fix: ensure Serialize writes nothing at all if not dirty and add test so it's never missed again(!).

fixes unchanged objects sending an unnecessary 1 byte dirty mask after recent dirtybits improvement.

Assets/Mirror/Core/NetworkIdentity.cs

@@ -947,8 +947,11 @@ internal void Serialize(bool initialState, NetworkWriter ownerWriter, NetworkWri
             //  11 components fit into 2 bytes. (previously 11 bytes)
             //  16 components fit into 3 bytes. (previously 16 bytes)
             // TODO imer: client knows amount of comps, write N bytes instead
-            Compression.CompressVarUInt(ownerWriter,     ownerMask);
+
-            Compression.CompressVarUInt(observersWriter, observerMask);
+            // if nothing dirty, then don't even write the mask.
+            // otherwise, every unchanged object would send a 1 byte dirty mask!
+            if (ownerMask    != 0) Compression.CompressVarUInt(ownerWriter,     ownerMask);
+            if (observerMask != 0) Compression.CompressVarUInt(observersWriter, observerMask);
 
             // serialize all components
             for (int i = 0; i < components.Length; ++i)

Assets/Mirror/Tests/Editor/NetworkIdentitySerializationTests.cs

@@ -199,5 +199,28 @@ public void SerializationMismatch()
             // should still work fine. that's the whole point.
             Assert.That(clientComp.value, Is.EqualTo("42"));
         }
+
+        // ensure Serialize writes nothing if not dirty.
+        // previously after the dirty mask improvement, it would write a 1 byte
+        // 0-dirty-mask. instead, we need to ensure it writes nothing.
+        // too easy to miss, with too significant bandwidth implications.
+        [Test]
+        public void Serialize_NotInitial_NotDirty_WritesNothing()
+        {
+            // create spawned so that isServer/isClient is set properly
+            CreateNetworkedAndSpawn(
+                out _, out NetworkIdentity serverIdentity, out SerializeTest1NetworkBehaviour serverComp1, out SerializeTest2NetworkBehaviour serverComp2,
+                out _, out NetworkIdentity clientIdentity, out SerializeTest1NetworkBehaviour clientComp1, out SerializeTest2NetworkBehaviour clientComp2);
+
+            // change nothing
+            // serverComp.value = "42";
+
+            // serialize server object.
+            // 'initial' would write everything.
+            // instead, try 'not initial' with 0 dirty bits
+            serverIdentity.Serialize(false, ownerWriter, observersWriter);
+            Assert.That(ownerWriter.Position, Is.EqualTo(0));
+            Assert.That(observersWriter.Position, Is.EqualTo(0));
+        }
     }
 }