Customizing playable director editor in Inspector window.

I was trying to add button in playable director, and found out this code below is ruining original layout of playable director.

Code (CSharp):

1.  
2. [CustomEditor(typeof(PlayableDirector))]
3. public class FooEditor : Editor
4. {
5.     public override void OnInspectorGUI()
6.     {
7.         base.OnInspectorGUI();
8.        
9.         GUILayout.Button("testButton");
10.     }
11. }

and it looks like below...



I think I need to inherit original editor for playable director, but I can't find it. Does anyone know about this?

Thank you in advance.

 

I found the editor code here, but it is internal class....
What should I do...?
I don't want to mess up original layout. Only thing I want is just putting a tiny button at the bottom.

 

Do you mean this?