fix: handle deferred transitions aborted before initialization (#18776)

- make deferred transition facade methods safe before the inner
animation initializes
- report the transition's initial position from the facade until
initialization
- add a runtime regression test for an out-only `crossfade` branch
restored in the same update

Fixes https://github.com/sveltejs/svelte/issues/18773

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/tidy-ducks-reset.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle deferred transitions aborted before initialization

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

@@ -359,9 +359,9 @@ function animate(element, options, counterpart, t2, on_begin, on_finish) {
 				aborted = true;
 				a?.abort();
 			},
-			deactivate: () => a.deactivate(),
-			reset: () => a.reset(),
-			t: () => a.t()
+			deactivate: () => a?.deactivate(),
+			reset: () => a?.reset(),
+			t: () => a?.t() ?? 1 - t2
 		};
 	}
 

packages/svelte/tests/runtime-runes/samples/deferred-outro-restored/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [button] = target.querySelectorAll('button');
+
+		button.click();
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, '<button>Reproduce</button><p>Deferred outro</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/deferred-outro-restored/main.svelte

@@ -0,0 +1,24 @@
+<script>
+	import { crossfade } from 'svelte/transition';
+
+	let visible = $state(true);
+	let restore = $state(false);
+	const [send] = crossfade({});
+
+	$effect(() => {
+		if (restore) visible = true;
+	});
+</script>
+
+<button
+	onclick={() => {
+		visible = false;
+		restore = true;
+	}}
+>
+	Reproduce
+</button>
+
+{#if visible}
+	<p out:send={{ key: 'item' }}>Deferred outro</p>
+{/if}