fix: prevent spread attribute from overriding class directive

.changeset/red-coats-grin.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent spread attribute from overriding class directive

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

@@ -219,7 +219,8 @@ export function RegularElement(node, context) {
 			node_id,
 			attributes_id,
 			(node.metadata.svg || node.metadata.mathml || is_custom_element_node(node)) && b.true,
-			node.name.includes('-') && b.true
+			node.name.includes('-') && b.true,
+			context.state
 		);
 
 		// If value binding exists, that one takes care of calling $.init_select

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

@@ -102,7 +102,8 @@ export function SvelteElement(node, context) {
 			element_id,
 			attributes_id,
 			b.binary('!==', b.member(element_id, 'namespaceURI'), b.id('$.NAMESPACE_SVG')),
-			b.call(b.member(b.member(element_id, 'nodeName'), 'includes'), b.literal('-'))
+			b.call(b.member(b.member(element_id, 'nodeName'), 'includes'), b.literal('-')),
+			context.state
 		);
 	}
 

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

@@ -1,6 +1,6 @@
 /** @import { Expression, Identifier, ObjectExpression } from 'estree' */
 /** @import { AST, Namespace } from '#compiler' */
-/** @import { ComponentContext } from '../../types' */
+/** @import { ComponentClientTransformState, ComponentContext } from '../../types' */
 import { normalize_attribute } from '../../../../../../utils.js';
 import { is_ignored } from '../../../../../state.js';
 import { get_attribute_expression, is_event_attribute } from '../../../../../utils/ast.js';
@@ -16,6 +16,7 @@ import { build_template_literal, build_update } from './utils.js';
  * @param {Identifier} attributes_id
  * @param {false | Expression} preserve_attribute_case
  * @param {false | Expression} is_custom_element
+ * @param {ComponentClientTransformState} state
  */
 export function build_set_attributes(
 	attributes,
@@ -24,9 +25,9 @@ export function build_set_attributes(
 	element_id,
 	attributes_id,
 	preserve_attribute_case,
-	is_custom_element
+	is_custom_element,
+	state
 ) {
-	let needs_isolation = false;
 	let has_state = false;
 
 	/** @type {ObjectExpression['properties']} */
@@ -50,12 +51,17 @@ export function build_set_attributes(
 
 			has_state ||= attribute.metadata.expression.has_state;
 		} else {
-			values.push(b.spread(/** @type {Expression} */ (context.visit(attribute))));
-
 			// objects could contain reactive getters -> play it safe and always assume spread attributes are reactive
 			has_state = true;
 
-			needs_isolation ||= attribute.metadata.expression.has_call;
+			let value = /** @type {Expression} */ (context.visit(attribute));
+
+			if (attribute.metadata.expression.has_call) {
+				const id = b.id(state.scope.generate('spread_with_call'));
+				state.init.push(b.const(id, create_derived(state, b.thunk(value))));
+				value = b.call('$.get', id);
+			}
+			values.push(b.spread(value));
 		}
 	}
 
@@ -72,14 +78,7 @@ export function build_set_attributes(
 
 	if (has_state) {
 		context.state.init.push(b.let(attributes_id));
-
 		const update = b.stmt(b.assignment('=', attributes_id, call));
-
-		if (needs_isolation) {
-			context.state.init.push(build_update(update));
-			return false;
-		}
-
 		context.state.update.push(update);
 		return true;
 	}

packages/svelte/tests/runtime-runes/samples/dynamic-spread-and-attribute-directive/_config.js

@@ -0,0 +1,25 @@
+import { flushSync } from 'svelte';
+import { test, ok } from '../../test';
+
+export default test({
+	test({ target, logs, assert }) {
+		const input = target.querySelector('input');
+
+		ok(input);
+
+		assert.deepEqual(logs, ['get_rest']);
+
+		assert.ok(input.classList.contains('dark'));
+		assert.equal(input.dataset.rest, 'true');
+
+		flushSync(() => {
+			input.focus();
+		});
+
+		assert.ok(input.classList.contains('dark'));
+		assert.ok(input.classList.contains('focused'));
+		assert.equal(input.dataset.rest, 'true');
+
+		assert.deepEqual(logs, ['get_rest']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/dynamic-spread-and-attribute-directive/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let focused = $state(false)
+
+	function get_rest() {
+		console.log("get_rest");
+		return {
+			"data-rest": "true"
+		}
+	}
+
+</script>
+
+<input 
+	onfocus={() => focused = true} 
+	onblur={() => focused = false} 
+	class:dark={true} 
+	class={`${focused ? 'focused' : ''}`} 
+	{...get_rest()} 
+>