fix: null out effect.b in destroy_effect to prevent memory leak

.changeset/fix-memory-leak-boundary.md

@@ -0,0 +1,9 @@
+---
+'svelte': patch
+---
+
+fix: null out `effect.b` (boundary reference) in `destroy_effect` to prevent memory leak
+
+When a dynamic component switches (e.g., `<Component />`), old branch effects are destroyed but were still retaining a reference to the `Boundary` instance via the `effect.b` field. This prevented the garbage collector from reclaiming memory for destroyed component subtrees, causing memory to grow without bound when repeatedly changing a mounted component's props.
+
+Fixes #17881

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

@@ -559,6 +559,7 @@ export function destroy_effect(effect, remove_dom = true) {
 		effect.fn =
 		effect.nodes =
 		effect.ac =
+		effect.b =
 			null;
 }
 

packages/svelte/tests/runtime-runes/samples/dynamic-component-memory-leak/About.svelte

@@ -0,0 +1 @@
+<p>About</p>

packages/svelte/tests/runtime-runes/samples/dynamic-component-memory-leak/Home.svelte

@@ -0,0 +1 @@
+<p>Home</p>

packages/svelte/tests/runtime-runes/samples/dynamic-component-memory-leak/_config.js

@@ -0,0 +1,26 @@
+import { flushSync } from 'svelte';
+
+export default {
+	html: '<p>Home</p>',
+
+	async test({ assert, target, component }) {
+		// Switch to About
+		component.page = 'About';
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<p>About</p>');
+
+		// Switch back to Home
+		component.page = 'Home';
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<p>Home</p>');
+
+		// Rapidly switch multiple times to stress test cleanup
+		for (let i = 0; i < 10; i++) {
+			component.page = i % 2 === 0 ? 'About' : 'Home';
+			flushSync();
+		}
+
+		// Should end on Home (10 is even, 0-indexed last is About → Home)
+		assert.htmlEqual(target.innerHTML, '<p>Home</p>');
+	}
+};

packages/svelte/tests/runtime-runes/samples/dynamic-component-memory-leak/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	import Home from './Home.svelte';
+	import About from './About.svelte';
+	let { page = 'Home' } = $props();
+
+	const components = { Home, About };
+	let Component = $derived(components[page]);
+</script>
+
+<Component />