fix: ensure undefined class still applies scoping class, if necessary

fixes #15635

Author: dummdidumm

.changeset/weak-knives-battle.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure `undefined` class still applies scoping class, if necessary

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

@@ -14,7 +14,11 @@ export function set_class(dom, is_html, value, hash, prev_classes, next_classes)
 	// @ts-expect-error need to add __className to patched prototype
 	var prev = dom.__className;
 
-	if (hydrating || prev !== value) {
+	if (
+		hydrating ||
+		prev !== value ||
+		prev === undefined // for edge case of `class={undefined}`
+	) {
 		var next_class_name = to_class(value, hash, next_classes);
 
 		if (!hydrating || next_class_name !== dom.getAttribute('class')) {

packages/svelte/tests/css/samples/undefined-with-scope/expected.html

@@ -1 +1,2 @@
-<p class="svelte-xyz">Foo</p>
+<p class="svelte-xyz">Foo</p>
+<p class="svelte-xyz">Bar</p>

packages/svelte/tests/css/samples/undefined-with-scope/input.svelte

@@ -1,3 +1,4 @@
 <style>p { color: red; }</style>
 
-<p class={undefined}>Foo</p>
+<p class={undefined}>Foo</p>
+<p class="{undefined}">Bar</p>