fix: Don't destroy boundary contents on error unless the boundary is an error boundary (#16746)

* fix: Don't destroy contents of boundaries on errors if they're not error boundaries

* changeset

* test

* prettier

* prettier

* simplify test

* oops

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/tasty-trainers-sell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't destroy contents of `svelte:boundary` unless the boundary is an error boundary

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -285,6 +285,12 @@ export class Boundary {
 		var onerror = this.#props.onerror;
 		let failed = this.#props.failed;
 
+		// If we have nothing to capture the error, or if we hit an error while
+		// rendering the fallback, re-throw for another boundary to handle
+		if (this.#is_creating_fallback || (!onerror && !failed)) {
+			throw error;
+		}
+
 		if (this.#main_effect) {
 			destroy_effect(this.#main_effect);
 			this.#main_effect = null;
@@ -346,12 +352,6 @@ export class Boundary {
 			}
 		};
 
-		// If we have nothing to capture the error, or if we hit an error while
-		// rendering the fallback, re-throw for another boundary to handle
-		if (this.#is_creating_fallback || (!onerror && !failed)) {
-			throw error;
-		}
-
 		var previous_reaction = active_reaction;
 
 		try {

packages/svelte/tests/runtime-runes/samples/non-error-boundary-preserve-on-error/_config.js

@@ -0,0 +1,20 @@
+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	test({ assert, target }) {
+		const [button] = target.querySelectorAll('button');
+
+		assert.throws(() => {
+			flushSync(() => button.click());
+		}, /oops/);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>throw</button>
+				<p>some content</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/non-error-boundary-preserve-on-error/main.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+	let should_throw = $state(false);
+
+	function throw_error() {
+		throw new Error('oops');
+	}
+</script>
+
+<button onclick={() => should_throw = true}>
+	throw
+</button>
+
+<svelte:boundary>
+	<p>some content</p>
+
+	{#if should_throw}
+		{throw_error()}
+	{/if}
+</svelte:boundary>