fix: address regression with untrack (#15079)

* fix: address regression with untrack

* add test

Author: trueadm

.changeset/ninety-chefs-know.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: address regression with untrack

packages/svelte/src/internal/client/reactivity/sources.js

@@ -116,7 +116,7 @@ export function mutable_state(v, immutable = false) {
  */
 /*#__NO_SIDE_EFFECTS__*/
 function push_derived_source(source) {
-	if (active_reaction !== null && (active_reaction.f & DERIVED) !== 0) {
+	if (active_reaction !== null && !untracking && (active_reaction.f & DERIVED) !== 0) {
 		if (derived_sources === null) {
 			set_derived_sources([source]);
 		} else {

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

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `3`
+});

packages/svelte/tests/runtime-runes/samples/untracked-derived-local/main.svelte

@@ -0,0 +1,47 @@
+<script module>
+	import { untrack } from 'svelte'
+	import { SvelteMap } from 'svelte/reactivity'
+
+	class Foo {
+		id
+		updateTime = $state(Date.now())
+
+		constructor(id) {
+			this.id = id
+		}
+	}
+
+	class Store {
+		cache = new SvelteMap()
+		ids = $state([1, 2, 3])
+
+		getOrDefault(id) {
+			let ret = this.cache.get(id)
+
+			if (ret) {
+				return ret
+			}
+
+			ret = untrack(() => {
+				ret = new Foo(id)
+				this.cache.set(id, ret)
+				return ret
+			})
+			this.cache.get(id)
+
+			return ret
+		}
+
+		get values() {
+			return this.ids.map(id => this.getOrDefault(id)).sort((a, b) => b.updateTime - a.updateTime)
+		}
+	}
+
+	const store = new Store()
+</script>
+
+<script>
+	const test = $derived.by(() => store.values.length)
+</script>
+
+{test}