fix: improve hydration of altered html

.changeset/angry-pigs-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve hydration of altered html

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

@@ -3,10 +3,11 @@ import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from '../hyd
 import { create_text, get_first_child, get_next_sibling } from '../operations.js';
 import { block } from '../../reactivity/effects.js';
 import { COMMENT_NODE, HEAD_EFFECT } from '#client/constants';
-import { HYDRATION_START } from '../../../../constants.js';
+import { HYDRATION_END, HYDRATION_ERROR, HYDRATION_START } from '../../../../constants.js';
+import { hydration_mismatch } from '../../warnings.js';
 
 /**
- * @type {Node | undefined}
+ * @type {Node | null | undefined}
  */
 let head_anchor;
 
@@ -32,15 +33,15 @@ export function head(render_fn) {
 
 		// There might be multiple head blocks in our app, so we need to account for each one needing independent hydration.
 		if (head_anchor === undefined) {
-			head_anchor = /** @type {TemplateNode} */ (get_first_child(document.head));
+			head_anchor = get_first_child(document.head);
 		}
 
 		while (
 			head_anchor !== null &&
 			(head_anchor.nodeType !== COMMENT_NODE ||
 				/** @type {Comment} */ (head_anchor).data !== HYDRATION_START)
 		) {
-			head_anchor = /** @type {TemplateNode} */ (get_next_sibling(head_anchor));
+			head_anchor = get_next_sibling(head_anchor);
 		}
 
 		// If we can't find an opening hydration marker, skip hydration (this can happen
@@ -58,6 +59,43 @@ export function head(render_fn) {
 
 	try {
 		block(() => render_fn(anchor), HEAD_EFFECT);
+		check_end();
+	} catch (error) {
+		// Remount only this svelte:head
+		if (was_hydrating && head_anchor != null) {
+			hydration_mismatch();
+			// Here head_anchor is the node next after HYDRATION_START
+			/** @type {Node | null} */
+			var node = head_anchor.previousSibling;
+			// Remove nodes that failed to hydrate
+			var depth = 0;
+			while (node !== null) {
+				var prev = /** @type {TemplateNode} */ (node);
+				node = get_next_sibling(node);
+				prev.remove();
+				if (prev.nodeType === COMMENT_NODE) {
+					var data = /** @type {Comment} */ (prev).data;
+					if (data === HYDRATION_END) {
+						depth -= 1;
+						if (depth === 0) break;
+					} else if (data === HYDRATION_START) {
+						depth += 1;
+					}
+				}
+			}
+			// Setup hydration for the next svelte:head
+			if (node === null) {
+				head_anchor = null;
+			} else {
+				head_anchor = set_hydrate_node(/** @type {TemplateNode} */ (node));
+			}
+
+			set_hydrating(false);
+			anchor = document.head.appendChild(create_text());
+			block(() => render_fn(anchor), HEAD_EFFECT);
+		} else {
+			throw error;
+		}
 	} finally {
 		if (was_hydrating) {
 			set_hydrating(true);
@@ -66,3 +104,11 @@ export function head(render_fn) {
 		}
 	}
 }
+
+// treeshaking of hydrate node fails when this is directly in the try-catch
+function check_end() {
+	if (hydrating && /** @type {Comment|null} */ (hydrate_node)?.data !== HYDRATION_END) {
+		hydration_mismatch();
+		throw HYDRATION_ERROR;
+	}
+}

packages/svelte/tests/hydration/samples/head-corrupted-2/_config.js

@@ -0,0 +1,18 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true,
+	test(assert, target, snapshot, component, window) {
+		assert.equal(window.document.querySelectorAll('meta').length, 5);
+
+		const [button] = target.getElementsByTagName('button');
+		button.click();
+		flushSync();
+
+		/** @type {NodeList} */
+		const metas = window.document.querySelectorAll('meta[name=count]');
+		assert.equal(metas.length, 4);
+		metas.forEach((meta) => assert.equal(/** @type {HTMLMetaElement} */ (meta).content, '2'));
+	}
+});

packages/svelte/tests/hydration/samples/head-corrupted-2/_expected_head.html

@@ -0,0 +1,2 @@
+<!--[--><meta name="count" content="1"><!----><!--]--><!--[--><meta name="count" content="1"><!----><!--]-->
+<!----><meta name="count" content="1"> <meta name="will-be-missing"> <meta name="count" content="1">

packages/svelte/tests/hydration/samples/head-corrupted-2/_override_head.html

@@ -0,0 +1,5 @@
+<!--[--><meta name="count" content="1" /><!----><!--]--><!--[--><meta name="count" content="1" />
+<meta name="count" content="1" /><!----><!--]--><!--[--><meta
+	name="count"
+	content="1"
+/><!----><!--]-->

packages/svelte/tests/hydration/samples/head-corrupted-2/head.svelte

@@ -0,0 +1,6 @@
+<script>
+	let { children } = $props();
+</script>
+<svelte:head>
+	{@render children()}
+</svelte:head>

packages/svelte/tests/hydration/samples/head-corrupted-2/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import Head from './head.svelte';
+
+	let count = $state(1);
+</script>
+
+<Head>
+	<meta name="count" content={count}>
+</Head>
+<Head>
+	<meta name="count" content={count}>
+	<meta name="will-be-missing">
+	<meta name="count" content={count}>
+</Head>
+<Head>
+	<meta name="count" content={count}>
+</Head>
+
+<button onclick={() => count++}>inc</button>

packages/svelte/tests/hydration/samples/head-corrupted-3/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true,
+	test(assert, target, snapshot, component, window) {
+		assert.equal(window.document.querySelectorAll('meta').length, 2);
+	}
+});

packages/svelte/tests/hydration/samples/head-corrupted-3/_expected_head.html

@@ -0,0 +1 @@
+<meta name="description" content="some description"> <meta name="keywords" content="some keywords">

packages/svelte/tests/hydration/samples/head-corrupted-3/_override_head.html

@@ -0,0 +1,2 @@
+<!--[--><meta name="description" content="some description" />
+<meta name="foreign" content="alien" /> <meta name="keywords" content="some keywords" /><!--]-->

packages/svelte/tests/hydration/samples/head-corrupted-3/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	let content = "some keywords"
+</script>
+
+<svelte:head>
+	<meta name="description" content="some description" />
+	<meta name="keywords" {content} />
+</svelte:head>
+
+<div>Just a dummy page.</div>

packages/svelte/tests/hydration/samples/head-corrupted/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true,
+	test(assert, target, snapshot, component, window) {
+		assert.equal(window.document.querySelectorAll('meta').length, 2);
+	}
+});

packages/svelte/tests/hydration/samples/head-corrupted/_expected_head.html

@@ -0,0 +1 @@
+<!--[--><meta name="description" content="some description"> <meta name="keywords" content="some keywords">

packages/svelte/tests/hydration/samples/head-corrupted/_override_head.html

@@ -0,0 +1 @@
+<!--[--><meta name="description" content="some description" /><!--]-->

packages/svelte/tests/hydration/samples/head-corrupted/main.svelte

@@ -0,0 +1,6 @@
+<svelte:head>
+	<meta name="description" content="some description" />
+	<meta name="keywords" content="some keywords" />
+</svelte:head>
+
+<div>Just a dummy page.</div>