Allow relative $PUBLIC_URL in dev mode

As long as client side routing isn't 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

packages/react-dev-utils/__tests__/getPublicUrlOrPath.test.js

@@ -14,10 +14,10 @@ const tests = [
   { dev: true, homepage: '/', expect: '/' },
   { dev: true, homepage: '/test', expect: '/test/' },
   { dev: true, homepage: '/test/', expect: '/test/' },
-  { dev: true, homepage: './', expect: '/' },
-  { dev: true, homepage: '../', expect: '/' },
-  { dev: true, homepage: '../test', expect: '/' },
-  { dev: true, homepage: './test/path', expect: '/' },
+  { dev: true, homepage: './', expect: './' },
+  { dev: true, homepage: '../', expect: '../' },
+  { dev: true, homepage: '../test', expect: '../test/' },
+  { dev: true, homepage: './test/path', expect: './test/path/' },
   { dev: true, homepage: 'https://create-react-app.dev/', expect: '/' },
   {
     dev: true,
@@ -28,10 +28,10 @@ const tests = [
   { dev: true, publicUrl: '/', expect: '/' },
   { dev: true, publicUrl: '/test', expect: '/test/' },
   { dev: true, publicUrl: '/test/', expect: '/test/' },
-  { dev: true, publicUrl: './', expect: '/' },
-  { dev: true, publicUrl: '../', expect: '/' },
-  { dev: true, publicUrl: '../test', expect: '/' },
-  { dev: true, publicUrl: './test/path', expect: '/' },
+  { dev: true, publicUrl: './', expect: './' },
+  { dev: true, publicUrl: '../', expect: '../' },
+  { dev: true, publicUrl: '../test', expect: '../test/' },
+  { dev: true, publicUrl: './test/path', expect: './test/path/' },
   { dev: true, publicUrl: 'https://create-react-app.dev/', expect: '/' },
   {
     dev: true,
@@ -42,10 +42,15 @@ const tests = [
   { dev: true, publicUrl: '/', homepage: '/test', expect: '/' },
   { dev: true, publicUrl: '/test', homepage: '/path', expect: '/test/' },
   { dev: true, publicUrl: '/test/', homepage: '/test/path', expect: '/test/' },
-  { dev: true, publicUrl: './', homepage: '/test', expect: '/' },
-  { dev: true, publicUrl: '../', homepage: '/test', expect: '/' },
-  { dev: true, publicUrl: '../test', homepage: '/test', expect: '/' },
-  { dev: true, publicUrl: './test/path', homepage: '/test', expect: '/' },
+  { dev: true, publicUrl: './', homepage: '/test', expect: './' },
+  { dev: true, publicUrl: '../', homepage: '/test', expect: '../' },
+  { dev: true, publicUrl: '../test', homepage: '/test', expect: '../test/' },
+  {
+    dev: true,
+    publicUrl: './test/path',
+    homepage: '/test',
+    expect: './test/path/',
+  },
   {
     dev: true,
     publicUrl: 'https://create-react-app.dev/',
@@ -67,11 +72,15 @@ const tests = [
   { dev: false, homepage: '../', expect: '../' },
   { dev: false, homepage: '../test', expect: '../test/' },
   { dev: false, homepage: './test/path', expect: './test/path/' },
-  { dev: false, homepage: 'https://create-react-app.dev/', expect: '/' },
+  {
+    dev: false,
+    homepage: 'https://create-react-app.dev/',
+    expect: 'https://create-react-app.dev/',
+  },
   {
     dev: false,
     homepage: 'https://create-react-app.dev/test',
-    expect: '/test/',
+    expect: 'https://create-react-app.dev/test/',
   },
   // PRODUCTION with publicUrl
   { dev: false, publicUrl: '/', expect: '/' },

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 '/';