From f835a572093722db8508ab7f4f2aae68d74422b5 Mon Sep 17 00:00:00 2001
From: 7nik <kifiranet@gmail.com>
Date: Mon, 15 Sep 2025 18:24:01 +0300
Subject: [PATCH] fix: correctly SSR hidden="until-found"

---
 .changeset/pink-gifts-sell.md                                | 5 +++++
 packages/svelte/src/internal/shared/attributes.js            | 4 ++++
 packages/svelte/src/utils.js                                 | 1 -
 .../samples/attribute-spread-hidden-2/_expected.html         | 1 +
 .../samples/attribute-spread-hidden-2/main.svelte            | 3 +++
 5 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/pink-gifts-sell.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/_expected.html
 create mode 100644 packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/main.svelte

diff --git a/.changeset/pink-gifts-sell.md b/.changeset/pink-gifts-sell.md
new file mode 100644
index 0000000000..f3f91ff7d9
--- /dev/null
+++ b/.changeset/pink-gifts-sell.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly SSR hidden="until-found"
diff --git a/packages/svelte/src/internal/shared/attributes.js b/packages/svelte/src/internal/shared/attributes.js
index c8758c1d4d..a96e71ff6f 100644
--- a/packages/svelte/src/internal/shared/attributes.js
+++ b/packages/svelte/src/internal/shared/attributes.js
@@ -23,6 +23,10 @@ const replacements = {
  */
 export function attr(name, value, is_boolean = false) {
 	if (value == null || (!value && is_boolean)) return '';
+	// attribute hidden for values other than "until-found" behaves like a boolean attribute
+	if (name === 'hidden' && value !== 'until-found') {
+		is_boolean = true;
+	}
 	const normalized = (name in replacements && replacements[name].get(value)) || value;
 	const assignment = is_boolean ? '' : `="${escape_html(normalized, true)}"`;
 	return ` ${name}${assignment}`;
diff --git a/packages/svelte/src/utils.js b/packages/svelte/src/utils.js
index f8c39253ac..f8a7e8d46d 100644
--- a/packages/svelte/src/utils.js
+++ b/packages/svelte/src/utils.js
@@ -154,7 +154,6 @@ const DOM_BOOLEAN_ATTRIBUTES = [
 	'default',
 	'disabled',
 	'formnovalidate',
-	'hidden',
 	'indeterminate',
 	'inert',
 	'ismap',
diff --git a/packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/_expected.html b/packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/_expected.html
new file mode 100644
index 0000000000..80937efaee
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/_expected.html
@@ -0,0 +1 @@
+<!--[--><div>A</div><div hidden>B</div><div hidden="until-found">C</div><!--]-->
diff --git a/packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/main.svelte b/packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/main.svelte
new file mode 100644
index 0000000000..6738b34054
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/attribute-spread-hidden-2/main.svelte
@@ -0,0 +1,3 @@
+<div {...{ hidden: false }}>A</div>
+<div {...{ hidden: true }}>B</div>
+<div {...{ hidden: 'until-found' }}>C</div>
\ No newline at end of file