Unity3d editor window to show recent selections

SelectionHistory.cs

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

public class SelectionHistory : EditorWindow
{
    private const int NumberOfItemsToKeep = 6;
    private const string MultiSelectPrefix = "+";
    private const int ItemsPerRow = 2;

    private static readonly Color[] ItemColors = new Color[] { Color.green, Color.cyan, Color.red, Color.blue, Color.yellow, Color.magenta };
    private static int NextColorIndex = 0;

    [MenuItem("BigRobot/Selection History")]
    public static void ShowWindow()
    {
        GetWindow(typeof(SelectionHistory));
    }

    private class SelectionDetail
    {
        public SelectionDetail(string name, int[] iDs)
        {
            Name = name;
            IDs = iDs;
            Color = ItemColors[NextColorIndex++];

            if (NextColorIndex >= ItemColors.Length)
            {
                NextColorIndex = 0;
            }
        }

        public string Name { get; private set; }
        public int[] IDs { get; private set; }
        public Color Color { get; private set; }
    }

    private List<SelectionDetail> _selections = new List<SelectionDetail>();

    private void OnGUI()
    {
        if (_selections.Count == 0)
        {
            EditorGUILayout.LabelField("No selections yet...");
        }
        else
        {
            EditorGUILayout.BeginHorizontal();
            var count = 0;

            foreach (var s  in _selections)
            {
                GUI.backgroundColor = s.Color;

                if (GUILayout.Button(s.Name, GUILayout.ExpandWidth(false)))
                {
                    MoveToTop(s);

                    Selection.instanceIDs = s.IDs;

                    break;
                }

                if (++count % ItemsPerRow == 0)
                {
                    EditorGUILayout.EndHorizontal();
                    EditorGUILayout.BeginHorizontal();
                }
            }

            EditorGUILayout.EndHorizontal();
        }
    }

    private void OnSelectionChange()
    {
        var ids = Selection.instanceIDs;
        var match = _selections.SingleOrDefault(s => s.IDs.SequenceEqual(ids));
        
        if (match == null)
        {
            var isMultiSelect = ids.Length > 1;
            var name = isMultiSelect ? string.Concat(MultiSelectPrefix, Selection.activeObject.name) : Selection.activeObject.name;
            var sel = new SelectionDetail(name, ids);

            AddNewItem(sel);
        }
        else
        {
            MoveToTop(match);
        }

        Repaint();
    }

    private void MoveToTop(SelectionDetail item)
    {
        _selections.Remove(item);
        _selections.Insert(0, item);
    }

    private void AddNewItem(SelectionDetail item)
    {
        _selections.Insert(0, item);

        _selections = _selections.Where(i => i != null).Take(NumberOfItemsToKeep).ToList();
    }
}