From 181fb2ad496dcd7e1915032b553115c805034bbf Mon Sep 17 00:00:00 2001
From: Paolo Ricciuti <ricciutipaolo@gmail.com>
Date: Wed, 5 Mar 2025 00:36:26 +0100
Subject: [PATCH] fix: correctly set `is_updating` before flushing root effects
 (#15442)

* fix: correctly set `is_updating` before flushing root effects

* rename for consistency with update_effect

* use var

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/proud-poems-brake.md               |  5 +++
 .../svelte/src/internal/client/runtime.js     |  4 ++
 packages/svelte/tests/signals/test.ts         | 41 +++++++++++++++++++
 3 files changed, 50 insertions(+)
 create mode 100644 .changeset/proud-poems-brake.md

diff --git a/.changeset/proud-poems-brake.md b/.changeset/proud-poems-brake.md
new file mode 100644
index 0000000000..3f4e076e22
--- /dev/null
+++ b/.changeset/proud-poems-brake.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly set `is_updating` before flushing root effects
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 486c819f36..9f721f9ec4 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -644,8 +644,11 @@ function infinite_loop_guard() {
 }
 
 function flush_queued_root_effects() {
+	var was_updating_effect = is_updating_effect;
+
 	try {
 		var flush_count = 0;
+		is_updating_effect = true;
 
 		while (queued_root_effects.length > 0) {
 			if (flush_count++ > 1000) {
@@ -670,6 +673,7 @@ function flush_queued_root_effects() {
 		}
 	} finally {
 		is_flushing = false;
+		is_updating_effect = was_updating_effect;
 
 		last_scheduled_effect = null;
 		if (DEV) {
diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index 046f833e0e..ef4cf16d3b 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -1135,4 +1135,45 @@ describe('signals', () => {
 			destroy();
 		};
 	});
+
+	test('unowned deriveds correctly update', () => {
+		const log: any[] = [];
+
+		return () => {
+			const a = state(0);
+			const b = state(0);
+			const c = derived(() => {
+				return $.get(a);
+			});
+			const d = derived(() => {
+				return $.get(b);
+			});
+
+			const destroy = effect_root(() => {
+				const e = derived(() => {
+					return $.get(c) === 1 && $.get(d) === 1;
+				});
+				render_effect(() => {
+					log.push($.get(e));
+				});
+			});
+
+			assert.deepEqual(log, [false]);
+
+			set(a, 1);
+			set(b, 1);
+
+			flushSync();
+
+			assert.deepEqual(log, [false, true]);
+
+			set(b, 9);
+
+			flushSync();
+
+			assert.deepEqual(log, [false, true, false]);
+
+			destroy();
+		};
+	});
 });