save original root transform to linked prefab + fix unit test

Author: vkovec

com.unity.formats.fbx/Editor/ConvertToNestedPrefab.cs

@@ -451,10 +451,6 @@ public static GameObject Convert(
             // replace hierarchy in the scene
             if (!isPrefabAsset && toConvert != null)
             {
-                // keep the transform of the root of the prefab instance the same
-                var jsonTransform = EditorJsonUtility.ToJson(toConvert.transform);
-                EditorJsonUtility.FromJsonOverwrite(jsonTransform, fbxInstance.transform);
-
                 Undo.DestroyObjectImmediate(toConvert);
                 Undo.RegisterCreatedObjectUndo(fbxInstance, UndoConversionCreateObject);
                 SceneManagement.EditorSceneManager.MarkSceneDirty(fbxInstance.scene);
@@ -869,10 +865,10 @@ internal static void CopyComponents(GameObject to, GameObject from, GameObject r
                 }
 
                 // ignore MeshFilter and Transform, but still ensure scene references are maintained.
-                // Don't need to copy regular transform as the values should already be correct in the FBX.
+                // Don't need to copy regular transform (except for the root object) as the values should already be correct in the FBX.
                 // Furthermore, copying transform values may result in overrides in the prefab, which is undesired as if
                 // the transform is updated in the FBX, it won't be in the prefab.
-                if (fromComponent is MeshFilter || fromComponent is Transform)
+                if (fromComponent is MeshFilter || (fromComponent is Transform && from != root))
                 {
                     FixSceneReferences(fromComponent, to.GetComponent(fromComponent.GetType()), root);
                     continue;

com.unity.formats.fbx/Tests/FbxTests/ConvertToModelTest.cs

@@ -433,6 +433,7 @@ public void TestStaticHelpers()
                 b2.transform.parent = b.transform; // in alpha order
                 a.AddComponent<BoxCollider> ();
                 a1.transform.localPosition = new Vector3 (1, 2, 3);
+                a.transform.localPosition = new Vector3(4, 5, 6);
 
                 Assert.AreNotEqual(b.GetComponent<MeshFilter>().sharedMesh, a.GetComponent<MeshFilter>().sharedMesh);
                 Assert.IsFalse (b.GetComponent<BoxCollider> ());
@@ -441,11 +442,12 @@ public void TestStaticHelpers()
 
                 ConvertToNestedPrefab.UpdateFromSourceRecursive (b, a);
 
-                // everything except the mesh + materials should change
+                // everything except the mesh + materials and child transforms should change
                 Assert.AreNotEqual(b.GetComponent<MeshFilter>().sharedMesh, a.GetComponent<MeshFilter>().sharedMesh);
                 Assert.IsTrue (b.GetComponent<BoxCollider> ());
                 Assert.AreEqual ("BB", b.transform.GetChild (1).name);
-                Assert.AreEqual (a1.transform.localPosition, b1.transform.localPosition);
+                Assert.AreEqual(a.transform.localPosition, b.transform.localPosition);
+                Assert.AreNotEqual (a1.transform.localPosition, b1.transform.localPosition);
             }
 
             // Test GetFbxAssetOrNull