fix: respect trailing slash option when navigating from the root page of the basepath (#11388)

* add failing test

* only resolve root page trailing slash during navigation

* changeset

* fix test

---------

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

Author: teemingc

.changeset/five-garlics-greet.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: respect the trailing slash option when navigating from the basepath root page

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

@@ -441,10 +441,19 @@ async function get_navigation_result_from_branch({
 }) {
 	/** @type {import('types').TrailingSlash} */
 	let slash = 'never';
-	for (const node of branch) {
-		if (node?.slash !== undefined) slash = node.slash;
+
+	// 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
+	if (base && (url.pathname === base || url.pathname === base + '/')) {
+		slash = 'always';
+	} else {
+		for (const node of branch) {
+			if (node?.slash !== undefined) slash = node.slash;
+		}
 	}
+
 	url.pathname = normalize_path(url.pathname, slash);
+
 	// eslint-disable-next-line
 	url.search = url.search; // turn `/?` into `/`
 
@@ -694,12 +703,7 @@ async function load_node({ loader, parent, url, params, route, server_data_node
 		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 always `/a/b/c/`, regardless of
-		// the `trailingSlash` route option, so that relative paths to JS and CSS work
-		slash:
-			base && (url.pathname === base || url.pathname === base + '/')
-				? 'always'
-				: node.universal?.trailingSlash ?? server_data_node?.slash
+		slash: node.universal?.trailingSlash ?? server_data_node?.slash
 	};
 }
 

packages/kit/test/apps/options-2/src/routes/+page.svelte

@@ -6,3 +6,5 @@
 
 <p data-testid="base">base: {base}</p>
 <p data-testid="assets">assets: {assets}</p>
+
+<a href="{base}/hello" data-testid="link">Go to /hello</a>

packages/kit/test/apps/options-2/test/test.js

@@ -37,6 +37,21 @@ test.describe('paths', () => {
 		expect(await page.textContent('[data-testid="base"]')).toBe(`base: ${base}`);
 		expect(await page.textContent('[data-testid="assets"]')).toBe(`assets: ${base}`);
 	});
+
+	test('serves /basepath with trailing slash always', async ({ page }) => {
+		await page.goto('/basepath');
+		expect(new URL(page.url()).pathname).toBe('/basepath/');
+	});
+
+	test('respects trailing slash option when navigating from /basepath', async ({
+		page,
+		clicknav
+	}) => {
+		await page.goto('/basepath');
+		expect(new URL(page.url()).pathname).toBe('/basepath/');
+		await clicknav('[data-testid="link"]');
+		expect(new URL(page.url()).pathname).toBe('/basepath/hello');
+	});
 });
 
 test.describe('Service worker', () => {