Don't lose `class:` directive classes on an element with `{...spread}` attributes when updating (#3781)

* include all class: directive updates on elements with spreads (#3421)

* add test

* update changelog

CHANGELOG.md

@@ -5,6 +5,7 @@
 * Fix `{#each}` context not shadowing outer scope when using `bind:` ([#1565](https://github.com/sveltejs/svelte/issues/1565))
 * Fix edge cases in matching selectors against elements ([#1710](https://github.com/sveltejs/svelte/issues/1710))
 * Allow exiting a reactive block early with `break $` ([#2828](https://github.com/sveltejs/svelte/issues/2828))
+* Don't lose `class:` directive classes on an element with `{...spread}` attributes when updating ([#3421](https://github.com/sveltejs/svelte/issues/3421))
 * Check attributes have changed before setting them to avoid image flicker ([#3579](https://github.com/sveltejs/svelte/pull/3579))
 * Fix generating malformed code for `{@debug}` tags with no dependencies ([#3588](https://github.com/sveltejs/svelte/issue/3588))
 * Fix generated code in specific case involving compound ifs and child components ([#3595](https://github.com/sveltejs/svelte/issue/3595))

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -809,6 +809,7 @@ export default class ElementWrapper extends Wrapper {
 	}
 
 	add_classes(block: Block) {
+		const has_spread = this.node.attributes.some(attr => attr.is_spread);
 		this.node.classes.forEach(class_directive => {
 			const { expression, name } = class_directive;
 			let snippet;
@@ -824,7 +825,9 @@ export default class ElementWrapper extends Wrapper {
 
 			block.chunks.hydrate.push(updater);
 
-			if ((dependencies && dependencies.size > 0) || this.class_dependencies.length) {
+			if (has_spread) {
+				block.chunks.update.push(updater);
+			} else if ((dependencies && dependencies.size > 0) || this.class_dependencies.length) {
 				const all_dependencies = this.class_dependencies.concat(...dependencies);
 				const condition = changed(all_dependencies);
 

test/runtime/samples/spread-element-class/_config.js

@@ -0,0 +1,7 @@
+export default {
+	html: `<div class='foo bar'>hello</div>`,
+	test({ assert, component, target }) {
+		component.blah = 'goodbye';
+		assert.htmlEqual(target.innerHTML, `<div class='foo bar'>goodbye</div>`);
+	}
+};

test/runtime/samples/spread-element-class/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let blah = 'hello';
+</script>
+
+<div class='foo' class:bar={true} {...{}}>{blah}</div>