From 8e280992f0bf7f90cb656f054ab04a665739c1ae Mon Sep 17 00:00:00 2001
From: floriskn <48930050+floriskn@users.noreply.github.com>
Date: Mon, 16 Jun 2025 11:52:48 +0200
Subject: [PATCH 1/4] fix: ensure undefined attributes are removed during
 hydration

Attributes that are `undefined` on the client should be removed during hydration,
even if their value hasn't changed compared to `prev_value`.
---
 .../svelte/src/internal/client/dom/elements/attributes.js   | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index fcce0b444f49..e61f9b5679ec 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -345,7 +345,11 @@ export function set_attributes(element, prev, next, css_hash, skip_warning = fal
 		}
 
 		var prev_value = current[key];
-		if (value === prev_value) continue;
+
+		// Skip if value is unchanged, unless it's `undefined` and the element still has the attribute
+		if (value === prev_value && !(value === undefined && element.hasAttribute(key))) {
+			continue;
+		}
 
 		current[key] = value;
 

From b58169da8b359433b71e3210bee39add32bc7526 Mon Sep 17 00:00:00 2001
From: floriskn <48930050+floriskn@users.noreply.github.com>
Date: Mon, 16 Jun 2025 11:59:09 +0200
Subject: [PATCH 2/4] Create plenty-wasps-sleep.md

---
 .changeset/plenty-wasps-sleep.md | 10 ++++++++++
 1 file changed, 10 insertions(+)
 create mode 100644 .changeset/plenty-wasps-sleep.md

diff --git a/.changeset/plenty-wasps-sleep.md b/.changeset/plenty-wasps-sleep.md
new file mode 100644
index 000000000000..446bf4653bb2
--- /dev/null
+++ b/.changeset/plenty-wasps-sleep.md
@@ -0,0 +1,10 @@
+---
+'svelte': patch
+---
+
+fix: ensure undefined attributes are removed during hydration
+
+Attributes set server-side but undefined on the client should be removed during hydration.
+This ensures expected cleanup behavior and resolves a mismatch between SSR and client-side rendering.
+
+Fixes sveltejs/kit#13890

From 3fc29e1e3ea8865bb7898b0671c6e4d469b52d54 Mon Sep 17 00:00:00 2001
From: floriskn <48930050+floriskn@users.noreply.github.com>
Date: Mon, 16 Jun 2025 16:27:42 +0200
Subject: [PATCH 3/4] added test

---
 .../samples/removes-undefined-attributes/_config.js   | 11 +++++++++++
 .../removes-undefined-attributes/_expected.html       |  1 +
 .../samples/removes-undefined-attributes/main.svelte  |  9 +++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 packages/svelte/tests/hydration/samples/removes-undefined-attributes/_config.js
 create mode 100644 packages/svelte/tests/hydration/samples/removes-undefined-attributes/_expected.html
 create mode 100644 packages/svelte/tests/hydration/samples/removes-undefined-attributes/main.svelte

diff --git a/packages/svelte/tests/hydration/samples/removes-undefined-attributes/_config.js b/packages/svelte/tests/hydration/samples/removes-undefined-attributes/_config.js
new file mode 100644
index 000000000000..bc74f23aac60
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/removes-undefined-attributes/_config.js
@@ -0,0 +1,11 @@
+import { test } from '../../test';
+
+export default test({
+	server_props: {
+		browser: false
+	},
+
+	props: {
+		browser: true
+	}
+});
diff --git a/packages/svelte/tests/hydration/samples/removes-undefined-attributes/_expected.html b/packages/svelte/tests/hydration/samples/removes-undefined-attributes/_expected.html
new file mode 100644
index 000000000000..cc789c8f5142
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/removes-undefined-attributes/_expected.html
@@ -0,0 +1 @@
+<!--[--><div></div><!--]-->
diff --git a/packages/svelte/tests/hydration/samples/removes-undefined-attributes/main.svelte b/packages/svelte/tests/hydration/samples/removes-undefined-attributes/main.svelte
new file mode 100644
index 000000000000..1a587eeeebc0
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/removes-undefined-attributes/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	const { browser } = $props();
+
+	const attributes = {
+		"data-test": browser ? undefined : ""
+	};
+</script>
+
+<div {...attributes}></div>

From 574def131957c4358e85917771405bfaef5b6ab9 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Tue, 17 Jun 2025 12:46:40 -0400
Subject: [PATCH 4/4] Update .changeset/plenty-wasps-sleep.md

---
 .changeset/plenty-wasps-sleep.md | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/.changeset/plenty-wasps-sleep.md b/.changeset/plenty-wasps-sleep.md
index 446bf4653bb2..aba96bc419b0 100644
--- a/.changeset/plenty-wasps-sleep.md
+++ b/.changeset/plenty-wasps-sleep.md
@@ -2,9 +2,4 @@
 'svelte': patch
 ---
 
-fix: ensure undefined attributes are removed during hydration
-
-Attributes set server-side but undefined on the client should be removed during hydration.
-This ensures expected cleanup behavior and resolves a mismatch between SSR and client-side rendering.
-
-Fixes sveltejs/kit#13890
+fix: remove undefined attributes on hydration