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

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly set `is_updating` before flushing root effects

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) {

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();
+		};
+	});
 });