fix: only re-run directly applied attachment if it changed

.changeset/slimy-drinks-divide.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: only re-run directly applied attachment if it changed

packages/svelte/src/internal/client/dom/elements/attachments.js

@@ -1,15 +1,33 @@
-import { effect } from '../../reactivity/effects.js';
+/** @import { Effect } from '#client' */
+import { block, branch, effect, destroy_effect } from '../../reactivity/effects.js';
+
+// TODO in 6.0 or 7.0, when we remove legacy mode, we can simplify this by
+// getting rid of the block/branch stuff and just letting the effect rip.
+// see https://github.com/sveltejs/svelte/pull/15962
 
 /**
  * @param {Element} node
  * @param {() => (node: Element) => void} get_fn
  */
 export function attach(node, get_fn) {
-	effect(() => {
-		const fn = get_fn();
+	/** @type {false | undefined | ((node: Element) => void)} */
+	var fn = undefined;
+
+	/** @type {Effect | null} */
+	var e;
+
+	block(() => {
+		if (fn !== (fn = get_fn())) {
+			if (e) {
+				destroy_effect(e);
+				e = null;
+			}
 
-		// we use `&&` rather than `?.` so that things like
-		// `{@attach DEV && something_dev_only()}` work
-		return fn && fn(node);
+			if (fn) {
+				e = branch(() => {
+					effect(() => /** @type {(node: Element) => void} */ (fn)(node));
+				});
+			}
+		}
 	});
 }

packages/svelte/tests/runtime-legacy/samples/attachment-in-mutated-state/_config.js

@@ -0,0 +1,16 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, logs }) {
+		assert.deepEqual(logs, ['up']);
+
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		assert.deepEqual(logs, ['up']);
+
+		flushSync(() => button?.click());
+		assert.deepEqual(logs, ['up', 'down']);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/attachment-in-mutated-state/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	let state = {
+		count: 0,
+		attachment(){
+			console.log('up');
+			return () => console.log('down');
+		}
+	};
+</script>
+
+<button onclick={() => state.count++}>{state.count}</button>
+
+{#if state.count < 2}
+	<div {@attach state.attachment}></div>
+{/if}