fix: ensure transitions are applied to nested elements (#14080)

.changeset/modern-adults-lay.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure transitions are applied to nested elements

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -63,6 +63,7 @@ import { SvelteWindow } from './visitors/SvelteWindow.js';
 import { TaggedTemplateExpression } from './visitors/TaggedTemplateExpression.js';
 import { Text } from './visitors/Text.js';
 import { TitleElement } from './visitors/TitleElement.js';
+import { TransitionDirective } from './visitors/TransitionDirective.js';
 import { UpdateExpression } from './visitors/UpdateExpression.js';
 import { UseDirective } from './visitors/UseDirective.js';
 import { VariableDeclarator } from './visitors/VariableDeclarator.js';
@@ -172,6 +173,7 @@ const visitors = {
 	SvelteWindow,
 	TaggedTemplateExpression,
 	Text,
+	TransitionDirective,
 	TitleElement,
 	UpdateExpression,
 	UseDirective,

packages/svelte/src/compiler/phases/2-analyze/visitors/TransitionDirective.js

@@ -0,0 +1,14 @@
+/** @import { AST } from '#compiler' */
+/** @import { Context } from '../types' */
+
+import { mark_subtree_dynamic } from './shared/fragment.js';
+
+/**
+ * @param {AST.TransitionDirective} node
+ * @param {Context} context
+ */
+export function TransitionDirective(node, context) {
+	mark_subtree_dynamic(context.path);
+
+	context.next();
+}

packages/svelte/tests/runtime-runes/samples/transition-static-subtree/_config.js

@@ -0,0 +1,23 @@
+import { flushSync } from '../../../../src/index-client.js';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, raf, target }) {
+		assert.htmlEqual(target.innerHTML, '<button>Toggle</button><div><span>123</span></div>');
+
+		const btn1 = target.querySelector('button');
+		btn1?.click();
+		flushSync();
+		raf.tick(250);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>Toggle</button><div><span style="opacity: 0.5;">123</span></div>'
+		);
+
+		flushSync();
+		raf.tick(500);
+
+		assert.htmlEqual(target.innerHTML, '<button>Toggle</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/transition-static-subtree/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	function fade(_) {
+		return {
+			duration: 500,
+			css: t => `opacity: ${t}`,
+		}
+	}
+
+	let visible = $state(true);
+</script>
+
+<button onclick={() => visible = !visible}>Toggle</button>
+
+{#if visible}
+	<div>
+		<span transition:fade>123</span>
+	</div>
+{/if}