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] 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 fcce0b444f..e61f9b5679 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;