always add trailing slash to base path

.changeset/great-stingrays-flash.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: use correct relative paths when rendering base path

packages/kit/src/runtime/server/page/render.js

@@ -95,23 +95,12 @@ export async function render_response({
 
 	// if appropriate, use relative paths for greater portability
 	if (paths.relative !== false && !state.prerendering?.fallback) {
-		const segments = event.url.pathname.slice(paths.base.length).split('/');
+		const segments = event.url.pathname.slice(paths.base.length).split('/').slice(2);
 
-		if (segments.length === 1 && paths.base !== '') {
-			// if we're on `/my-base-path`, relative links need to start `./my-base-path` rather than `.`
-			base = `./${paths.base.split('/').at(-1)}`;
+		base = segments.map(() => '..').join('/') || '.';
 
-			base_expression = `new URL(${s(base)}, location).pathname`;
-		} else {
-			base =
-				segments
-					.slice(2)
-					.map(() => '..')
-					.join('/') || '.';
-
-			// resolve e.g. '../..' against current location, then remove trailing slash
-			base_expression = `new URL(${s(base)}, location).pathname.slice(0, -1)`;
-		}
+		// resolve e.g. '../..' against current location, then remove trailing slash
+		base_expression = `new URL(${s(base)}, location).pathname.slice(0, -1)`;
 
 		if (!paths.assets || (paths.assets[0] === '/' && paths.assets !== SVELTE_KIT_ASSETS)) {
 			assets = base;

packages/kit/src/runtime/server/respond.js

@@ -172,7 +172,11 @@ export async function respond(request, options, manifest, state) {
 	try {
 		// determine whether we need to redirect to add/remove a trailing slash
 		if (route && !is_data_request) {
-			if (route.page) {
+			// if `paths.base === '/a/b/c`, then the root route is `/a/b/c/`,
+			// regardless of the `trailingSlash` route option
+			if (url.pathname === base || url.pathname === base + '/') {
+				trailing_slash = 'always';
+			} else if (route.page) {
 				const nodes = await Promise.all([
 					// we use == here rather than === because [undefined] serializes as "[null]"
 					...route.page.layouts.map((n) => (n == undefined ? n : manifest._.nodes[n]())),

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

@@ -27,7 +27,7 @@ test.describe('paths', () => {
 	test('uses relative paths during SSR', async ({ page, javaScriptEnabled }) => {
 		await page.goto('/basepath');
 
-		let base = javaScriptEnabled ? '/basepath' : './basepath';
+		let base = javaScriptEnabled ? '/basepath' : '.';
 		expect(await page.textContent('[data-testid="base"]')).toBe(`base: ${base}`);
 		expect(await page.textContent('[data-testid="assets"]')).toBe(`assets: ${base}`);