gistfile1.cs

using System;
using UnityEditor;
using UnityEngine;
using System.Collections.Generic;

public class Question3_practical : EditorWindow
{
    [MenuItem("Window/Question3_practical")]
    static void Open()
    {
        GetWindow<Question3_practical>();
    }

    private List<Rect> rects = new List<Rect>();

    void OnGUI()
    {
        if (Event.current.type == EventType.ContextClick)
        {
            GenericMenu menu = new GenericMenu();
            menu.AddItem(new GUIContent("new Window"), false, () =>
            {
                rects.Add(new Rect(0, 0, 100, 100));
                Repaint();
            });
            menu.ShowAsContext();
        }
        BeginWindows();
        for (int index = 0; index < rects.Count; index++)
        {
            rects[index] = GUI.Window(index, rects[index], (id) =>
            {
                if (Event.current.type == EventType.ContextClick)
                {
                    GenericMenu menu = new GenericMenu();
                    menu.AddItem(new GUIContent("Delete Window"), false, (data) =>
                    {
                        rects.Remove((Rect) data);
                        Repaint();
                    }, rects[id]);
                    menu.ShowAsContext();
                }
                GUI.DragWindow(new Rect(0, 0, 100, 16));
            }, index.ToString());
        }
        EndWindows();
    }
}