fix: correctly SSR hidden="until-found" (#16773)

.changeset/pink-gifts-sell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly SSR hidden="until-found"

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}`;

packages/svelte/src/utils.js

@@ -154,7 +154,6 @@ const DOM_BOOLEAN_ATTRIBUTES = [
 	'default',
 	'disabled',
 	'formnovalidate',
-	'hidden',
 	'indeterminate',
 	'inert',
 	'ismap',

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><!--]-->

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>