Fix the bug

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

@@ -21,6 +21,7 @@ export const INSPECT_EFFECT = 1 << 18;
 export const HEAD_EFFECT = 1 << 19;
 export const EFFECT_HAS_DERIVED = 1 << 20;
 export const EFFECT_IS_UPDATING = 1 << 21;
+export const TEMPLATE_EFFECT = 1 << 22;
 
 export const STATE_SYMBOL = Symbol('$state');
 export const LEGACY_PROPS = Symbol('legacy props');

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

@@ -32,7 +32,8 @@ import {
 	HEAD_EFFECT,
 	MAYBE_DIRTY,
 	EFFECT_HAS_DERIVED,
-	BOUNDARY_EFFECT
+	BOUNDARY_EFFECT,
+	TEMPLATE_EFFECT
 } from '#client/constants';
 import { set } from './sources.js';
 import * as e from '../errors.js';
@@ -329,6 +330,7 @@ export function render_effect(fn) {
 /**
  * @param {(...expressions: any) => void | (() => void)} fn
  * @param {Array<() => any>} thunks
+ * @param {<T>(fn: () => T) => Derived<T>} d
  * @returns {Effect}
  */
 export function template_effect(fn, thunks = [], d = derived) {
@@ -343,12 +345,12 @@ export function template_effect(fn, thunks = [], d = derived) {
 			define_property(inner, 'name', { value: '{expression}' });
 
 			const deriveds = thunks.map(d);
-			block(inner);
+			block(inner, TEMPLATE_EFFECT);
 		});
 	}
 
 	const deriveds = thunks.map(d);
-	return block(() => fn(...deriveds.map(get)));
+	return block(() => fn(...deriveds.map(get)), TEMPLATE_EFFECT);
 }
 
 /**
@@ -606,7 +608,7 @@ function resume_children(effect, local) {
 
 	// If a dependency of this effect changed while it was paused,
 	// schedule the effect to update
-	if (check_dirtiness(effect)) {
+	if (check_dirtiness(effect, true)) {
 		set_signal_status(effect, DIRTY);
 		schedule_effect(effect);
 	}

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

@@ -22,7 +22,8 @@ import {
 	ROOT_EFFECT,
 	LEGACY_DERIVED_PROP,
 	DISCONNECTED,
-	EFFECT_IS_UPDATING
+	EFFECT_IS_UPDATING,
+	TEMPLATE_EFFECT
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
@@ -151,9 +152,10 @@ export function increment_write_version() {
  * Determines whether a derived or effect is dirty.
  * If it is MAYBE_DIRTY, will set the status to CLEAN
  * @param {Reaction} reaction
+ * @param {boolean} [resuming]
  * @returns {boolean}
  */
-export function check_dirtiness(reaction) {
+export function check_dirtiness(reaction, resuming = false) {
 	var flags = reaction.f;
 
 	if ((flags & DIRTY) !== 0) {
@@ -203,6 +205,17 @@ export function check_dirtiness(reaction) {
 				dependency = dependencies[i];
 
 				if (check_dirtiness(/** @type {Derived} */ (dependency))) {
+					/* Don't execute deriveds of template effects when unpausing, for example when outer resumes
+
+					{#if outer}
+					  {#if inner}
+					  	{inner.func()}
+					  {/if}
+					{/if}
+
+					inner might be undefined, so don't eagerly execute `inner.func()`
+					 */
+					if (resuming && (reaction.f & TEMPLATE_EFFECT) !== 0) return true;
 					update_derived(/** @type {Derived} */ (dependency));
 				}