fix(adapter-vercel): make normalization redirects root-relative

.changeset/tidy-vercel-isr-pathnames.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+fix: make Vercel ISR normalization redirects root-relative

packages/adapter-vercel/files/serverless.js

@@ -33,10 +33,27 @@ export default {
 			request = new Request(url, request);
 		}
 
-		return server.respond(request, {
-			getClientAddress() {
-				return /** @type {string} */ (request.headers.get('x-forwarded-for'));
-			}
-		});
+		return server
+			.respond(request, {
+				getClientAddress() {
+					return /** @type {string} */ (request.headers.get('x-forwarded-for'));
+				}
+			})
+			.then((response) => {
+				const location = response.headers.get('location');
+
+				if (
+					response.headers.has('x-sveltekit-normalize') &&
+					location &&
+					!location.startsWith('/') &&
+					pathname !== null
+				) {
+					const headers = new Headers(response.headers);
+					headers.set('location', new URL(`/${location}`, request.url).href);
+					return new Response(response.body, { status: response.status, headers });
+				}
+
+				return response;
+			});
 	}
 };

packages/adapter-vercel/test/apps/basic/test/test.ts

@@ -62,9 +62,9 @@ test('ISR page with trailingSlash always loads without errors', async ({ page })
 		}
 	});
 
-	await page.goto('/isr-trailing-slash/');
+	await page.goto('/isr-trailing-slash-absolute/');
 
-	expect(new URL(page.url()).pathname).toBe('/isr-trailing-slash/');
+	expect(new URL(page.url()).pathname).toBe('/isr-trailing-slash-absolute/');
 	await expect(page.locator('h1')).toContainText('ISR Trailing Slash Page');
 	expect(errors).toEqual([]);
 });