fix: ensure nested blocks are inert during outro transitions (#10126)

* fix: ensure nested blocks are inert during outro transitions

* lint

.changeset/spotty-pens-agree.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure nested blocks are inert during outro transitions

packages/svelte/src/internal/client/runtime.js

@@ -10,6 +10,7 @@ import {
 } from '../../constants.js';
 import { readonly } from './proxy/readonly.js';
 import { READONLY_SYMBOL, STATE_SYMBOL, proxy, unstate } from './proxy/proxy.js';
+import { EACH_BLOCK, IF_BLOCK } from './block.js';
 
 export const SOURCE = 1;
 export const DERIVED = 1 << 1;
@@ -1019,6 +1020,28 @@ export function mark_subtree_inert(signal, inert) {
 		if (!inert && (flags & IS_EFFECT) !== 0 && (flags & CLEAN) === 0) {
 			schedule_effect(/** @type {import('./types.js').EffectSignal} */ (signal), false);
 		}
+		// Nested if block effects
+		const block = signal.b;
+		if (block !== null) {
+			const type = block.t;
+			if (type === IF_BLOCK) {
+				const consequent_effect = block.ce;
+				if (consequent_effect !== null) {
+					mark_subtree_inert(consequent_effect, inert);
+				}
+				const alternate_effect = block.ae;
+				if (alternate_effect !== null) {
+					mark_subtree_inert(alternate_effect, inert);
+				}
+			} else if (type === EACH_BLOCK) {
+				const items = block.v;
+				for (let { e: each_item_effect } of items) {
+					if (each_item_effect !== null) {
+						mark_subtree_inert(each_item_effect, inert);
+					}
+				}
+			}
+		}
 	}
 	const references = signal.r;
 	if (references !== null) {

packages/svelte/tests/runtime-runes/samples/if-transition-inert/_config.js

@@ -0,0 +1,16 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>hide</button><div>hello</div>`,
+
+	async test({ assert, target }) {
+		const [btn1, btn2] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn1.click();
+		});
+
+		assert.htmlEqual(target.innerHTML, `<button>hide</button><div style="opacity: 0;">hello</div>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/if-transition-inert/main.svelte

@@ -0,0 +1,17 @@
+<script>
+  import { fade } from "svelte/transition";
+
+  let state = $state("hello");
+</script>
+
+<button onclick={() => state = ''}>hide</button>
+
+{#if state}
+  <div in:fade={{ duration: 2000 }} out:fade={{ duration: 2000 }}>
+    {#if true}
+      {state}
+    {/if}
+  </div>
+{/if}
+
+