fix: make `$effect.active()` true when updating deriveds

.changeset/serious-goats-tap.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make `$effect.active()` true when updating deriveds

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

@@ -26,7 +26,8 @@ import {
 	EFFECT_RAN,
 	BLOCK_EFFECT,
 	ROOT_EFFECT,
-	EFFECT_TRANSPARENT
+	EFFECT_TRANSPARENT,
+	DERIVED
 } from '../constants.js';
 import { set } from './sources.js';
 import { remove } from '../dom/reconciler.js';
@@ -118,7 +119,9 @@ function create_effect(type, fn, sync) {
  * @returns {boolean}
  */
 export function effect_active() {
-	return current_effect ? (current_effect.f & (BRANCH_EFFECT | ROOT_EFFECT)) === 0 : false;
+	if (current_effect) return (current_effect.f & (BRANCH_EFFECT | ROOT_EFFECT)) === 0;
+	if (current_reaction) return (current_reaction.f & DERIVED) !== 0;
+	return false;
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/effect-active-derived/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>toggle (false)</button>`,
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+		flushSync(() => btn?.click());
+
+		assert.htmlEqual(target.innerHTML, `<button>toggle (true)</button><p>bar is true</p>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/effect-active-derived/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	let foo = $state(false)
+	let bar = $derived(foo ? $effect.active() : false);
+</script>
+
+<button onclick={() => foo = !foo}>
+	toggle ({foo})
+</button>
+
+{#if bar}
+	<p>bar is true</p>
+{/if}