Disable SSE in browser

Author: wtgodbe

src/SignalR/clients/csharp/Http.Connections.Client/src/HttpConnection.Log.cs

@@ -66,6 +66,9 @@ private static class Log
             private static readonly Action<ILogger, HttpTransportType, Exception> _transportStarted =
                 LoggerMessage.Define<HttpTransportType>(LogLevel.Debug, new EventId(18, "TransportStarted"), "Transport '{Transport}' started.");
 
+            private static readonly Action<ILogger, Exception> _serverSentEventsNotSupportedByBrowser =
+                LoggerMessage.Define(LogLevel.Debug, new EventId(19, "ServerSentEventsNotSupportedByBrowser"), "Skipping ServerSentEvents because they are not supported by the browser.");
+
             public static void Starting(ILogger logger)
             {
                 _starting(logger, null);

src/SignalR/clients/csharp/Http.Connections.Client/src/HttpConnection.cs

@@ -37,6 +37,7 @@ public partial class HttpConnection : ConnectionContext, IConnectionInherentKeep
         private bool _started;
         private bool _disposed;
         private bool _hasInherentKeepAlive;
+        private bool _isRunningInBrowser;
 
         private readonly HttpClient _httpClient;
         private readonly HttpConnectionOptions _httpConnectionOptions;
@@ -154,6 +155,8 @@ public HttpConnection(HttpConnectionOptions httpConnectionOptions, ILoggerFactor
             _logScope = new ConnectionLogScope();
 
             Features.Set<IConnectionInherentKeepAliveFeature>(this);
+
+            _isRunningInBrowser = Utils.IsRunningInBrowser();
         }
 
         // Used by unit tests
@@ -365,6 +368,13 @@ private async Task SelectAndStartTransport(TransferFormat transferFormat, Cancel
                         continue;
                     }
 
+                    if (transportType == HttpTransportType.ServerSentEvents && _isRunningInBrowser)
+                    {
+                        Log.ServerSentEventsNotSupportedByBrowser(_logger);
+                        transportExceptions.Add(new TransportFailedException("ServerSentEvents", "The transport is not supported in the browser."));
+                        continue;
+                    }
+
                     try
                     {
                         if ((transportType & _httpConnectionOptions.Transports) == 0)