Allow relative $PUBLIC_URL in dev mode

As long as client side routing isn't actively used, a relative
$PUBLIC_URL should work correctly in both development and production.
There's no reason it should be limited to production.

Closes facebook#8623

Also see coder/code-server#2565

Author: nhooyr

packages/react-dev-utils/getPublicUrlOrPath.js

@@ -14,7 +14,7 @@ module.exports = getPublicUrlOrPath;
 /**
  * Returns a URL or a path with slash at the end
  * In production can be URL, abolute path, relative path
- * In development always will be an absolute path
+ * In development can be a relative or absolute path
  * In development can use `path` module functions for operations
  *
  * @param {boolean} isEnvDevelopment
@@ -31,34 +31,31 @@ function getPublicUrlOrPath(isEnvDevelopment, homepage, envPublicUrl) {
       ? envPublicUrl
       : envPublicUrl + '/';
 
+    // Some apps do not use client-side routing with pushState.
+    // For these, "$PUBLIC_URL" can be set to "." to enable relative asset paths.
+    if (envPublicUrl.startsWith(".")) {
+      return envPublicUrl;
+    }
+
     // validate if `envPublicUrl` is a URL or path like
     // `stubDomain` is ignored if `envPublicUrl` contains a domain
     const validPublicUrl = new URL(envPublicUrl, stubDomain);
-
-    return isEnvDevelopment
-      ? envPublicUrl.startsWith('.')
-        ? '/'
-        : validPublicUrl.pathname
-      : // Some apps do not use client-side routing with pushState.
-        // For these, "homepage" can be set to "." to enable relative asset paths.
-        envPublicUrl;
+    return isEnvDevelopment ? validPublicUrl.pathname : envPublicUrl;
   }
 
   if (homepage) {
     // strip last slash if exists
     homepage = homepage.endsWith('/') ? homepage : homepage + '/';
 
+    // Some apps do not use client-side routing with pushState.
+    // For these, homepage can be set to "." to enable relative asset paths.
+    if (homepage.startsWith(".")) {
+      return homepage;
+    }
+
     // validate if `homepage` is a URL or path like and use just pathname
     const validHomepagePathname = new URL(homepage, stubDomain).pathname;
-    return isEnvDevelopment
-      ? homepage.startsWith('.')
-        ? '/'
-        : validHomepagePathname
-      : // Some apps do not use client-side routing with pushState.
-      // For these, "homepage" can be set to "." to enable relative asset paths.
-      homepage.startsWith('.')
-      ? homepage
-      : validHomepagePathname;
+    return isEnvDevelopment ? validHomepagePathname : homepage;
   }
 
   return '/';