fix: never set custom element props as attributes (#12622)

* fix: never set custom element props as attributes

* changeset

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/fifty-toys-invite.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: never set custom element props as attributes inside templates

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -2152,6 +2152,7 @@ export const template_visitors = {
 				}
 
 				if (
+					!is_custom_element &&
 					attribute.name !== 'autofocus' &&
 					(attribute.value === true || is_text_attribute(attribute))
 				) {

packages/svelte/tests/runtime-legacy/samples/attribute-custom-element-inheritance/_config.js

@@ -1,8 +1,7 @@
 import { test } from '../../test';
 
 export default test({
-	skip: true, // TODO: needs fixing
-
+	mode: ['client'],
 	html: `
 		<my-custom-inheritance-element>Hello World!</my-custom-inheritance-element>
 	`