Fix language switcher `trailingSlash` edge case (#2616)

8 files changed, 83 insertions, 33 deletions

diff --git a/.changeset/thick-cooks-call.md b/.changeset/thick-cooks-call.md
new file mode 100644
index 00000000..6b3e49ca
--- /dev/null
+++ b/.changeset/thick-cooks-call.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes an edge case to correctly avoid a trailing slash when navigating from a root locale homepage to another language via Starlight’s language switcher when `trailingSlash: 'never'` is set
diff --git a/packages/starlight/__tests__/basics/localizedUrl.test.ts b/packages/starlight/__tests__/basics/localizedUrl.test.ts
index f65529cb..480699cd 100644
--- a/packages/starlight/__tests__/basics/localizedUrl.test.ts
+++ b/packages/starlight/__tests__/basics/localizedUrl.test.ts
@@ -4,23 +4,23 @@ import { localizedUrl } from '../../utils/localizedUrl';
 describe('with `build.output: "directory"`', () => {
 	test('it has no effect in a monolingual project', () => {
 		const url = new URL('https://example.com/en/guide/');
-		expect(localizedUrl(url, undefined).href).toBe(url.href);
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe(url.href);
 	});
 
 	test('has no effect on index route in a monolingual project', () => {
 		const url = new URL('https://example.com/');
-		expect(localizedUrl(url, undefined).href).toBe(url.href);
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe(url.href);
 	});
 });
 
 describe('with `build.output: "file"`', () => {
 	test('it has no effect in a monolingual project', () => {
 		const url = new URL('https://example.com/en/guide.html');
-		expect(localizedUrl(url, undefined).href).toBe(url.href);
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe(url.href);
 	});
 
 	test('has no effect on index route in a monolingual project', () => {
 		const url = new URL('https://example.com/index.html');
-		expect(localizedUrl(url, undefined).href).toBe(url.href);
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe(url.href);
 	});
 });
diff --git a/packages/starlight/__tests__/i18n-non-root-single-locale/localizedUrl.test.ts b/packages/starlight/__tests__/i18n-non-root-single-locale/localizedUrl.test.ts
index 49ef69aa..3ebf6de5 100644
--- a/packages/starlight/__tests__/i18n-non-root-single-locale/localizedUrl.test.ts
+++ b/packages/starlight/__tests__/i18n-non-root-single-locale/localizedUrl.test.ts
@@ -4,23 +4,23 @@ import { localizedUrl } from '../../utils/localizedUrl';
 describe('with `build.output: "directory"`', () => {
 	test('it has no effect in a monolingual project with a non-root single locale', () => {
 		const url = new URL('https://example.com/fr/guide/');
-		expect(localizedUrl(url, 'fr').href).toBe(url.href);
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe(url.href);
 	});
 
 	test('has no effect on index route in a monolingual project with a non-root single locale', () => {
 		const url = new URL('https://example.com/fr/');
-		expect(localizedUrl(url, 'fr').href).toBe(url.href);
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe(url.href);
 	});
 });
 
 describe('with `build.output: "file"`', () => {
 	test('it has no effect in a monolingual project with a non-root single locale', () => {
 		const url = new URL('https://example.com/fr/guide.html');
-		expect(localizedUrl(url, 'fr').href).toBe(url.href);
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe(url.href);
 	});
 
 	test('has no effect on index route in a monolingual project with a non-root single locale', () => {
 		const url = new URL('https://example.com/fr.html');
-		expect(localizedUrl(url, 'fr').href).toBe(url.href);
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe(url.href);
 	});
 });
diff --git a/packages/starlight/__tests__/i18n-root-locale/localizedUrl.test.ts b/packages/starlight/__tests__/i18n-root-locale/localizedUrl.test.ts
index 51ed335d..326a6c3d 100644
--- a/packages/starlight/__tests__/i18n-root-locale/localizedUrl.test.ts
+++ b/packages/starlight/__tests__/i18n-root-locale/localizedUrl.test.ts
@@ -4,73 +4,110 @@ import { localizedUrl } from '../../utils/localizedUrl';
 describe('with `build.output: "directory"`', () => {
 	test('it has no effect if locale matches', () => {
 		const url = new URL('https://example.com/en/guide/');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it has no effect if locale matches for index', () => {
 		const url = new URL('https://example.com/en/');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it changes locale to requested locale', () => {
 		const url = new URL('https://example.com/en/guide/');
-		expect(localizedUrl(url, 'ar').href).toBe('https://example.com/ar/guide/');
+		expect(localizedUrl(url, 'ar', 'ignore').href).toBe('https://example.com/ar/guide/');
 	});
 
 	test('it changes locale to requested locale for index', () => {
 		const url = new URL('https://example.com/en/');
-		expect(localizedUrl(url, 'ar').href).toBe('https://example.com/ar/');
+		expect(localizedUrl(url, 'ar', 'ignore').href).toBe('https://example.com/ar/');
 	});
 
 	test('it can change to root locale', () => {
 		const url = new URL('https://example.com/en/guide/');
-		expect(localizedUrl(url, undefined).href).toBe('https://example.com/guide/');
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe('https://example.com/guide/');
 	});
 
 	test('it can change from root locale', () => {
 		const url = new URL('https://example.com/guide/');
-		expect(localizedUrl(url, 'en').href).toBe('https://example.com/en/guide/');
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe('https://example.com/en/guide/');
+	});
+});
+
+describe('with `trailingSlash: "never"`', () => {
+	test('it has no effect if locale matches', () => {
+		const url = new URL('https://example.com/en/guide');
+		expect(localizedUrl(url, 'en', 'never').href).toBe(url.href);
+	});
+
+	test('it has no effect if locale matches for index', () => {
+		const url = new URL('https://example.com/en');
+		expect(localizedUrl(url, 'en', 'never').href).toBe(url.href);
+	});
+
+	test('it changes locale to requested locale', () => {
+		const url = new URL('https://example.com/en/guide');
+		expect(localizedUrl(url, 'ar', 'never').href).toBe('https://example.com/ar/guide');
+	});
+
+	test('it changes locale to requested locale for index', () => {
+		const url = new URL('https://example.com/en');
+		expect(localizedUrl(url, 'ar', 'never').href).toBe('https://example.com/ar');
+	});
+
+	test('it can change to root locale', () => {
+		const url = new URL('https://example.com/en/guide');
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe('https://example.com/guide');
+	});
+
+	test('it can change from root locale', () => {
+		const url = new URL('https://example.com/guide');
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe('https://example.com/en/guide');
+	});
+
+	test('it can change from root index', () => {
+		const url = new URL('https://example.com');
+		expect(localizedUrl(url, 'en', 'never').href).toBe('https://example.com/en');
 	});
 });
 
 describe('with `build.output: "file"`', () => {
 	test('it has no effect if locale matches', () => {
 		const url = new URL('https://example.com/en/guide.html');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it has no effect if locale matches for index', () => {
 		const url = new URL('https://example.com/en.html');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it changes locale to requested locale', () => {
 		const url = new URL('https://example.com/en/guide.html');
-		expect(localizedUrl(url, 'ar').href).toBe('https://example.com/ar/guide.html');
+		expect(localizedUrl(url, 'ar', 'ignore').href).toBe('https://example.com/ar/guide.html');
 	});
 
 	test('it changes locale to requested locale for index', () => {
 		const url = new URL('https://example.com/en.html');
-		expect(localizedUrl(url, 'ar').href).toBe('https://example.com/ar.html');
+		expect(localizedUrl(url, 'ar', 'ignore').href).toBe('https://example.com/ar.html');
 	});
 
 	test('it can change to root locale', () => {
 		const url = new URL('https://example.com/en/guide.html');
-		expect(localizedUrl(url, undefined).href).toBe('https://example.com/guide.html');
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe('https://example.com/guide.html');
 	});
 
 	test('it can change to root locale from index', () => {
 		const url = new URL('https://example.com/en.html');
-		expect(localizedUrl(url, undefined).href).toBe('https://example.com/index.html');
+		expect(localizedUrl(url, undefined, 'ignore').href).toBe('https://example.com/index.html');
 	});
 
 	test('it can change from root locale', () => {
 		const url = new URL('https://example.com/guide.html');
-		expect(localizedUrl(url, 'en').href).toBe('https://example.com/en/guide.html');
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe('https://example.com/en/guide.html');
 	});
 
 	test('it can change from root locale from index', () => {
 		const url = new URL('https://example.com/index.html');
-		expect(localizedUrl(url, 'en').href).toBe('https://example.com/en.html');
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe('https://example.com/en.html');
 	});
 });
diff --git a/packages/starlight/__tests__/i18n/localizedUrl.test.ts b/packages/starlight/__tests__/i18n/localizedUrl.test.ts
index fb4ec2d7..b138af37 100644
--- a/packages/starlight/__tests__/i18n/localizedUrl.test.ts
+++ b/packages/starlight/__tests__/i18n/localizedUrl.test.ts
@@ -4,43 +4,43 @@ import { localizedUrl } from '../../utils/localizedUrl';
 describe('with `build.output: "directory"`', () => {
 	test('it has no effect if locale matches', () => {
 		const url = new URL('https://example.com/en/guide/');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it has no effect if locale matches for index', () => {
 		const url = new URL('https://example.com/en/');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it changes locale to requested locale', () => {
 		const url = new URL('https://example.com/en/guide/');
-		expect(localizedUrl(url, 'fr').href).toBe('https://example.com/fr/guide/');
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe('https://example.com/fr/guide/');
 	});
 
 	test('it changes locale to requested locale for index', () => {
 		const url = new URL('https://example.com/en/');
-		expect(localizedUrl(url, 'fr').href).toBe('https://example.com/fr/');
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe('https://example.com/fr/');
 	});
 });
 
 describe('with `build.output: "file"`', () => {
 	test('it has no effect if locale matches', () => {
 		const url = new URL('https://example.com/en/guide.html');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it has no effect if locale matches for index', () => {
 		const url = new URL('https://example.com/en.html');
-		expect(localizedUrl(url, 'en').href).toBe(url.href);
+		expect(localizedUrl(url, 'en', 'ignore').href).toBe(url.href);
 	});
 
 	test('it changes locale to requested locale', () => {
 		const url = new URL('https://example.com/en/guide.html');
-		expect(localizedUrl(url, 'fr').href).toBe('https://example.com/fr/guide.html');
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe('https://example.com/fr/guide.html');
 	});
 
 	test('it changes locale to requested locale for index', () => {
 		const url = new URL('https://example.com/en.html');
-		expect(localizedUrl(url, 'fr').href).toBe('https://example.com/fr.html');
+		expect(localizedUrl(url, 'fr', 'ignore').href).toBe('https://example.com/fr.html');
 	});
 });
diff --git a/packages/starlight/components/Head.astro b/packages/starlight/components/Head.astro
index d0fe067d..3a66e7c5 100644
--- a/packages/starlight/components/Head.astro
+++ b/packages/starlight/components/Head.astro
@@ -1,5 +1,6 @@
 ---
 import type { z } from 'astro/zod';
+import context from 'virtual:starlight/project-context';
 import config from 'virtual:starlight/user-config';
 import { version } from '../package.json';
 import type { HeadConfigSchema } from '../schemas/head';
@@ -66,7 +67,7 @@ if (canonical && config.isMultilingual) {
 			attrs: {
 				rel: 'alternate',
 				hreflang: localeOpts.lang,
-				href: localizedUrl(canonical, locale).href,
+				href: localizedUrl(canonical, locale, context.trailingSlash).href,
 			},
 		});
 	}
diff --git a/packages/starlight/components/LanguageSelect.astro b/packages/starlight/components/LanguageSelect.astro
index 42999eac..dd9dbb6c 100644
--- a/packages/starlight/components/LanguageSelect.astro
+++ b/packages/starlight/components/LanguageSelect.astro
@@ -1,4 +1,5 @@
 ---
+import context from 'virtual:starlight/project-context';
 import config from 'virtual:starlight/user-config';
 import { localizedUrl } from '../utils/localizedUrl';
 import Select from './Select.astro';
@@ -8,7 +9,7 @@ import type { Props } from '../props';
  * Get the equivalent of the current page path for the passed locale.
  */
 function localizedPathname(locale: string | undefined): string {
-	return localizedUrl(Astro.url, locale).pathname;
+	return localizedUrl(Astro.url, locale, context.trailingSlash).pathname;
 }
 ---
 
diff --git a/packages/starlight/utils/localizedUrl.ts b/packages/starlight/utils/localizedUrl.ts
index e6df9068..5b9debee 100644
--- a/packages/starlight/utils/localizedUrl.ts
+++ b/packages/starlight/utils/localizedUrl.ts
@@ -1,10 +1,15 @@
 import config from 'virtual:starlight/user-config';
 import { stripTrailingSlash } from './path';
+import type { AstroConfig } from 'astro';
 
 /**
  * Get the equivalent of the passed URL for the passed locale.
  */
-export function localizedUrl(url: URL, locale: string | undefined): URL {
+export function localizedUrl(
+	url: URL,
+	locale: string | undefined,
+	trailingSlash: AstroConfig['trailingSlash']
+): URL {
 	// Create a new URL object to void mutating the global.
 	url = new URL(url);
 	if (!config.locales) {
@@ -41,5 +46,6 @@ export function localizedUrl(url: URL, locale: string | undefined): URL {
 	}
 	// Restore base
 	if (hasBase) url.pathname = base + url.pathname;
+	if (trailingSlash === 'never') url.pathname = stripTrailingSlash(url.pathname);
 	return url;
 }