fix: m_ListAtLastReset and dynamically spawned objects (#2065)

Author: jeffreyrainy

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -6,6 +6,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 Additional documentation and release notes are available at [Multiplayer Documentation](https://docs-multiplayer.unity3d.com).
 
+## [Unreleased]
+
+### Changed
+
+### Fixed
+
+- Fixed NetworkLists not populating on client. NetworkList now uses the most recent list as opposed to the list at the end of previous frame, when sending full updates to dynamically spawned NetworkObject. The difference in behaviour is required as scene management spawns those objects at a different time in the frame, relative to updates. (#2062)
+
 ## [1.0.0] - 2022-06-27
 
 ### Changed

com.unity.netcode.gameobjects/Runtime/NetworkVariable/Collections/NetworkList.cs

@@ -122,10 +122,26 @@ public override void WriteDelta(FastBufferWriter writer)
         /// <inheritdoc />
         public override void WriteField(FastBufferWriter writer)
         {
-            writer.WriteValueSafe((ushort)m_ListAtLastReset.Length);
-            for (int i = 0; i < m_ListAtLastReset.Length; i++)
+            // The listAtLastReset mechanism was put in place to deal with duplicate adds
+            // upon initial spawn. However, it causes issues with in-scene placed objects
+            // due to difference in spawn order. In order to address this, we pick the right
+            // list based on the type of object.
+            bool isSceneObject = m_NetworkBehaviour.NetworkObject.IsSceneObject != false;
+            if (isSceneObject)
             {
-                NetworkVariableSerialization<T>.Write(writer, ref m_ListAtLastReset.ElementAt(i));
+                writer.WriteValueSafe((ushort)m_ListAtLastReset.Length);
+                for (int i = 0; i < m_ListAtLastReset.Length; i++)
+                {
+                    NetworkVariableSerialization<T>.Write(writer, ref m_ListAtLastReset.ElementAt(i));
+                }
+            }
+            else
+            {
+                writer.WriteValueSafe((ushort)m_List.Length);
+                for (int i = 0; i < m_List.Length; i++)
+                {
+                    NetworkVariableSerialization<T>.Write(writer, ref m_List.ElementAt(i));
+                }
             }
         }