From 4dfad737861c66aedf27fd3a8d3b74935dcb6e63 Mon Sep 17 00:00:00 2001
From: 7nik <kifiranet@gmail.com>
Date: Thu, 26 Jun 2025 19:51:57 +0300
Subject: [PATCH 1/4] fix: remount at any hydration error

---
 .changeset/hip-eagles-yawn.md                 |  5 ++++
 packages/svelte/src/internal/client/render.js | 23 ++++++++++---------
 .../whitespace-at-block-start/Nested.svelte   |  1 +
 .../whitespace-at-block-start/_config.js      | 21 +++++++++++++++++
 .../whitespace-at-block-start/_expected.html  |  1 +
 .../whitespace-at-block-start/_override.html  |  2 ++
 .../whitespace-at-block-start/main.svelte     |  7 ++++++
 .../samples/keyed-each-dev-unique/_config.js  |  1 +
 .../inspect-state-unsafe-mutation/_config.js  |  1 +
 .../samples/props-bound-fallback/_config.js   |  1 +
 10 files changed, 52 insertions(+), 11 deletions(-)
 create mode 100644 .changeset/hip-eagles-yawn.md
 create mode 100644 packages/svelte/tests/hydration/samples/whitespace-at-block-start/Nested.svelte
 create mode 100644 packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js
 create mode 100644 packages/svelte/tests/hydration/samples/whitespace-at-block-start/_expected.html
 create mode 100644 packages/svelte/tests/hydration/samples/whitespace-at-block-start/_override.html
 create mode 100644 packages/svelte/tests/hydration/samples/whitespace-at-block-start/main.svelte

diff --git a/.changeset/hip-eagles-yawn.md b/.changeset/hip-eagles-yawn.md
new file mode 100644
index 000000000000..c987e7c7f2eb
--- /dev/null
+++ b/.changeset/hip-eagles-yawn.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: remount at any hydration error
diff --git a/packages/svelte/src/internal/client/render.js b/packages/svelte/src/internal/client/render.js
index ff6844453dcc..dfb40f179661 100644
--- a/packages/svelte/src/internal/client/render.js
+++ b/packages/svelte/src/internal/client/render.js
@@ -136,20 +136,21 @@ export function hydrate(component, options) {
 
 		return /**  @type {Exports} */ (instance);
 	} catch (error) {
-		if (error === HYDRATION_ERROR) {
-			if (options.recover === false) {
-				e.hydration_failed();
-			}
-
-			// If an error occured above, the operations might not yet have been initialised.
-			init_operations();
-			clear_text_content(target);
+		if (error !== HYDRATION_ERROR) {
+			// eslint-disable-next-line no-console
+			console.error('Failed to hydrate: ', error);
+		}
 
-			set_hydrating(false);
-			return mount(component, options);
+		if (options.recover === false) {
+			e.hydration_failed();
 		}
 
-		throw error;
+		// If an error occured above, the operations might not yet have been initialised.
+		init_operations();
+		clear_text_content(target);
+
+		set_hydrating(false);
+		return mount(component, options);
 	} finally {
 		set_hydrating(was_hydrating);
 		set_hydrate_node(previous_hydrate_node);
diff --git a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/Nested.svelte b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/Nested.svelte
new file mode 100644
index 000000000000..70bf63ad9de2
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/Nested.svelte
@@ -0,0 +1 @@
+<p>nested</p>
\ No newline at end of file
diff --git a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js
new file mode 100644
index 000000000000..e01861a8231b
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js
@@ -0,0 +1,21 @@
+import { test } from '../../test';
+
+/** @type {string[]} */
+let logs = [];
+/** @type {typeof console['error']} */
+let console_error;
+
+export default test({
+	before_test() {
+		console_error = console.error;
+		console.error = (...args) => logs.push(args.join(''));
+	},
+	after_test() {
+		console.error = console_error;
+	},
+	test({ deepEqual }) {
+		deepEqual(logs, [
+			"Failed to hydrate: HierarchyRequestError: Node can't be inserted in a #text parent."
+		]);
+	}
+});
diff --git a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_expected.html b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_expected.html
new file mode 100644
index 000000000000..46f8e8a7ace4
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_expected.html
@@ -0,0 +1 @@
+<main><p>nested</p><!----></main>
\ No newline at end of file
diff --git a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_override.html b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_override.html
new file mode 100644
index 000000000000..90ca4ef4b8c9
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_override.html
@@ -0,0 +1,2 @@
+<!--[-->
+<main><p>nested</p><!----></main><!--]-->
\ No newline at end of file
diff --git a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/main.svelte b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/main.svelte
new file mode 100644
index 000000000000..f1f962b95848
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/main.svelte
@@ -0,0 +1,7 @@
+<script>
+	import Nested from './Nested.svelte';
+</script>
+
+<main>
+	<Nested />
+</main>
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js b/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js
index 6e64245d861e..97ccaebac252 100644
--- a/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js
@@ -1,6 +1,7 @@
 import { test } from '../../test';
 
 export default test({
+	mode: ['server', 'client'],
 	compileOptions: {
 		dev: true
 	},
diff --git a/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js b/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
index dcf3a8bc3d3f..27b9cc7fecab 100644
--- a/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
@@ -2,6 +2,7 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
+	mode: ['client'],
 	compileOptions: {
 		dev: true
 	},
diff --git a/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js b/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js
index d04d8884ca37..26cb426127c7 100644
--- a/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js
@@ -5,6 +5,7 @@ import { test } from '../../test';
 // uses a prop it does not write to but has a fallback value
 export default test({
 	accessors: false, // so that prop actually becomes $.prop and not $.prop_source
+	mode: ['server', 'client'],
 	html: `<button>0</button><span>0</span>`,
 
 	test({ assert, target }) {

From bda8955687acd3bfd11b7835c43738346a6f2e6f Mon Sep 17 00:00:00 2001
From: 7nik <kifiranet@gmail.com>
Date: Fri, 27 Jun 2025 11:40:18 +0300
Subject: [PATCH 2/4] re-throw svelte error immediately

---
 packages/svelte/src/internal/client/render.js                 | 4 ++++
 .../runtime-legacy/samples/keyed-each-dev-unique/_config.js   | 1 -
 .../samples/inspect-state-unsafe-mutation/_config.js          | 1 -
 .../runtime-runes/samples/props-bound-fallback/_config.js     | 1 -
 4 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/render.js b/packages/svelte/src/internal/client/render.js
index dfb40f179661..7df1d419e067 100644
--- a/packages/svelte/src/internal/client/render.js
+++ b/packages/svelte/src/internal/client/render.js
@@ -136,6 +136,10 @@ export function hydrate(component, options) {
 
 		return /**  @type {Exports} */ (instance);
 	} catch (error) {
+		// re-throw Svelte errors - they are certainly not related to hydration
+		if (error instanceof Error && error.message.includes('https://svelte.dev/e/')) {
+			throw error;
+		}
 		if (error !== HYDRATION_ERROR) {
 			// eslint-disable-next-line no-console
 			console.error('Failed to hydrate: ', error);
diff --git a/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js b/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js
index 97ccaebac252..6e64245d861e 100644
--- a/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique/_config.js
@@ -1,7 +1,6 @@
 import { test } from '../../test';
 
 export default test({
-	mode: ['server', 'client'],
 	compileOptions: {
 		dev: true
 	},
diff --git a/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js b/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
index 27b9cc7fecab..dcf3a8bc3d3f 100644
--- a/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
@@ -2,7 +2,6 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
-	mode: ['client'],
 	compileOptions: {
 		dev: true
 	},
diff --git a/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js b/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js
index 26cb426127c7..d04d8884ca37 100644
--- a/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js
@@ -5,7 +5,6 @@ import { test } from '../../test';
 // uses a prop it does not write to but has a fallback value
 export default test({
 	accessors: false, // so that prop actually becomes $.prop and not $.prop_source
-	mode: ['server', 'client'],
 	html: `<button>0</button><span>0</span>`,
 
 	test({ assert, target }) {

From 0300fbca904e97fcb8abb56795487cd86ff24fe7 Mon Sep 17 00:00:00 2001
From: 7nik <kifiranet@gmail.com>
Date: Fri, 27 Jun 2025 11:49:16 +0300
Subject: [PATCH 3/4] log error as warn

---
 packages/svelte/src/internal/client/render.js |  2 +-
 .../whitespace-at-block-start/_config.js      | 21 ++++---------------
 2 files changed, 5 insertions(+), 18 deletions(-)

diff --git a/packages/svelte/src/internal/client/render.js b/packages/svelte/src/internal/client/render.js
index 7df1d419e067..f09625d9510c 100644
--- a/packages/svelte/src/internal/client/render.js
+++ b/packages/svelte/src/internal/client/render.js
@@ -142,7 +142,7 @@ export function hydrate(component, options) {
 		}
 		if (error !== HYDRATION_ERROR) {
 			// eslint-disable-next-line no-console
-			console.error('Failed to hydrate: ', error);
+			console.warn('Failed to hydrate: ', error);
 		}
 
 		if (options.recover === false) {
diff --git a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js
index e01861a8231b..457eeb2201d2 100644
--- a/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js
+++ b/packages/svelte/tests/hydration/samples/whitespace-at-block-start/_config.js
@@ -1,21 +1,8 @@
 import { test } from '../../test';
 
-/** @type {string[]} */
-let logs = [];
-/** @type {typeof console['error']} */
-let console_error;
-
 export default test({
-	before_test() {
-		console_error = console.error;
-		console.error = (...args) => logs.push(args.join(''));
-	},
-	after_test() {
-		console.error = console_error;
-	},
-	test({ deepEqual }) {
-		deepEqual(logs, [
-			"Failed to hydrate: HierarchyRequestError: Node can't be inserted in a #text parent."
-		]);
-	}
+	errors: [
+		'Failed to hydrate: ',
+		new DOMException("Node can't be inserted in a #text parent.", 'HierarchyRequestError')
+	]
 });

From ab587bff3516643e4c734c4f9abbdb6e942dda6c Mon Sep 17 00:00:00 2001
From: 7nik <kifiranet@gmail.com>
Date: Fri, 27 Jun 2025 12:33:06 +0300
Subject: [PATCH 4/4] fix?

---
 packages/svelte/src/internal/client/render.js | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/render.js b/packages/svelte/src/internal/client/render.js
index f09625d9510c..3a489f9f927b 100644
--- a/packages/svelte/src/internal/client/render.js
+++ b/packages/svelte/src/internal/client/render.js
@@ -137,7 +137,10 @@ export function hydrate(component, options) {
 		return /**  @type {Exports} */ (instance);
 	} catch (error) {
 		// re-throw Svelte errors - they are certainly not related to hydration
-		if (error instanceof Error && error.message.includes('https://svelte.dev/e/')) {
+		if (
+			error instanceof Error &&
+			error.message.split('\n').some((line) => line.startsWith('https://svelte.dev/e/'))
+		) {
 			throw error;
 		}
 		if (error !== HYDRATION_ERROR) {