hacky fix

packages/svelte/src/internal/client/dom/elements/bindings/this.js

@@ -1,5 +1,7 @@
 import { STATE_SYMBOL } from '#client/constants';
+import { capture_signals } from '../../../legacy.js';
 import { effect, render_effect } from '../../../reactivity/effects.js';
+import { old_values } from '../../../reactivity/sources.js';
 import { untrack } from '../../../runtime.js';
 import { queue_micro_task } from '../../task.js';
 
@@ -48,12 +50,16 @@ export function bind_this(element_or_component = {}, update, get_value, get_part
 		});
 
 		return () => {
-			// We cannot use effects in the teardown phase, we we use a microtask instead.
-			queue_micro_task(() => {
-				if (parts && is_bound_this(get_value(...parts), element_or_component)) {
-					update(null, ...parts);
-				}
-			});
+			const signals = capture_signals(get_value);
+
+			if (parts && is_bound_this(get_value(...parts), element_or_component)) {
+				update(null, ...parts);
+			}
+
+			if (signals && signals.size === 1) {
+				const [signal] = [...signals];
+				old_values.set(signal, element_or_component);
+			}
 		};
 	});
 

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

@@ -13,7 +13,7 @@ export let captured_signals = null;
  * @template T
  * @param {() => T} fn
  */
-function capture_signals(fn) {
+export function capture_signals(fn) {
 	var previous_captured_signals = captured_signals;
 
 	try {

packages/svelte/tests/runtime-runes/samples/if-block-const-destroyed-external-reader/Inner.svelte

@@ -0,0 +1,9 @@
+<script>
+	let { data } = $props();
+
+	const processed = $derived(data.toUpperCase());
+
+	export function getProcessed() {
+		return processed;
+	}
+</script>

packages/svelte/tests/runtime-runes/samples/if-block-const-destroyed-external-reader/_config.js

@@ -0,0 +1,15 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	ssrHtml: '<button>clear</button><p></p>',
+	html: '<button>clear</button><p>HELLO</p>',
+
+	async test({ assert, target }) {
+		const [button] = target.querySelectorAll('button');
+
+		flushSync(() => button.click());
+
+		assert.htmlEqual(target.innerHTML, '<button>clear</button><p></p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/if-block-const-destroyed-external-reader/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import Inner from './Inner.svelte';
+
+	let value = $state('hello');
+
+	let innerComp = $state();
+
+	// Reads Inner's derived value from outside the {#if} block, keeping it
+	// connected in the reactive graph even after the branch is destroyed.
+	const externalView = $derived(innerComp?.getProcessed() ?? '');
+</script>
+
+{#if value}
+	{@const result = value}
+	<Inner data={result} bind:this={innerComp} />
+{/if}
+
+<button onclick={() => (value = undefined)}>clear</button>
+<p>{externalView}</p>

packages/svelte/tests/runtime-runes/samples/if-block-const-inert-derived/_config.js

@@ -0,0 +1,18 @@
+import { flushSync, tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: '<button>clear</button><div><p>hello</p></div>',
+
+	async test({ assert, target, raf, logs }) {
+		const [button] = target.querySelectorAll('button');
+
+		flushSync(() => button.click());
+		assert.deepEqual(logs, ['hello']);
+
+		// Let the transition finish and clean up
+		raf.tick(100);
+
+		assert.htmlEqual(target.innerHTML, '<button>clear</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/if-block-const-inert-derived/main.svelte

@@ -0,0 +1,32 @@
+<script>
+	let value = $state('hello');
+
+	function compute(v) {
+		console.log(v);
+		return { data: v, ready: true };
+	}
+
+	function fade(node) {
+		return {
+			duration: 100,
+			tick: (t) => {
+				node.style.opacity = String(t);
+			}
+		};
+	}
+</script>
+
+<button
+	onclick={() => {
+		value = undefined;
+	}}>clear</button
+>
+
+{#if value}
+	{@const result = compute(value)}
+	{#if result.ready}
+		<div out:fade|global>
+			<p>{result.data}</p>
+		</div>
+	{/if}
+{/if}