @@ function flush_queued_effects(effects) {
 
 		if ((effect.f & (DESTROYED | INERT)) === 0 && is_dirty(effect)) {
 			var n = current_batch ? current_batch.current.size : 0;
+			var ran = effect.f & EFFECT_RAN;
 
 			update_effect(effect);
 
@@ -622,6 +624,7 @@ function flush_queued_effects(effects) {
 			// if state is written in a user effect, abort and re-schedule, lest we run
 			// effects that should be removed as a result of the state change
 			if (
+				// ran &&
 				current_batch !== null &&
 				current_batch.current.size > n &&
 				(effect.f & USER_EFFECT) !== 0

diff --git a/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/Comp-1.svelte b/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/Comp-1.svelte
@@ -0,0 +1,7 @@
+<script>
+	let { data } = $props();
+</script>
+
+{#each data.obj.arr as i}
+	<p>{i}</p>
+{/each}
diff --git a/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/Comp-2.svelte b/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/Comp-2.svelte
@@ -0,0 +1 @@
+<p>Comp 2</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/_config.js b/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/_config.js
@@ -0,0 +1,11 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [btn] = target.querySelectorAll('button');
+		btn.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<button>Change</button> <p>Comp 2</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/main.svelte b/packages/svelte/tests/runtime-runes/samples/svelte-component-props-update/main.svelte
@@ -0,0 +1,16 @@
+<script>
+	import Comp_1 from './Comp-1.svelte';
+	import Comp_2 from './Comp-2.svelte';
+
+	let Comp = $state.raw(Comp_1);
+	let data = $state.raw({ obj: { arr: [1, 2, 3] } });
+
+	function change() {
+		Comp = Comp_2;
+		data = {};
+	}
+</script>
+
+<button onclick={change}>Change</button>
+
+<Comp {data} />