fix: add dummy anchor for dynamic component HMR wrappers (#12252)

* add failing test

* add dummy anchor for HMR wrappers

* lint

Author: Rich-Harris

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -653,9 +653,10 @@ function collect_parent_each_blocks(context) {
  * @param {import('#compiler').Component | import('#compiler').SvelteComponent | import('#compiler').SvelteSelf} node
  * @param {string} component_name
  * @param {import('../types.js').ComponentContext} context
+ * @param {import('estree').Expression} anchor
  * @returns {import('estree').Statement}
  */
-function serialize_inline_component(node, component_name, context) {
+function serialize_inline_component(node, component_name, context, anchor = context.state.node) {
 	/** @type {Array<import('estree').Property[] | import('estree').Expression>} */
 	const props_and_spreads = [];
 
@@ -946,13 +947,13 @@ function serialize_inline_component(node, component_name, context) {
 				node_id,
 				b.thunk(/** @type {import('estree').Expression} */ (context.visit(node.expression))),
 				b.arrow(
-					[b.id(component_name)],
+					[b.id('$$anchor'), b.id(component_name)],
 					b.block([
 						...binding_initializers,
 						b.stmt(
 							context.state.options.dev
-								? b.call('$.validate_dynamic_component', b.thunk(prev(node_id)))
-								: prev(node_id)
+								? b.call('$.validate_dynamic_component', b.thunk(prev(b.id('$$anchor'))))
+								: prev(b.id('$$anchor'))
 						)
 					])
 				)
@@ -970,12 +971,12 @@ function serialize_inline_component(node, component_name, context) {
 		);
 
 		statements.push(
-			b.stmt(b.call('$.css_props', context.state.node, b.thunk(b.object(custom_css_props)))),
-			b.stmt(fn(b.member(context.state.node, b.id('lastChild'))))
+			b.stmt(b.call('$.css_props', anchor, b.thunk(b.object(custom_css_props)))),
+			b.stmt(fn(b.member(anchor, b.id('lastChild'))))
 		);
 	} else {
 		context.state.template.push('<!>');
-		statements.push(b.stmt(fn(context.state.node)));
+		statements.push(b.stmt(fn(anchor)));
 	}
 
 	return statements.length > 1 ? b.block(statements) : statements[0];
@@ -3025,7 +3026,7 @@ export const template_visitors = {
 	Component(node, context) {
 		if (node.metadata.dynamic) {
 			// Handle dynamic references to what seems like static inline components
-			const component = serialize_inline_component(node, '$$component', context);
+			const component = serialize_inline_component(node, '$$component', context, b.id('$$anchor'));
 			context.state.init.push(
 				b.stmt(
 					b.call(
@@ -3036,7 +3037,7 @@ export const template_visitors = {
 						b.thunk(
 							/** @type {import('estree').Expression} */ (context.visit(b.member_id(node.name)))
 						),
-						b.arrow([b.id('$$component')], b.block([component]))
+						b.arrow([b.id('$$anchor'), b.id('$$component')], b.block([component]))
 					)
 				)
 			);

packages/svelte/src/internal/client/dev/hmr.js

@@ -18,7 +18,7 @@ export function hmr(source) {
 		/** @type {import("#client").Effect} */
 		let effect;
 
-		block(null, 0, () => {
+		block(anchor, 0, () => {
 			const component = get(source);
 
 			if (effect) {

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

@@ -1,11 +1,13 @@
+import { DEV } from 'esm-env';
 import { block, branch, pause_effect } from '../../reactivity/effects.js';
+import { empty } from '../operations.js';
 
 /**
  * @template P
  * @template {(props: P) => void} C
  * @param {import('#client').TemplateNode} anchor
  * @param {() => C} get_component
- * @param {(component: C) => import('#client').Dom | void} render_fn
+ * @param {(anchor: import('#client').TemplateNode, component: C) => import('#client').Dom | void} render_fn
  * @returns {void}
  */
 export function component(anchor, get_component, render_fn) {
@@ -15,6 +17,11 @@ export function component(anchor, get_component, render_fn) {
 	/** @type {import('#client').Effect | null} */
 	let effect;
 
+	var component_anchor = anchor;
+
+	// create a dummy anchor for the HMR wrapper, if such there be
+	if (DEV) component_anchor = empty();
+
 	block(anchor, 0, () => {
 		if (component === (component = get_component())) return;
 
@@ -24,7 +31,8 @@ export function component(anchor, get_component, render_fn) {
 		}
 
 		if (component) {
-			effect = branch(() => render_fn(component));
+			if (DEV) anchor.before(component_anchor);
+			effect = branch(() => render_fn(component_anchor, component));
 		}
 	});
 }

packages/svelte/tests/runtime-runes/samples/hmr-removal/Child.svelte

@@ -0,0 +1 @@
+<p>hello</p>

packages/svelte/tests/runtime-runes/samples/hmr-removal/_config.js

@@ -0,0 +1,21 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true,
+		hmr: true
+	},
+
+	html: `<button>toggle</button>`,
+
+	test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		assert.htmlEqual(target.innerHTML, `<button>toggle</button><p>hello</p>`);
+
+		flushSync(() => button?.click());
+		assert.htmlEqual(target.innerHTML, `<button>toggle</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/hmr-removal/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Child from './Child.svelte';
+
+	let open = $state(false);
+</script>
+
+<button onclick={() => (open = !open)}>toggle</button>
+
+{#if open}
+	<Child />
+{/if}