Mouse clicks inside editor GUI

I have made an EditorWindow which draws a GUI.Box. How do I get mouse coordinates relative to this box when the user clicks inside of it?

You could abuse GUI.BeginGroup to offset Event.current.mousePosition:

GUI.Box(boxRect, "");
GUI.BeginGroup(boxRect);
Debug.Log(Event.current.mousePosition); //position will be relative to top left of box
GUI.EndGroup();