gistfile1.cs

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

public class Question6_practical : EditorWindow
{
    private Dictionary<int, Rect> windowRects = new Dictionary<int, Rect>();

    [MenuItem("Window/Question6_practical")]

    static void Open()
    {
        GetWindow<Question6_practical>();
    }

    void OnEnable()
    {
        AddRect(0, 1, 2, 3, 4, 5);
    }

    void OnGUI()
    {
        BeginWindows();

        for (int i = 0; i < windowRects.Count; i++)
        {
            int windowID = windowRects.Keys.ElementAt(i);

            windowRects[windowID] = GUI.Window(windowID, windowRects[windowID], (id) => GUI.DragWindow(), windowID.ToString());

            if (i != 0)
            {
                int preWindowID = windowRects.Keys.ElementAt(i - 1);
                DrawLine(windowRects[preWindowID], windowRects[windowID]);
            }
        }

        EndWindows();
    }

    void AddRect(params int[] windowIDs)
    {
        foreach (int windowID in windowIDs)
        {
            windowRects.Add(windowID, new Rect(10 * windowID, 10 * windowID, 100, 100));
        }
    }

    void DrawLine(Rect start, Rect end)
    {
        Vector2 startPos = new Vector2(start.x + start.width * 0.5f, start.y + start.height * 0.5f);
        Vector2 endPos = new Vector2(end.x + end.width * 0.5f, end.y + end.height * 0.5f);
        Handles.color = Color.black;
        Handles.DrawLine(startPos, endPos);
    }
}