fix: prevent reactive snippet from reinitializing unnecessarily

.changeset/light-pens-watch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent reactive snippet from reinitializing unnecessarily

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -1734,18 +1734,20 @@ export const template_visitors = {
 		if (node.argument) {
 			args.push(b.thunk(/** @type {import('estree').Expression} */ (context.visit(node.argument))));
 		}
-		const snippet_function = /** @type {import('estree').Expression} */ (
+
+		let snippet_function = /** @type {import('estree').Expression} */ (
 			context.visit(node.expression)
 		);
-		const init = b.call(
-			context.state.options.dev ? b.call('$.validate_snippet', snippet_function) : snippet_function,
-			...args
-		);
+		if (context.state.options.dev) {
+			snippet_function = b.call('$.validate_snippet', snippet_function);
+		}
 
 		if (is_reactive) {
-			context.state.init.push(b.stmt(b.call('$.snippet_effect', b.thunk(init))));
+			context.state.init.push(
+				b.stmt(b.call('$.snippet_effect', b.thunk(snippet_function), ...args))
+			);
 		} else {
-			context.state.init.push(b.stmt(init));
+			context.state.init.push(b.stmt(b.call(snippet_function, ...args)));
 		}
 	},
 	AnimateDirective(node, { state, visit }) {

packages/svelte/src/internal/client/render.js

@@ -3170,13 +3170,18 @@ export function sanitize_slots(props) {
 }
 
 /**
- * @param {() => void} create_snippet
+ * @param {() => Function} get_snippet
+ * @param {Node} node
+ * @param {() => any} args
  * @returns {void}
  */
-export function snippet_effect(create_snippet) {
+export function snippet_effect(get_snippet, node, args) {
 	const block = create_snippet_block();
 	render_effect(() => {
-		create_snippet();
+		// Only rerender when the snippet function itself changes,
+		// not when an eagerly-read prop inside the snippet function changes
+		const snippet = get_snippet();
+		untrack(() => snippet(node, args));
 		return () => {
 			if (block.d !== null) {
 				remove(block.d);

packages/svelte/tests/runtime-runes/samples/snippet-reactive-args/_config.js

@@ -0,0 +1,62 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<p>snippet: 0</p>
+		<button>toggle</button>
+		<button>increase count</button>
+	`,
+	props: {
+		get log() {
+			return [];
+		}
+	},
+
+	async test({ assert, target, component }) {
+		const [toggle, increment] = target.querySelectorAll('button');
+
+		await increment?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>snippet: 1</p>
+				<button>toggle</button>
+				<button>increase count</button>
+			`
+		);
+		assert.deepEqual(component.log, []);
+
+		await toggle?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>component: 1</p>
+				<button>toggle</button>
+				<button>increase count</button>
+			`
+		);
+		assert.deepEqual(component.log, [1]);
+
+		await increment?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>component: 2</p>
+				<button>toggle</button>
+				<button>increase count</button>
+			`
+		);
+		assert.deepEqual(component.log, [1]);
+
+		await toggle?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>snippet: 2</p>
+				<button>toggle</button>
+				<button>increase count</button>
+			`
+		);
+		assert.deepEqual(component.log, [1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/snippet-reactive-args/inner.svelte

@@ -0,0 +1,6 @@
+<script>
+    let { count, log } = $props();
+    log.push(count);
+</script>
+
+<p>component: {count}</p>

packages/svelte/tests/runtime-runes/samples/snippet-reactive-args/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	import Inner from "./inner.svelte";
+
+	let { log } = $props();
+
+	let count = $state(0);
+	let show_foo = $state(true);
+	let snippet = $derived(show_foo ? foo : bar);
+</script>
+
+{#snippet foo({count})}
+	<p>snippet: {count}</p>
+{/snippet}
+
+{#snippet bar(props)}
+	<Inner {...props}></Inner>
+{/snippet}
+
+{@render snippet({ count, log })}
+
+<button onclick={() => show_foo = !show_foo}>toggle</button>
+<button onclick={() => count++}>increase count</button>