fix: correctly trailing slash redirect when navigating from root page

.changeset/lovely-plums-move.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: correctly handle trailing slash redirect when navigating from the root page

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

@@ -660,10 +660,10 @@ export function create_client(app, target) {
 			server: server_data_node,
 			universal: node.universal?.load ? { type: 'data', data, uses } : null,
 			data: data ?? server_data_node?.data ?? null,
-			// if `paths.base === '/a/b/c`, then the root route is `/a/b/c/`,
-			// regardless of the `trailingSlash` route option
+			// 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
 			slash:
-				url.pathname === base || url.pathname === base + '/'
+				base && (url.pathname === base || url.pathname === base + '/')
 					? 'always'
 					: node.universal?.trailingSlash ?? server_data_node?.slash
 		};

packages/kit/test/apps/basics/src/routes/+page.svelte

@@ -6,3 +6,7 @@
 </script>
 
 <h1>the answer is {data.answer}</h1>
+
+<a href="/routing/trailing-slash/never/"
+	>URL with trailing slash that should redirect to remove it</a
+>

packages/kit/test/apps/basics/test/cross-platform/client.test.js

@@ -742,6 +742,16 @@ test.describe('Routing', () => {
 		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/ignore/');
 		await expect(page.locator('p')).toHaveText('/routing/trailing-slash/ignore/');
 	});
+
+	test('trailing slash redirect works when navigating from root page', async ({
+		page,
+		clicknav
+	}) => {
+		await page.goto('/');
+		await clicknav('a[href="/routing/trailing-slash/never/"]');
+		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/never');
+		await expect(page.locator('p')).toHaveText('/routing/trailing-slash/never');
+	});
 });
 
 test.describe('Shadow DOM', () => {