fix: preserve trailing slash when error boundary truncates branch

When a page's load function throws an error and the branch is truncated
to the nearest error boundary, the page-level trailingSlash config is
lost. This causes the URL to change (e.g. /test/ → /test).

Default to 'ignore' instead of 'never' when an error is present, so
the URL pathname stays unchanged. If a layout above the error boundary
has its own trailingSlash config, it still overrides the default.

Fixes #13516

Author: FrankFMY

packages/kit/src/runtime/client/client.js

@@ -633,7 +633,7 @@ async function initialize(result, target, hydrate) {
  */
 function get_navigation_result_from_branch({ url, params, branch, status, error, route, form }) {
 	/** @type {import('types').TrailingSlash} */
-	let slash = 'never';
+	let slash = error ? 'ignore' : 'never';
 
 	// if `paths.base === '/a/b/c`, then the root route is always `/a/b/c/`, regardless of
 	// the `trailingSlash` route option, so that relative paths to JS and CSS work

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/error/+error.svelte

@@ -0,0 +1,5 @@
+<script>
+	import { page } from '$app/state';
+</script>
+
+<h1>Error: {page.error?.message}</h1>

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/error/always-error/+page.js

@@ -0,0 +1,7 @@
+import { error } from '@sveltejs/kit';
+
+export const trailingSlash = 'always';
+
+export function load() {
+	error(500, 'trailing slash error test');
+}

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/error/always-error/+page.svelte

@@ -1132,6 +1132,15 @@ test.describe('Routing', () => {
 		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/never');
 		await expect(page.locator('p')).toHaveText('/routing/trailing-slash/never');
 	});
+
+	test('trailing slash is preserved when error boundary truncates branch', async ({
+		page,
+		app
+	}) => {
+		await page.goto('/routing/trailing-slash');
+		await app.goto('/routing/trailing-slash/error/always-error/');
+		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/error/always-error/');
+	});
 });
 
 test.describe('Shadow DOM', () => {