fix: add css hash to custom element rendered with svelte:element (#12715)

* fix: add css hash to custom element rendered with `svelte:element`

* simplify

* skip arg where possible

* drive-by improvements — remove some unnecessary arguments where possible

---------

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

Author: paoloricciuti

.changeset/chilly-carpets-switch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add css hash to custom element rendered with `svelte:element`

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

@@ -470,10 +470,8 @@ function build_element_spread_attributes(
 			attribute.type === 'SpreadAttribute' && attribute.metadata.expression.has_call;
 	}
 
-	const lowercase_attributes =
-		element.metadata.svg || element.metadata.mathml || is_custom_element_node(element)
-			? b.false
-			: b.true;
+	const preserve_attribute_case =
+		element.metadata.svg || element.metadata.mathml || is_custom_element_node(element);
 	const id = context.state.scope.generate('attributes');
 
 	const update = b.stmt(
@@ -485,8 +483,8 @@ function build_element_spread_attributes(
 				element_id,
 				b.id(id),
 				b.object(values),
-				lowercase_attributes,
-				b.literal(context.state.analysis.css.hash),
+				context.state.analysis.css.hash !== '' && b.literal(context.state.analysis.css.hash),
+				preserve_attribute_case && b.true,
 				is_ignored(element, 'hydration_attribute_changed') && b.true
 			)
 		)

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

@@ -200,7 +200,7 @@ function build_dynamic_element_attributes(attributes, context, element_id) {
 					element_id,
 					b.id(id),
 					b.object(values),
-					b.literal(context.state.analysis.css.hash)
+					context.state.analysis.css.hash !== '' && b.literal(context.state.analysis.css.hash)
 				)
 			)
 		);
@@ -221,7 +221,7 @@ function build_dynamic_element_attributes(attributes, context, element_id) {
 				element_id,
 				b.literal(null),
 				b.object(values),
-				b.literal(context.state.analysis.css.hash)
+				context.state.analysis.css.hash !== '' && b.literal(context.state.analysis.css.hash)
 			)
 		)
 	);

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -147,13 +147,19 @@ export function set_custom_element_data(node, prop, value) {
  * @param {Element & ElementCSSInlineStyle} element
  * @param {Record<string, any> | undefined} prev
  * @param {Record<string, any>} next New attributes - this function mutates this object
- * @param {boolean} lowercase_attributes
- * @param {string} css_hash
+ * @param {string} [css_hash]
+ * @param {boolean} preserve_attribute_case
  * @param {boolean} [skip_warning]
  * @returns {Record<string, any>}
  */
-export function set_attributes(element, prev, next, lowercase_attributes, css_hash, skip_warning) {
-	var has_hash = css_hash.length !== 0;
+export function set_attributes(
+	element,
+	prev,
+	next,
+	css_hash,
+	preserve_attribute_case = false,
+	skip_warning
+) {
 	var current = prev || {};
 	var is_option_element = element.tagName === 'OPTION';
 
@@ -163,8 +169,8 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
 		}
 	}
 
-	if (has_hash && !next.class) {
-		next.class = '';
+	if (css_hash !== undefined) {
+		next.class = next.class ? next.class + ' ' + css_hash : css_hash;
 	}
 
 	var setters = setters_cache.get(element.nodeName);
@@ -267,7 +273,7 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
 			element.value = element[key] = element.__value = value;
 		} else {
 			var name = key;
-			if (lowercase_attributes) {
+			if (!preserve_attribute_case) {
 				name = normalize_attribute(name);
 			}
 
@@ -279,11 +285,6 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
 					element[name] = value;
 				}
 			} else if (typeof value !== 'function') {
-				if (has_hash && name === 'class') {
-					if (value) value += ' ';
-					value += css_hash;
-				}
-
 				set_attribute(element, name, value);
 			}
 		}
@@ -309,7 +310,7 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
  * @param {Element} node
  * @param {Record<string, any> | undefined} prev
  * @param {Record<string, any>} next The new attributes - this function mutates this object
- * @param {string} css_hash
+ * @param {string} [css_hash]
  */
 export function set_dynamic_element_attributes(node, prev, next, css_hash) {
 	if (node.tagName.includes('-')) {
@@ -319,6 +320,10 @@ export function set_dynamic_element_attributes(node, prev, next, css_hash) {
 			}
 		}
 
+		if (css_hash !== undefined) {
+			next.class = next.class ? next.class + ' ' + css_hash : css_hash;
+		}
+
 		for (key in next) {
 			set_custom_element_data(node, key, next[key]);
 		}
@@ -330,8 +335,8 @@ export function set_dynamic_element_attributes(node, prev, next, css_hash) {
 		/** @type {Element & ElementCSSInlineStyle} */ (node),
 		prev,
 		next,
-		node.namespaceURI !== NAMESPACE_SVG,
-		css_hash
+		css_hash,
+		node.namespaceURI !== NAMESPACE_SVG
 	);
 }
 

packages/svelte/tests/runtime-runes/samples/svelte-element-custom-element-css-hash/_config.js

@@ -0,0 +1,14 @@
+import { ok, test } from '../../test';
+
+export default test({
+	html: `<custom-element class="red svelte-p153w3"></custom-element><custom-element class="red svelte-p153w3"></custom-element>`,
+
+	async test({ assert, target }) {
+		const [el, el2] = target.querySelectorAll('custom-element');
+		ok(el);
+		ok(el2);
+
+		assert.deepEqual(el.className, 'red svelte-p153w3');
+		assert.deepEqual(el2.className, 'red svelte-p153w3');
+	}
+});

packages/svelte/tests/runtime-runes/samples/svelte-element-custom-element-css-hash/main.svelte

@@ -0,0 +1,8 @@
+<svelte:element this={'custom-element'} class="red"></svelte:element>
+<custom-element class="red"></custom-element>
+
+<style>
+	.red {
+		color: red;
+	}
+</style>