#unitytips: Scene Selection Toolbar - http://diegogiacomelli.com.br/unitytips-scene-selection-toolbar

SceneSelectionToolbar.cs

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

/// <summary>
/// #unitytips: Scene Selection Toolbar - http://diegogiacomelli.com.br/unitytips-scene-selection-toolbar
/// </summary>
[InitializeOnLoad]
public static class SceneSelectionToolbar
{
    static List<SceneInfo> _scenes;
    static SceneInfo _sceneOpened;    
    static int _selectedIndex;
    static string[] _displayedOptions;

    static SceneSelectionToolbar()
    {
        LoadFromPlayerPrefs();
        ToolbarExtender.LeftToolbarGUI.Add(OnToolbarGUI);
        EditorSceneManager.sceneOpened += HandleSceneOpened;            
    } 

    static void OnToolbarGUI()
    {
        GUILayout.FlexibleSpace();
        
        _selectedIndex = EditorGUILayout.Popup(_selectedIndex, _displayedOptions); ;

        GUI.enabled = _selectedIndex == 0;
        if (GUILayout.Button("+"))
            AddScene(_sceneOpened);

        GUI.enabled = _selectedIndex > 0;
        if (GUILayout.Button("-"))
            RemoveScene(_sceneOpened);

        GUI.enabled = true;
        if (GUI.changed && _selectedIndex > 0 && _scenes.Count > _selectedIndex - 1)
            EditorSceneManager.OpenScene(_scenes[_selectedIndex - 1].Path);
    }

    static void RefreshDisplayedOptions()
    {
        _displayedOptions = new string[_scenes.Count + 1];
        _displayedOptions[0] = "Click on '+' to add current scene";

        for (int i = 0; i < _scenes.Count; i++)
            _displayedOptions[i + 1] = _scenes[i].Name;
    }

    static void HandleSceneOpened(Scene scene, OpenSceneMode mode) => SetOpenedScene(scene);

    static void SetOpenedScene(SceneInfo scene)
    {
        if (scene == null || string.IsNullOrEmpty(scene.Path))
            return;

        for (int i = 0; i < _scenes.Count; i++)
        {
            if (_scenes[i].Path == scene.Path)
            {
                _sceneOpened = _scenes[i];
                _selectedIndex = i + 1;
                SaveToPlayerPrefs(true);
                return;
            }
        }

        _sceneOpened = scene;
        _selectedIndex = 0;
        SaveToPlayerPrefs(true);
    }

    static void SetOpenedScene(Scene scene) => SetOpenedScene(new SceneInfo(scene));

    static void AddScene(SceneInfo scene)
    {
        if (scene == null)
            return;

        if (_scenes.Any(s => s.Path == scene.Path))
            RemoveScene(scene);

        _scenes.Add(scene);
        _selectedIndex = _scenes.Count;
        SetOpenedScene(scene);
        RefreshDisplayedOptions();
        SaveToPlayerPrefs();
    }

    static void RemoveScene(SceneInfo scene)
    {
        _scenes.Remove(scene);
        _selectedIndex = 0;                
        RefreshDisplayedOptions();
        SaveToPlayerPrefs(); 
    }

    static void SaveToPlayerPrefs(bool onlyLatestOpenedScene = false)
    {
        if (!onlyLatestOpenedScene)
        {
            var serialized = string.Join(";", _scenes.Where(s => !string.IsNullOrEmpty(s.Path)).Select(s => s.Path));
            SetPref("SceneSelectionToolbar.Scenes", serialized);
        }

        if(_sceneOpened != null)
            SetPref("SceneSelectionToolbar.LatestOpenedScene", _sceneOpened.Path);
    }

    static void LoadFromPlayerPrefs()
    {
        var serialized = GetPref("SceneSelectionToolbar.Scenes");
        
        _scenes = serialized.Split(new char[] { ';' }, StringSplitOptions.RemoveEmptyEntries).Select(s => new SceneInfo(s)).ToList();

        if (_scenes == null)
            _scenes = new List<SceneInfo>();        

        serialized = GetPref("SceneSelectionToolbar.LatestOpenedScene");

        if (!string.IsNullOrEmpty(serialized))
            SetOpenedScene(new SceneInfo(serialized));
        
        RefreshDisplayedOptions();
    }

    static void SetPref(string name, string value) => EditorPrefs.SetString($"{Application.productName}_{name}", value);
    static string GetPref(string name) => EditorPrefs.GetString($"{Application.productName}_{name}");

    [Serializable]
    class SceneInfo
    {
        public SceneInfo() { }
        public SceneInfo(Scene scene)
        {
            Name = scene.name;
            Path = scene.path;
        }

        public SceneInfo(string path)
        {
            Name = System.IO.Path.GetFileNameWithoutExtension(path);
            Path = path;
        }

        public string Name;
        public string Path;
    }
}

Ah, there is a problem with this, if you add a scene. Then move that scene to a different folder, it then fails. If I try to remove the scene entry from the dropdown and then readd, after a editor reload it then does not restore the new location of the scene and fails again. This makes opening that scene from the dropdown impossible. :(

There is an critical bug in this code - when switching scenes, everything that was done in the current scene is not saved

Small fix for the line 43-44:

if (GUI.changed && _selectedIndex > 0 && _scenes.Count > _selectedIndex - 1)
{
    if (EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
    { 
       EditorSceneManager.OpenScene(_scenes[_selectedIndex - 1].Path);
    }
}

Thanks @PasterLak .. what a coincidence as I was just looking at the scene move issue I described above. I have a fix for it so will fork and add your suggestion too as the code I've added is around your addition.

Please see my fork which includes fixes for the two above issues:
https://gist.github.com/danieldownes/d9cef8b2c9136cf95be150357e74c7e3

..I would Pull-Request back if/when its possible with a gist.