fix: Shut down NetworkManager when reloading the domain

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -22,6 +22,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- `NetworkManager` will now perform a proper shutdown when the domain is reloaded in play mode, instead of being left in some half-initialized state that leaks memory and socket handles. (#4068)
 - Issue where a NullReferenceException was thrown when a non-authority failed to spawn a NetworkObject. (#4067)
 - Issue when FastBufferReader is attempting to read a string and all or a portion of the character count has already been read by user script, it could read a character length that results in a negative byte length which could result in an editor crash. (#4052)
 - Issue where NetworkRigidbodyBase was not applying rotation correctly when using Rigidbody2D. (#4012)

com.unity.netcode.gameobjects/Editor/NetworkManagerHelper.cs

@@ -30,6 +30,10 @@ private static void InitializeOnload()
             Singleton = new NetworkManagerHelper();
 
             NetworkManager.NetworkManagerHelper = Singleton;
+
+            AssemblyReloadEvents.beforeAssemblyReload -= AssemblyReloadEvents_beforeAssemblyReload;
+            AssemblyReloadEvents.beforeAssemblyReload += AssemblyReloadEvents_beforeAssemblyReload;
+
             EditorApplication.playModeStateChanged -= EditorApplication_playModeStateChanged;
             EditorApplication.hierarchyChanged -= EditorApplication_hierarchyChanged;
 
@@ -55,6 +59,19 @@ private static void InitializeOnload()
             };
         }
 
+        private static void AssemblyReloadEvents_beforeAssemblyReload()
+        {
+            if (Application.isPlaying)
+            {
+                var networkManager = NetworkManager.Singleton;
+                if (networkManager != null && (networkManager.IsServer || networkManager.IsClient))
+                {
+                    networkManager.Shutdown();
+                    networkManager.ShutdownInternal();
+                }
+            }
+        }
+
         private static void EditorApplication_playModeStateChanged(PlayModeStateChange playModeStateChange)
         {
             switch (playModeStateChange)