fix: prevent incorrect trailing slash redirect for prerendered root page when paths.base is set (#10763)

fixes #9982

Author: teemingc

.changeset/rich-pandas-provide.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prerendered root page with `paths.base` config uses correct trailing slash option

packages/kit/src/exports/vite/preview/index.js

@@ -71,7 +71,19 @@ export async function preview(vite, vite_config, svelte_config) {
 
 		vite.middlewares.use((req, res, next) => {
 			const original_url = /** @type {string} */ (req.url);
-			const { pathname } = new URL(original_url, 'http://dummy');
+			const { pathname, search } = new URL(original_url, 'http://dummy');
+
+			// if `paths.base === '/a/b/c`, then the root route is `/a/b/c/`,
+			// regardless of the `trailingSlash` route option
+			if (base.length > 1 && pathname === base) {
+				let location = base + '/';
+				if (search) location += search;
+				res.writeHead(307, {
+					location
+				});
+				res.end();
+				return;
+			}
 
 			if (pathname.startsWith(base)) {
 				next();

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

@@ -572,7 +572,12 @@ 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,
-			slash: node.universal?.trailingSlash ?? server_data_node?.slash
+			// if `paths.base === '/a/b/c`, then the root route is `/a/b/c/`,
+			// regardless of the `trailingSlash` route option
+			slash:
+				url.pathname === base || url.pathname === base + '/'
+					? 'always'
+					: node.universal?.trailingSlash ?? server_data_node?.slash
 		};
 	}