partial fix

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

@@ -1,6 +1,6 @@
 import { DEV } from 'esm-env';
-import { CLEAN, DERIVED, UNINITIALIZED, UNOWNED } from '../constants.js';
-import { current_reaction, current_effect } from '../runtime.js';
+import { CLEAN, DERIVED, DESTROYED, UNINITIALIZED, UNOWNED } from '../constants.js';
+import { current_reaction, current_effect, set_signal_status } from '../runtime.js';
 import { default_equals, safe_equal } from './equality.js';
 
 /**
@@ -54,3 +54,9 @@ export function derived_safe_equal(fn) {
 	signal.eq = safe_equal;
 	return signal;
 }
+
+/** @param {import('#client').Derived} derived */
+export function destroy_derived(derived) {
+	// TODO what needs to happen here, if anything?
+	set_signal_status(derived, DESTROYED);
+}

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

@@ -17,6 +17,7 @@ import {
 	ROOT_EFFECT,
 	DESTROYED
 } from '../constants.js';
+import { destroy_derived } from './deriveds.js';
 
 /**
  * @param {import('./types.js').EffectType} type
@@ -248,7 +249,7 @@ function pause_children(effect, transitions, local) {
 }
 
 /**
- * @param {import('#client').Effect} effect TODO this isn't just block effects, it's deriveds etc too
+ * @param {import('#client').Effect} effect
  */
 export function destroy_effect(effect) {
 	if ((effect.f & DESTROYED) !== 0) return;
@@ -263,6 +264,21 @@ export function destroy_effect(effect) {
 		remove(effect.dom);
 	}
 
+	effect.v?.();
+
+	destroy_effect_children(effect);
+}
+
+/**
+ * @param {import('#client').Effect} effect
+ */
+export function destroy_effect_children(effect) {
+	if (effect.r) {
+		for (const derived of effect.r) {
+			destroy_derived(derived);
+		}
+	}
+
 	if (effect.children) {
 		for (const child of effect.children) {
 			destroy_effect(child);

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

@@ -8,7 +8,7 @@ import {
 	object_prototype
 } from './utils.js';
 import { unstate } from './proxy.js';
-import { pre_effect } from './reactivity/effects.js';
+import { destroy_effect_children, pre_effect } from './reactivity/effects.js';
 import {
 	EFFECT,
 	PRE_EFFECT,
@@ -379,6 +379,9 @@ function destroy_references(effect) {
 	}
 }
 
+/** @param {import('#client').Derived} effect */
+function destroy_effect(effect) {}
+
 /**
  * @param {import('#client').Effect} effect
  * @returns {void}
@@ -399,7 +402,7 @@ export function execute_effect(effect) {
 		if ((effect.f & BRANCH_EFFECT) === 0) {
 			// branch effects (i.e. {#if ...} blocks) need to keep their references
 			// TODO their children should detach themselves from signal.r when destroyed
-			destroy_references(effect);
+			destroy_effect_children(effect);
 		}
 
 		if (teardown !== null) {