Save Thumbnails for Scenes (unity editor script)

CustomThumbnail.cs

using System.IO;
using UnityEditor;
using UnityEngine;

namespace UnityLibrary
{
    [CustomEditor(typeof(SceneAsset))]
    public class CustomThumbnail : Editor
    {
        public override bool HasPreviewGUI() => true;

        public override void OnPreviewGUI(Rect r, GUIStyle background)
        {
            if (target == null) return;

            GUI.Label(r, (target as SceneAsset).name);

            var scenePath = Path.GetDirectoryName(AssetDatabase.GetAssetPath(target));
            var sceneName = Path.GetFileNameWithoutExtension((target as SceneAsset).name);

            // check if scene thumbnail exists (NOTE: this is a hidden file)
            var path = Path.Combine(scenePath, "." + sceneName + ".png");

            if (File.Exists(path))
            {
                Texture2D texture = new Texture2D(2, 2);
                texture.LoadImage(File.ReadAllBytes(path));
                GUI.DrawTexture(r, texture, ScaleMode.ScaleToFit);
            }
            else
            {
                //GUI.Label(r, "\n\nNo thumbnail found");
                var fileSize = File.Exists(scenePath) ? new FileInfo(scenePath).Length.ToString() : "???";
                var lastModified = File.Exists(scenePath) ? File.GetLastWriteTime(scenePath) : System.DateTime.Now;
                GUI.Label(r, "\n\nSize: " + fileSize + "\nLast modified: " + lastModified);
            }
        }
    }
}

SceneThumbnail.cs

using UnityEditor;
using UnityEngine;
using System.IO;

namespace UnityLibrary
{
    [InitializeOnLoad]
    static class SceneThumbnail
    {
        static bool saveAsHiddenFile = true;

        static SceneThumbnail()
        {
            UnityEditor.SceneManagement.EditorSceneManager.sceneSaved += OnSceneSaved;
        }

        // https://forum.unity.com/threads/editorscenemanager-scenesaved-example-no-code-examples-in-the-docs.488861/
        static void OnSceneSaved(UnityEngine.SceneManagement.Scene scene)
        {
            var sceneName = System.IO.Path.GetFileNameWithoutExtension(scene.path);

            // grab scene camera (or main camera if want to?)
            var camera = SceneView.lastActiveSceneView.camera;
            if (camera == null)
            {
                Debug.LogWarning("No SceneView.lastActiveSceneView.camera found..");
                return;
            }

            // render the camera to a texture
            var texture = new Texture2D(camera.pixelWidth, camera.pixelHeight);
            var renderTexture = new RenderTexture(camera.pixelWidth, camera.pixelHeight, 24);
            camera.targetTexture = renderTexture;
            camera.Render();
            RenderTexture.active = renderTexture;
            texture.ReadPixels(new Rect(0, 0, camera.pixelWidth, camera.pixelHeight), 0, 0);
            texture.Apply();
            camera.targetTexture = null;
            RenderTexture.active = null;
            Object.DestroyImmediate(renderTexture);

            // save the texture to a file
            var path = Path.Combine(Path.GetDirectoryName(scene.path), (saveAsHiddenFile ? "." : "") + sceneName + ".png");
            File.WriteAllBytes(path, texture.EncodeToPNG());
            //Debug.LogFormat("Saved thumbnail to '{0}'", path);

            AssetDatabase.Refresh();
        }
    }
}

forum thread
https://forum.unity.com/threads/2024-and-still-no-snapshot.1575175/#post-9754894