fix: ensure subscriptions are picked up correctly by deriveds

.changeset/fresh-penguins-impress.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure subscriptions are picked up correctly by deriveds

packages/svelte/src/reactivity/create-subscriber.js

@@ -82,6 +82,10 @@ export function createSubscriber(start) {
 						if (subscribers === 0) {
 							stop?.();
 							stop = undefined;
+							// Increment the version to ensure any dependent deriveds are marked dirty when the subscription is picked up again later.
+							// If we didn't do this then the comparison of write versions would determine that the derived has a later version than
+							// the subscriber, and it would not be re-run.
+							increment(version);
 						}
 					});
 				};

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

@@ -0,0 +1,45 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test: ({ assert, target }) => {
+		const [loading, increment] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<div>$value: 0</div>
+				<div>valueFromStore.current: 0</div>
+				<div>valueDerivedCurrent: 0</div>
+				<button>Loading</button>
+				<button>Increment</button>
+			`
+		);
+
+		loading.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<div>$value: Loading...</div>
+				<div>valueFromStore.current: Loading...</div>
+				<div>valueDerivedCurrent: Loading...</div>
+				<button>Loading</button>
+				<button>Increment</button>
+			`
+		);
+
+		increment.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<div>$value: 1</div>
+				<div>valueFromStore.current: 1</div>
+				<div>valueDerivedCurrent: 1</div>
+				<button>Loading</button>
+				<button>Increment</button>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/store-inside-derived/main.svelte

@@ -0,0 +1,36 @@
+<script>
+  import { fromStore, writable } from 'svelte/store';
+
+  let isLoading = $state(false);
+
+  const value = writable(0);
+  const valueFromStore = fromStore(value);
+  const valueDerivedCurrent = $derived(valueFromStore.current);
+</script>
+
+<div>
+	$value: {isLoading ? 'Loading...' : $value}
+</div>
+
+<div>
+	valueFromStore.current: {isLoading ? 'Loading...' : valueFromStore.current}
+</div>
+
+<div>
+	valueDerivedCurrent: {isLoading ? 'Loading...' : valueDerivedCurrent}
+</div>
+
+<button
+	onclick={() => {
+		isLoading = true;
+	}}>
+	Loading
+</button>
+
+<button
+	onclick={() => {
+		$value++;
+		isLoading = false;
+	}}>
+	Increment
+</button>