ActiveEditorTracker.MakeCustomEditor

using UnityEngine;
using UnityEditor;

public class CustomWindow : EditorWindow {

    [MenuItem("Window/Test")]
    public static void Test() {
        EditorWindow.GetWindow<CustomWindow>("Test");
    }

    GameObject activeGO;
    Editor editor;

    void Update() {
        if (activeGO == Selection.activeGameObject)
            return;

        activeGO = Selection.activeGameObject;
        editor = null;

        if (activeGO != null)
            editor = UnityEditor.ActiveEditorTracker.MakeCustomEditor(activeGO.transform);

        Repaint();
    }

    void OnGUI() {
        GameObject go = Selection.activeGameObject;
        if (go == null)
            return;

        // Use the registered editor for `Transform`
        EditorGUILayout.InspectorTitlebar(true, editor.target);
        editor.OnInspectorGUI();
    }

}