fix: preserve trailing slash when error boundary truncates branch

.changeset/fix-trailing-slash-error-boundary.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: preserve trailing slash when error boundary truncates branch

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/test/cross-platform/client.test.js

@@ -9,7 +9,7 @@
 test.describe.configure({ mode: 'parallel' });
 
 test.describe('a11y', () => {
 	test('resets focus', async ({ page, clicknav, browserName }) => {
 		const tab = browserName === 'webkit' ? 'Alt+Tab' : 'Tab';
 
 		await page.goto('/accessibility/a');
@@ -32,7 +32,7 @@
 		expect(await page.evaluate(() => document.documentElement.getAttribute('tabindex'))).toBe(null);
 	});
 
 	test('applies autofocus after a navigation', async ({ page, clicknav }) => {
 		await page.goto('/accessibility/autofocus/a');
 
 		await clicknav('[href="/accessibility/autofocus/b"]', {
@@ -95,7 +95,7 @@
 		).toBe(0);
 	});
 
 	test('keepfocus works', async ({ page }) => {
 		await page.goto('/keepfocus');
 
 		await Promise.all([
@@ -1069,7 +1069,7 @@
 		expect(requests.filter((r) => !r.includes('__route.js'))).toEqual([]);
 	});
 
 	test('responds to <form target="_blank"> submission with new tab', async ({ page }) => {
 		await page.goto('/routing/form-target-blank');
 
 		let tabs = page.context().pages();
@@ -1083,7 +1083,7 @@
 		expect(tabs.length > 1);
 	});
 
 	test('responds to <button formtarget="_blank" submission with new tab', async ({ page }) => {
 		await page.goto('/routing/form-target-blank');
 
 		let tabs = page.context().pages();
@@ -1132,6 +1132,15 @@
 		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', () => {