@pavelsavara's version of fix.

Author: ilonatommy

src/libraries/Common/tests/System/Net/Prerequisites/NetCoreServer/Handlers/EchoWebSocketHandler.cs

@@ -144,6 +144,22 @@ await socket.CloseAsync(
                     {
                         await Task.Delay(5000);
                     }
+                    else if (receivedMessage == ".receiveMessageAfterClose")
+                    {
+                        byte[] buffer = new byte[1024];
+                        string message1 = $"{receivedMessage} 1 {DateTime.Now.ToString("HH:mm:ss")}";
+                        buffer = System.Text.Encoding.UTF8.GetBytes(message1);
+                        var stamp = DateTime.Now.ToString("HH:mm:ss");
+                        Console.WriteLine($"[{stamp}]: Sending message: {message1}");
+                        await socket.SendAsync(
+                            new ArraySegment<byte>(buffer, 0, message1.Length),
+                            WebSocketMessageType.Text,
+                            true,
+                            CancellationToken.None);
+                        stamp = DateTime.Now.ToString("HH:mm:ss");
+                        Console.WriteLine($"[{stamp}] Closing the socket");
+                        await socket.CloseAsync(WebSocketCloseStatus.NormalClosure, receivedMessage, CancellationToken.None);
+                    }
                     else if (socket.State == WebSocketState.Open)
                     {
                         sendMessage = true;

src/libraries/System.Net.WebSockets.Client/src/System/Net/WebSockets/BrowserWebSockets/BrowserWebSocket.cs

@@ -385,12 +385,6 @@ private void CreateCore(Uri uri, List<string>? requestedSubProtocols)
 #endif
                         _closeStatus = (WebSocketCloseStatus)code;
                         _closeStatusDescription = reason;
-                        _closeReceived = true;
-                        WebSocketState state = State;
-                        if (state == WebSocketState.Connecting || state == WebSocketState.Open || state == WebSocketState.CloseSent)
-                        {
-                            FastState = WebSocketState.Closed;
-                        }
 #if FEATURE_WASM_THREADS
                     } //lock
 #endif

src/libraries/System.Net.WebSockets.Client/tests/CloseTest.cs

@@ -367,6 +367,35 @@ await cws.SendAsync(
             }
         }
 
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/28957", typeof(PlatformDetection), nameof(PlatformDetection.IsNotBrowser))]
+        [ConditionalTheory(nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
+        public async Task CloseOutputAsync_ServerInitiated_CanReceiveAfterClose(Uri server)
+        {
+            using (ClientWebSocket cws = await GetConnectedWebSocket(server, TimeOutMilliseconds, _output))
+            {
+                var cts = new CancellationTokenSource(TimeOutMilliseconds);
+
+                await cws.SendAsync(
+                    WebSocketData.GetBufferFromText(".receiveMessageAfterClose"),
+                    WebSocketMessageType.Text,
+                    true,
+                    cts.Token);
+
+                var recvBuffer = new ArraySegment<byte>(new byte[1024]);
+                await Task.Delay(2000); // even if we decrease to 100, it still fails with
+                // System.Net.WebSockets.WebSocketException : The WebSocket is in an invalid state ('Closed') for this operation. Valid states are: 'Open, CloseSent'
+                // in the manual demo we can wait even 6 sec and the message arrives just fine
+                var stamp = DateTime.Now.ToString("HH:mm:ss");
+                Console.WriteLine($"[{stamp}] Attempting to receive data... cws.State={cws.State}");
+                WebSocketReceiveResult recvResult = await cws.ReceiveAsync(recvBuffer, cts.Token);
+                var message = Encoding.UTF8.GetString(recvBuffer.ToArray(), 0, recvResult.Count);
+                stamp = DateTime.Now.ToString("HH:mm:ss");
+                Console.WriteLine($"[{stamp}] Received message={message}");
+                Assert.Contains(".shutdownAfterTwoMessages 1", message);
+                Console.WriteLine($"cws.State={cws.State}");
+            }
+        }
+
         [OuterLoop("Uses external servers", typeof(PlatformDetection), nameof(PlatformDetection.LocalEchoServerIsNotAvailable))]
         [ConditionalTheory(nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
         public async Task CloseOutputAsync_CloseDescriptionIsNull_Success(Uri server)

src/mono/wasm/runtime/web-socket.ts

@@ -175,15 +175,6 @@ export function ws_wasm_receive(ws: WebSocketExtension, buffer_ptr: VoidPtr, buf
         return null;
     }
 
-    const readyState = ws.readyState;
-    if (readyState == WebSocket.CLOSED) {
-        const receive_status_ptr = ws[wasm_ws_receive_status_ptr];
-        setI32(receive_status_ptr, 0); // count
-        setI32(<any>receive_status_ptr + 4, 2); // type:close
-        setI32(<any>receive_status_ptr + 8, 1);// end_of_message: true
-        return null;
-    }
-
     const { promise, promise_control } = createPromiseController<void>();
     const receive_promise_control = promise_control as ReceivePromiseControl;
     receive_promise_control.buffer_ptr = buffer_ptr;