Correctly compute 'startURL' when using a frontend development server. (#3299)

* Fix computing 'startURL' when 'FRONTEND_DEVSERVER_URL' is present.

* Respect provided URL when computing 'startURL' and 'FRONTEND_DEVSERVER_URL' is present.

* Update changelog.

* Correctly produce absolute URI in 'GetStartURL' when input is provided.

v3/internal/assetserver/assetserver.go

@@ -148,26 +148,19 @@ func GetStartURL(userURL string) (string, error) {
 		}
 		port := parsedURL.Port()
 		if port != "" {
-			baseURL.Host = net.JoinHostPort(baseURL.Host, port)
+			baseURL.Host = net.JoinHostPort(baseURL.Hostname(), port)
 			startURL = baseURL.String()
 		}
-	} else {
-		if userURL != "" {
-			// parse the url
-			parsedURL, err := url.Parse(userURL)
-			if err != nil {
-				return "", fmt.Errorf("Error parsing URL: " + err.Error())
-			}
-			if parsedURL.Scheme == "" {
-				startURL = baseURL.ResolveReference(&url.URL{Path: userURL}).String()
-				// if the original URL had a trailing slash, add it back
-				if strings.HasSuffix(userURL, "/") && !strings.HasSuffix(startURL, "/") {
-					startURL = startURL + "/"
-				}
-			} else {
-				startURL = userURL
-			}
-		}
 	}
+
+	if userURL != "" {
+		parsedURL, err := baseURL.Parse(userURL)
+		if err != nil {
+			return "", fmt.Errorf("Error parsing URL: " + err.Error())
+		}
+
+		startURL = parsedURL.String()
+	}
+
 	return startURL, nil
 }