fix: run boundary `onerror` callbacks in a microtask, in case they result in the boundary's destruction (#17561)

* fix: mark parent effect as dirty on error boundary initialization

This change ensures that if an error occurs while a parent effect is still initializing, the parent effect is marked as dirty and scheduled to re-run. This addresses scenarios where state updates in the onerror handler do not trigger a re-evaluation of the condition, improving error handling in Svelte components.

* chore: add changeset for #17553

* fix: run boundary  callbacks in a microtask, in case they result in the boundary's destruction

---------

Co-authored-by: frozenflux2 <snowflake30518@gmail.com>

.changeset/fix-if-block-boundary-onerror.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: run boundary `onerror` callbacks in a microtask, in case they result in the boundary's destruction

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

@@ -1,5 +1,6 @@
 /** @import { Effect, Source, TemplateNode, } from '#client' */
 import {
+	BLOCK_EFFECT,
 	BOUNDARY_EFFECT,
 	COMMENT_NODE,
 	DIRTY,
@@ -449,21 +450,16 @@ export class Boundary {
 			}
 		};
 
-		var previous_reaction = active_reaction;
+		queue_micro_task(() => {
-
 			try {
-			set_active_reaction(null);
 				calling_on_error = true;
 				onerror?.(error, reset);
 				calling_on_error = false;
 			} catch (error) {
 				invoke_error_boundary(error, this.#effect && this.#effect.parent);
-		} finally {
-			set_active_reaction(previous_reaction);
 			}
 
 			if (failed) {
-			queue_micro_task(() => {
 				this.#failed_effect = this.#run(() => {
 					Batch.ensure();
 					this.#is_creating_fallback = true;
@@ -483,8 +479,8 @@ export class Boundary {
 						this.#is_creating_fallback = false;
 					}
 				});
-			});
 			}
+		});
 	}
 }
 

packages/svelte/tests/runtime-runes/samples/error-boundary-24/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	throw new Error('child error');
+</script>
+
+<p>Child content</p>

packages/svelte/tests/runtime-runes/samples/error-boundary-24/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	test({ assert, target }) {
+		flushSync();
+
+		// When exception is set by onerror, the {#if !exception} block should hide
+		// and only the {#if exception} block should be visible
+		assert.htmlEqual(target.innerHTML, '<p>caught error: child error</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-24/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import Child from './Child.svelte';
+
+	let exception = $state();
+	const onerror = (e) => {
+		exception = e;
+	};
+</script>
+
+{#if !exception}
+	<p>condition is {String(!exception)}</p>
+	<svelte:boundary {onerror}>
+		<Child />
+	</svelte:boundary>
+{/if}
+
+{#if exception}
+	<p>caught error: {exception.message}</p>
+{/if}

packages/svelte/tests/runtime-runes/samples/error-boundary-25/Child.svelte

@@ -0,0 +1,8 @@
+<script>
+	// it's important for the test that this isn't an `Error`
+	throw 'child error'
+</script>
+
+<p>
+	boom
+</p>

packages/svelte/tests/runtime-runes/samples/error-boundary-25/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	test({ assert, target }) {
+		flushSync();
+
+		// When exception is set by onerror, the {#if !exception} block should hide
+		// and only the {#if exception} block should be visible
+		assert.htmlEqual(target.innerHTML, '<p>caught error: child error</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-25/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import Child from './Child.svelte';
+
+	let exception = $state();
+	const onerror = (e) => {
+		exception = e;
+	};
+</script>
+
+{#if !exception}
+	<p>condition is {String(!exception)}</p>
+	<svelte:boundary {onerror}>
+		<Child />
+	</svelte:boundary>
+{/if}
+
+{#if exception}
+	<p>caught error: {exception}</p>
+{/if}