fix: apply clsx logic to custom element class attributes

.changeset/happy-candles-clean.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: apply `clsx` logic to custom element `class` attributes

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

@@ -571,7 +571,9 @@ function build_element_attribute_update_assignment(
 				is_svg ? '$.set_svg_class' : is_mathml ? '$.set_mathml_class' : '$.set_class',
 				node_id,
 				value,
-				attribute.metadata.needs_clsx ? b.literal(context.state.analysis.css.hash) : undefined
+				attribute.metadata.needs_clsx && context.state.analysis.css.hash
+					? b.literal(context.state.analysis.css.hash)
+					: undefined
 			)
 		);
 	} else if (name === 'value') {
@@ -648,6 +650,14 @@ function build_custom_element_attribute_update_assignment(node_id, attribute, co
 	const name = attribute.name; // don't lowercase, as we set the element's property, which might be case sensitive
 	let { has_call, value } = build_attribute_value(attribute.value, context);
 
+	// We assume that noone's going to redefine the semantics of the class attribute on custom elements, i.e. it's still used for CSS classes
+	if (name === 'class' && attribute.metadata.needs_clsx) {
+		if (context.state.analysis.css.hash) {
+			value = b.array([value, b.literal(context.state.analysis.css.hash)]);
+		}
+		value = b.call('$.clsx', value);
+	}
+
 	const update = b.stmt(b.call('$.set_custom_element_data', node_id, b.literal(name), value));
 
 	if (attribute.metadata.expression.has_state) {

packages/svelte/tests/runtime-runes/samples/clsx/_config.js

@@ -1,3 +1,4 @@
+import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
@@ -14,27 +15,32 @@ export default test({
 		<div class="foo">child</div>
 		<div class="foo">child</div>
 
+		<applied-to-custom-element class="foo svelte-owbekl"></applied-to-custom-element>
+
 		<button>update</button>
 	`,
 	test({ assert, target }) {
 		const button = target.querySelector('button');
 
 		button?.click();
+		flushSync();
 
 		assert.htmlEqual(
 			target.innerHTML,
 			`
 				<div class="foo svelte-owbekl"></div>
 				<div class="foo svelte-owbekl"></div>
-				<div class="foo svelte-owbekl"></div>
-				<div class="foo svelte-owbekl"></div>
+				<div class="bar svelte-owbekl"></div>
+				<div class="bar svelte-owbekl"></div>
 				<div class="foo svelte-owbekl"></div>
 
 				<div class="foo">child</div>
 				<div class="foo">child</div>
+				<div class="bar">child</div>
+				<div class="bar">child</div>
 				<div class="foo">child</div>
-				<div class="foo">child</div>
-				<div class="foo">child</div>
+
+				<applied-to-custom-element class="bar svelte-owbekl"></applied-to-custom-element>
 
 				<button>update</button>
 			`

packages/svelte/tests/runtime-runes/samples/clsx/main.svelte

@@ -18,6 +18,8 @@
 <Child class={[ foo, bar ]} />
 <Child {...spread} />
 
+<applied-to-custom-element class={{ foo, bar }}></applied-to-custom-element>
+
 <button onclick={() => {
     foo = null;
     bar = 'bar';