Custom Editor : simple example required c#

I'd appreciate it if someone could show me how to create a custom editor that will display the mouse position in the inspector when the left mouse button is released in the Scene View.

I've tried for an hour or so but I must be missing something.

Thanks for any help.

This works to catch mouse down. Mouse up doesn't come through though -- looks like the scene view already swallowed it. I do get mouse up for the right button though, if that helps.

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(Transform))]
public class EditorTempTest : Editor
{
    void OnSceneGUI()
    {
        if (Event.current.type == EventType.MouseDown)
        {
            Debug.Log(Event.current.mousePosition);
        }
    }
}