fix: ensure untrack correctly retains the active reaction

.changeset/fresh-cycles-sneeze.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure untrack correctly retains the active reaction

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

@@ -16,7 +16,8 @@ import {
 	set_is_flushing_effect,
 	set_signal_status,
 	untrack,
-	skip_reaction
+	skip_reaction,
+	untracking
 } from '../runtime.js';
 import {
 	DIRTY,
@@ -43,8 +44,7 @@ import * as e from '../errors.js';
 import { DEV } from 'esm-env';
 import { define_property } from '../../shared/utils.js';
 import { get_next_sibling } from '../dom/operations.js';
-import { derived, derived_safe_equal, destroy_derived } from './deriveds.js';
-import { legacy_mode_flag } from '../../flags/index.js';
+import { derived, destroy_derived } from './deriveds.js';
 
 /**
  * @param {'$effect' | '$effect.pre' | '$inspect'} rune
@@ -166,7 +166,7 @@ function create_effect(type, fn, sync, push = true) {
  * @returns {boolean}
  */
 export function effect_tracking() {
-	if (active_reaction === null) {
+	if (active_reaction === null || untracking) {
 		return false;
 	}
 

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

@@ -17,7 +17,8 @@ import {
 	set_derived_sources,
 	check_dirtiness,
 	set_is_flushing_effect,
-	is_flushing_effect
+	is_flushing_effect,
+	untracking
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
 import {
@@ -148,6 +149,7 @@ export function mutate(source, value) {
 export function set(source, value) {
 	if (
 		active_reaction !== null &&
+		!untracking &&
 		is_runes() &&
 		(active_reaction.f & (DERIVED | BLOCK_EFFECT)) !== 0 &&
 		// If the source was created locally within the current derived, then

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

@@ -78,6 +78,8 @@ let dev_effect_stack = [];
 /** @type {null | Reaction} */
 export let active_reaction = null;
 
+export let untracking = false;
+
 /** @param {null | Reaction} reaction */
 export function set_active_reaction(reaction) {
 	active_reaction = reaction;
@@ -423,6 +425,7 @@ export function update_reaction(reaction) {
 	var previous_skip_reaction = skip_reaction;
 	var prev_derived_sources = derived_sources;
 	var previous_component_context = component_context;
+	var previous_untracking = untracking;
 	var flags = reaction.f;
 
 	new_deps = /** @type {null | Value[]} */ (null);
@@ -432,6 +435,7 @@ export function update_reaction(reaction) {
 	skip_reaction = !is_flushing_effect && (flags & UNOWNED) !== 0;
 	derived_sources = null;
 	component_context = reaction.ctx;
+	untracking = false;
 	read_version++;
 
 	try {
@@ -495,6 +499,7 @@ export function update_reaction(reaction) {
 		skip_reaction = previous_skip_reaction;
 		derived_sources = prev_derived_sources;
 		component_context = previous_component_context;
+		untracking = previous_untracking;
 	}
 }
 
@@ -934,7 +939,7 @@ export function get(signal) {
 	}
 
 	// Register the dependency on the current reaction signal.
-	if (active_reaction !== null) {
+	if (active_reaction !== null && !untracking) {
 		if (derived_sources !== null && derived_sources.includes(signal)) {
 			e.state_unsafe_local_read();
 		}
@@ -1085,12 +1090,12 @@ export function invalidate_inner_signals(fn) {
  * @returns {T}
  */
 export function untrack(fn) {
-	const previous_reaction = active_reaction;
+	var previous_untracking = untracking;
 	try {
-		active_reaction = null;
+		untracking = true;
 		return fn();
 	} finally {
-		active_reaction = previous_reaction;
+		untracking = previous_untracking;
 	}
 }
 

packages/svelte/tests/signals/test.ts

@@ -803,6 +803,46 @@ describe('signals', () => {
 		};
 	});
 
+	test('deriveds containing effects work correctly when used with untrack', () => {
+		return () => {
+			let a = render_effect(() => {});
+			let b = state(0);
+			let c;
+			let effects = [];
+
+			const destroy = effect_root(() => {
+				a = render_effect(() => {
+					c = derived(() => {
+						$.untrack(() => {
+							effects.push(
+								effect(() => {
+									$.get(b);
+								})
+							);
+						});
+						$.get(b);
+					});
+					$.get(c);
+				});
+			});
+
+			assert.deepEqual(c!.children?.length, 1);
+			assert.deepEqual(a.first, a.last);
+
+			set(b, 1);
+
+			flushSync();
+
+			assert.deepEqual(c!.children?.length, 1);
+			assert.deepEqual(a.first, a.last);
+
+			destroy();
+
+			assert.deepEqual(a.deriveds, null);
+			assert.deepEqual(a.first, null);
+		};
+	});
+
 	test('bigint states update correctly', () => {
 		return () => {
 			const count = state(0n);