NetworkClient.connection: type changed from NetworkConnection to explicit NetworkConnectionToServer to ensure correct usage. C# has a great type system, we should use to prevent issues.

Assets/Mirror/Core/NetworkClient.cs

@@ -43,7 +43,7 @@ public static partial class NetworkClient
             new Dictionary<uint, NetworkIdentity>();
 
         /// <summary>Client's NetworkConnection to server.</summary>
-        public static NetworkConnection connection { get; internal set; }
+        public static NetworkConnectionToServer connection { get; internal set; }
 
         /// <summary>True if client is ready (= joined world).</summary>
         // TODO redundant state. point it to .connection.isReady instead (& test)

Assets/Mirror/Tests/Common/FakeNetworkConnectionToServer.cs

@@ -0,0 +1,6 @@
+namespace Mirror.Tests
+{
+    public class FakeNetworkConnectionToServer : NetworkConnectionToServer
+    {
+    }
+}

Assets/Mirror/Tests/Editor/ClientSceneTests_OnSpawn.cs

@@ -534,7 +534,7 @@ public void ApplyPayload_LocalPlayerAddsIdentityToConnection()
                 payload = default,
             };
 
-            NetworkClient.connection = new FakeNetworkConnectionToClient();
+            NetworkClient.connection = new FakeNetworkConnectionToServer();
             NetworkClient.ready = true;
             NetworkClient.ApplySpawnPayload(identity, msg);