From 03f166117693c6daccb0bf46cf20464cbd3afae6 Mon Sep 17 00:00:00 2001
From: Matteo Battista <matteo.battista@gmail.com>
Date: Wed, 30 Apr 2025 08:47:21 +0200
Subject: [PATCH 1/4] fix: update_branch with (anchor).data possible undefined

---
 packages/svelte/src/internal/client/dom/blocks/if.js | 1 +
 1 file changed, 1 insertion(+)

diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 925abb9d9d46..2f408c009c82 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -57,6 +57,7 @@ export function if_block(node, fn, [root_index, hydrate_index] = [0, 0]) {
 		if (hydrating && hydrate_index !== -1) {
 			if (root_index === 0) {
 				const data = /** @type {Comment} */ (anchor).data;
+				if (!data) return;
 				if (data === HYDRATION_START) {
 					hydrate_index = 0;
 				} else if (data === HYDRATION_START_ELSE) {

From 84305cf2a911110a6f7d7fb519b223d6e87f3ba5 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 15 May 2025 11:46:07 -0400
Subject: [PATCH 2/4] error sooner

---
 .../svelte/src/internal/client/dom/blocks/each.js   |  3 ++-
 .../svelte/src/internal/client/dom/blocks/if.js     |  5 +++--
 .../svelte/src/internal/client/dom/hydration.js     | 13 +++++++++++++
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 92c953b541f8..2997664fa259 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -12,6 +12,7 @@ import {
 	hydrate_next,
 	hydrate_node,
 	hydrating,
+	read_hydration_instruction,
 	remove_nodes,
 	set_hydrate_node,
 	set_hydrating
@@ -160,7 +161,7 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		let mismatch = false;
 
 		if (hydrating) {
-			var is_else = /** @type {Comment} */ (anchor).data === HYDRATION_START_ELSE;
+			var is_else = read_hydration_instruction(anchor) === HYDRATION_START_ELSE;
 
 			if (is_else !== (length === 0)) {
 				// hydration mismatch — remove the server-rendered DOM and start over
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 2f408c009c82..bf1098c3f465 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -4,6 +4,7 @@ import {
 	hydrate_next,
 	hydrate_node,
 	hydrating,
+	read_hydration_instruction,
 	remove_nodes,
 	set_hydrate_node,
 	set_hydrating
@@ -56,8 +57,8 @@ export function if_block(node, fn, [root_index, hydrate_index] = [0, 0]) {
 
 		if (hydrating && hydrate_index !== -1) {
 			if (root_index === 0) {
-				const data = /** @type {Comment} */ (anchor).data;
-				if (!data) return;
+				const data = read_hydration_instruction(anchor);
+
 				if (data === HYDRATION_START) {
 					hydrate_index = 0;
 				} else if (data === HYDRATION_START_ELSE) {
diff --git a/packages/svelte/src/internal/client/dom/hydration.js b/packages/svelte/src/internal/client/dom/hydration.js
index 8523ff97d559..ab3256da82db 100644
--- a/packages/svelte/src/internal/client/dom/hydration.js
+++ b/packages/svelte/src/internal/client/dom/hydration.js
@@ -103,3 +103,16 @@ export function remove_nodes() {
 		node = next;
 	}
 }
+
+/**
+ *
+ * @param {TemplateNode} node
+ */
+export function read_hydration_instruction(node) {
+	if (!node || node.nodeType !== 8) {
+		w.hydration_mismatch();
+		throw HYDRATION_ERROR;
+	}
+
+	return /** @type {Comment} */ (node).data;
+}

From 941514506bea613de930e70bc38f18ced76310a6 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 15 May 2025 11:46:29 -0400
Subject: [PATCH 3/4] add tests

---
 .../samples/cloudflare-mirage-borking-2/_config.js       | 6 ++++++
 .../samples/cloudflare-mirage-borking-2/_expected.html   | 1 +
 .../samples/cloudflare-mirage-borking-2/_override.html   | 1 +
 .../samples/cloudflare-mirage-borking-2/main.svelte      | 5 +++++
 .../samples/cloudflare-mirage-borking/_config.js         | 6 ++++++
 .../samples/cloudflare-mirage-borking/_expected.html     | 1 +
 .../samples/cloudflare-mirage-borking/_override.html     | 1 +
 .../samples/cloudflare-mirage-borking/main.svelte        | 9 +++++++++
 8 files changed, 30 insertions(+)
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html
 create mode 100644 packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte

diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js
new file mode 100644
index 000000000000..56ba73b06408
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js
@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15819
+export default test({
+	expect_hydration_error: true
+});
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html
new file mode 100644
index 000000000000..5179fb04a5f7
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html
@@ -0,0 +1 @@
+<p>start</p><p>cond</p><!---->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html
new file mode 100644
index 000000000000..2a1c32328897
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html
@@ -0,0 +1 @@
+<!--[--> <p>start</p><!--[--><p>cond</p><!--]--><!--]-->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte
new file mode 100644
index 000000000000..bfb4f2cdb8cf
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	const cond = true;
+</script>
+
+<p>start</p>{#if cond}<p>cond</p>{/if}
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js
new file mode 100644
index 000000000000..56ba73b06408
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js
@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15819
+export default test({
+	expect_hydration_error: true
+});
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html
new file mode 100644
index 000000000000..f6c03b87c1c2
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html
@@ -0,0 +1 @@
+<p>start</p> pre123 mid<!---->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html
new file mode 100644
index 000000000000..c84efbb00bd2
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html
@@ -0,0 +1 @@
+<!--[--><p>start</p>pre<a>123</a> <!--[-->mid<!--]--><!--]-->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte
new file mode 100644
index 000000000000..2c9a94686eb3
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	let cond = true;
+</script>
+
+<p>start</p>
+pre123
+{#if cond}
+mid
+{/if}

From 2caf059768256adf8543bd6ba0155a3ff763c831 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 15 May 2025 11:50:29 -0400
Subject: [PATCH 4/4] changeset

---
 .changeset/silly-apples-remain.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/silly-apples-remain.md

diff --git a/.changeset/silly-apples-remain.md b/.changeset/silly-apples-remain.md
new file mode 100644
index 000000000000..10d43db55087
--- /dev/null
+++ b/.changeset/silly-apples-remain.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle more hydration mismatches