fix: ensure disconnected deriveds correctly connect again

.changeset/happy-plums-greet.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure disconnected deriveds correctly connect again

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

@@ -207,7 +207,9 @@ export function check_dirtiness(reaction) {
 				for (i = 0; i < length; i++) {
 					dependency = dependencies[i];
 
-					if (!dependency?.reactions?.includes(reaction)) {
+					// We always re-add all reactions (even duplicates) if the derived was
+					// previously disconnected
+					if (is_disconnected || !dependency?.reactions?.includes(reaction)) {
 						(dependency.reactions ??= []).push(reaction);
 					}
 				}

packages/svelte/tests/runtime-runes/samples/derived-disconnect/_config.js

@@ -0,0 +1,96 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		let [b1, b2, b3, b4, b5] = target.querySelectorAll('button');
+
+		b1?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 1</div>
+			<div>Name: a</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b2?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 2</div>
+			<div>Name: b</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b3?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 3</div>
+			<div>Name: c</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b4?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 4</div>
+			<div>Name: d</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b5?.click();
+		flushSync();
+
+		b5?.click();
+		flushSync();
+
+		[b1, b2, b3, b4, b5] = target.querySelectorAll('button');
+
+		b1?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 1</div>
+			<div>Name: a</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b2?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 2</div>
+			<div>Name: b</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b3?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 3</div>
+			<div>Name: c</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+
+		b4?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<main><div>Current ID: 4</div>
+			<div>Name: d</div><div><button>a</button></div><div><button>b</button></div><div><button>c</button></div><div><button>d</button></div><hr><div>
+			<button>Show / Hide</button></div></main>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-disconnect/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	const items = [
+		{ id: 1, name: "a" },
+		{ id: 2, name: "b" },
+		{ id: 3, name: "c" },
+		{ id: 4, name: "d" },
+	];
+	let currentId = $state(1);
+	let currentItem = $derived(items.find(item => item.id === currentId));
+	let visible = $state(true);
+</script>
+
+<main>
+	{#if visible}
+		<div>Current ID: {currentId}</div>
+		<div>Name: {currentItem.name}</div>
+		{#each items as item}
+			<div><button onclick={() => { currentId = item.id; }}>{item.name}</button></div>
+		{/each}
+	{/if}
+	<hr>
+	<div><button onclick={() => { visible = !visible; }}>Show / Hide</button></div>
+</main>