fix(v2): never remove trailing slash from site root like '/baseUrl/' (#5082)

* never apply trailingSlash to site root ('/baseUrl/') => only subroutes

* add deprecation comment for loadContext.baseUrl in favor of loadContext.siteConfig.baseUrl

* commit typo

* useless code

packages/docusaurus-utils-common/src/applyTrailingSlash.tsx

@@ -5,10 +5,20 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import type {DocusaurusConfig} from '@docusaurus/types';
+
+export type ApplyTrailingSlashParams = Pick<
+  DocusaurusConfig,
+  'trailingSlash' | 'baseUrl'
+>;
+
+// Trailing slash handling depends in some site configuration options
 export default function applyTrailingSlash(
   path: string,
-  trailingSlash: boolean | undefined,
+  options: ApplyTrailingSlashParams,
 ): string {
+  const {trailingSlash, baseUrl} = options;
+
   if (path.startsWith('#')) {
     // Never apply trailing slash to an anchor link
     return path;
@@ -34,7 +44,14 @@ export default function applyTrailingSlash(
   const [pathname] = path.split(/[#?]/);
 
   // Never transform '/' to ''
-  const newPathname =
-    pathname === '/' ? '/' : handleTrailingSlash(pathname, trailingSlash);
+  // Never remove the baseUrl trailing slash!
+  // If baseUrl = /myBase/, we want to emit /myBase/index.html and not /myBase.html !
+  // See https://github.com/facebook/docusaurus/issues/5077
+  const shouldNotApply = pathname === '/' || pathname === baseUrl;
+
+  const newPathname = shouldNotApply
+    ? pathname
+    : handleTrailingSlash(pathname, trailingSlash);
+
   return path.replace(pathname, newPathname);
 }