fix: avoid disconnecting deriveds that are still active

.changeset/dirty-worms-type.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid disconnecting deriveds that are still active

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

@@ -370,7 +370,14 @@ function remove_reaction(signal, dependency) {
 	}
 	// If the derived has no reactions, then we can disconnect it from the graph,
 	// allowing it to either reconnect in the future, or be GC'd by the VM.
-	if (reactions === null && (dependency.f & DERIVED) !== 0) {
+	if (
+		reactions === null &&
+		(dependency.f & DERIVED) !== 0 &&
+		// Destroying a child effect while updating a parent effect can cause a dependency to appear
+		// to be unused, when in fact it is used by the currently-updating parent. Checking `new_deps`
+		// allows us to skip the expensive work of disconnecting and immediately reconnecting it
+		(new_deps === null || !new_deps.includes(dependency))
+	) {
 		set_signal_status(dependency, MAYBE_DIRTY);
 		// If we are working with a derived that is owned by an effect, then mark it as being
 		// disconnected.

packages/svelte/tests/runtime-runes/samples/if-block-dependencies/_config.js

@@ -0,0 +1,19 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await Promise.resolve();
+
+		let [btn1] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn1?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`false\ntrue\n<button>Toggle</button>\nfirst:\nfalse\n<br>\nsecond:\ntrue`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/if-block-dependencies/main.svelte

@@ -0,0 +1,21 @@
+<script>
+	let first = $state(true)
+	let second = $state(false)
+	let derivedSecond = $derived(second)
+
+	queueMicrotask(() => {
+		first = false
+	});
+</script>
+
+{first} {second}
+
+<button onclick={() => {
+	second = true
+}}>Toggle</button>
+
+{#if first || derivedSecond}
+		first: {first}
+		<br />
+		second: {derivedSecond}
+{/if}