fix: address regressed memory leak

.changeset/sour-geese-listen.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: address regressed memory leak

packages/svelte/src/internal/client/dom/blocks/html.js

@@ -4,6 +4,7 @@ import { current_effect, get } from '../../runtime.js';
 import { is_array } from '../../utils.js';
 import { hydrate_nodes, hydrating } from '../hydration.js';
 import { create_fragment_from_html, remove } from '../reconciler.js';
+import { push_template_node } from '../template.js';
 
 /**
  * @param {import('#client').Effect} effect
@@ -37,7 +38,7 @@ export function html(anchor, get_value, svg, mathml) {
 	let value = derived(get_value);
 
 	render_effect(() => {
-		var dom = html_to_dom(anchor, get(value), svg, mathml);
+		var dom = html_to_dom(anchor, parent_effect, get(value), svg, mathml);
 
 		if (dom) {
 			return () => {
@@ -55,12 +56,13 @@ export function html(anchor, get_value, svg, mathml) {
  * inserts it before the target anchor and returns the new nodes.
  * @template V
  * @param {Element | Text | Comment} target
+ * @param {import('#client').Effect | null} effect
  * @param {V} value
  * @param {boolean} svg
  * @param {boolean} mathml
  * @returns {Element | Comment | (Element | Comment | Text)[]}
  */
-function html_to_dom(target, value, svg, mathml) {
+function html_to_dom(target, effect, value, svg, mathml) {
 	if (hydrating) return hydrate_nodes;
 
 	var html = value + '';
@@ -79,6 +81,9 @@ function html_to_dom(target, value, svg, mathml) {
 	if (node.childNodes.length === 1) {
 		var child = /** @type {Text | Element | Comment} */ (node.firstChild);
 		target.before(child);
+		if (effect !== null) {
+			push_template_node(child, effect);
+		}
 		return child;
 	}
 
@@ -92,5 +97,9 @@ function html_to_dom(target, value, svg, mathml) {
 		target.before(node);
 	}
 
+	if (effect !== null) {
+		push_template_node(nodes, effect);
+	}
+
 	return nodes;
 }

packages/svelte/src/internal/client/dom/blocks/svelte-element.js

@@ -10,8 +10,31 @@ import {
 } from '../../reactivity/effects.js';
 import { set_should_intro } from '../../render.js';
 import { current_each_item, set_current_each_item } from './each.js';
-import { current_component_context } from '../../runtime.js';
+import { current_component_context, current_effect } from '../../runtime.js';
 import { DEV } from 'esm-env';
+import { is_array } from '../../utils.js';
+import { push_template_node } from '../template.js';
+
+/**
+ * @param {import('#client').Effect} effect
+ * @param {Element} from
+ * @param {Element} to
+ * @returns {void}
+ */
+function swap_block_dom(effect, from, to) {
+	const dom = effect.dom;
+
+	if (is_array(dom)) {
+		for (let i = 0; i < dom.length; i++) {
+			if (dom[i] === from) {
+				dom[i] = to;
+				break;
+			}
+		}
+	} else if (dom === from) {
+		effect.dom = to;
+	}
+}
 
 /**
  * @param {Comment} anchor
@@ -23,6 +46,7 @@ import { DEV } from 'esm-env';
  * @returns {void}
  */
 export function element(anchor, get_tag, is_svg, render_fn, get_namespace, location) {
+	const parent_effect = /** @type {import('#client').Effect} */ (current_effect);
 	const filename = DEV && location && current_component_context?.function.filename;
 
 	/** @type {string | null} */
@@ -78,6 +102,7 @@ export function element(anchor, get_tag, is_svg, render_fn, get_namespace, locat
 
 		if (next_tag && next_tag !== current_tag) {
 			effect = branch(() => {
+				const prev_element = element;
 				element = hydrating
 					? /** @type {Element} */ (hydrate_start)
 					: ns
@@ -111,9 +136,12 @@ export function element(anchor, get_tag, is_svg, render_fn, get_namespace, locat
 
 				anchor.before(element);
 
-				return () => {
-					element?.remove();
-				};
+				if (prev_element) {
+					swap_block_dom(parent_effect, prev_element, element);
+					prev_element.remove();
+				} else if (!hydrating) {
+					push_template_node(element, parent_effect);
+				}
 			});
 		}
 

packages/svelte/src/internal/client/dom/template.js

@@ -4,17 +4,32 @@ import { create_fragment_from_html } from './reconciler.js';
 import { current_effect } from '../runtime.js';
 import { TEMPLATE_FRAGMENT, TEMPLATE_USE_IMPORT_NODE } from '../../../constants.js';
 import { effect } from '../reactivity/effects.js';
+import { is_array } from '../utils.js';
 
 /**
  * @template {import("#client").TemplateNode | import("#client").TemplateNode[]} T
  * @param {T} dom
+ * @param {import("#client").Effect} effect
  */
-function push_template_node(dom) {
-	var effect = /** @type {import('#client').Effect} */ (current_effect);
-
-	if (effect.dom === null) {
+export function push_template_node(
+	dom,
+	effect = /** @type {import('#client').Effect} */ (current_effect)
+) {
+	var current_dom = effect.dom;
+	if (current_dom === null) {
 		effect.dom = dom;
+	} else {
+		if (!is_array(current_dom)) {
+			current_dom = effect.dom = [current_dom];
+		}
+
+		if (is_array(dom)) {
+			current_dom.push(...dom);
+		} else {
+			current_dom.push(dom);
+		}
 	}
+	return dom;
 }
 
 /**
@@ -41,7 +56,15 @@ export function template(content, flags) {
 			if (!is_fragment) node = /** @type {Node} */ (node.firstChild);
 		}
 
-		return use_import_node ? document.importNode(node, true) : node.cloneNode(true);
+		var clone = use_import_node ? document.importNode(node, true) : node.cloneNode(true);
+
+		push_template_node(
+			is_fragment
+				? /** @type {import('#client').TemplateNode[]} */ ([...clone.childNodes])
+				: /** @type {import('#client').TemplateNode} */ (clone)
+		);
+
+		return clone;
 	};
 }
 
@@ -102,7 +125,15 @@ export function ns_template(content, flags, ns = 'svg') {
 			}
 		}
 
-		return node.cloneNode(true);
+		var clone = node.cloneNode(true);
+
+		push_template_node(
+			is_fragment
+				? /** @type {import('#client').TemplateNode[]} */ ([...clone.childNodes])
+				: /** @type {import('#client').TemplateNode} */ (clone)
+		);
+
+		return clone;
 	};
 }
 
@@ -177,7 +208,7 @@ function run_scripts(node) {
  */
 /*#__NO_SIDE_EFFECTS__*/
 export function text(anchor) {
-	if (!hydrating) return empty();
+	if (!hydrating) return push_template_node(empty());
 
 	var node = hydrate_start;
 
@@ -201,6 +232,7 @@ export function comment() {
 	var frag = document.createDocumentFragment();
 	var anchor = empty();
 	frag.append(anchor);
+	push_template_node([anchor]);
 
 	return frag;
 }
@@ -213,13 +245,9 @@ export function comment() {
  */
 export function append(anchor, dom) {
 	if (hydrating) return;
-
-	var effect = /** @type {import('#client').Effect} */ (current_effect);
-
-	effect.dom =
-		dom.nodeType === 11
-			? /** @type {import('#client').TemplateNode[]} */ ([...dom.childNodes])
-			: /** @type {Element | Comment} */ (dom);
+	// We intentionally do not assign the `dom` property of the effect here because it's far too
+	// late. If we try, we will capture additional DOM elements that we cannot control the lifecycle
+	// for and will inevitably cause memory leaks. See https://github.com/sveltejs/svelte/pull/11832
 
 	anchor.before(/** @type {Node} */ (dom));
 }