Lightmap Preview Mode in final render instead of using debug mode.

LightmapPreviewModeWindow.cs

// https://discussions.unity.com/t/decent-lightmapper-preview/1571658/2

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

public class LightmapPreviewModeWindow : EditorWindow {
	[MenuItem("Tools/Lightmap preview mode")]
	public static void Get() {
		GetWindow<LightmapPreviewModeWindow>().Show();
	}

	[InitializeOnLoadMethod]
	static void Init() {
		var s_ShowInteractiveLightBakingToggleCameraModes = typeof(SceneView).GetField("s_ShowInteractiveLightBakingToggleCameraModes", BindingFlags.NonPublic | BindingFlags.Static);
		(s_ShowInteractiveLightBakingToggleCameraModes.GetValue(null) as HashSet<DrawCameraMode>).Add(DrawCameraMode.Textured);
	}

	SceneView dummySceneInDebugMode;
	SceneView lastScene;

	void OnEnable2() {
		bool thisHasFocus = this.hasFocus;
		dummySceneInDebugMode = CreateWindow<SceneView>("Ignore", typeof(LightmapPreviewModeWindow));
		dummySceneInDebugMode.cameraMode = SceneView.GetBuiltinCameraMode(DrawCameraMode.BakedLightmap);
		dummySceneInDebugMode.debugDrawModesUseInteractiveLightBakingData = true;
		dummySceneInDebugMode.wantsLessLayoutEvents = true;
		if (thisHasFocus) {
			ShowTab();
		}
		lastScene = SceneView.lastActiveSceneView;
	}

	void OnDisable() {
		dummySceneInDebugMode?.Close();
		if (lastScene != null) lastScene.debugDrawModesUseInteractiveLightBakingData = false;
	}

	void OnGUI() {
		if (dummySceneInDebugMode == null) {
			OnEnable2();
			return;
		}
		GUILayout.Label("Lightmap preview mode", EditorStyles.boldLabel);
		EditorGUILayout.HelpBox("Real time lightmap preview to configure lights quickly", MessageType.Info);
		EditorGUILayout.HelpBox("Doesn't update Lightprobes", MessageType.Warning);
		EditorGUILayout.HelpBox("Close this window when done", MessageType.Info);
		var scene = SceneView.lastActiveSceneView;
		if (scene != lastScene) {
			if (lastScene != null) lastScene.debugDrawModesUseInteractiveLightBakingData = false;
		}
	}

	void Update() {
		lastScene = SceneView.lastActiveSceneView;
		if (lastScene != null) {
			lastScene.debugDrawModesUseInteractiveLightBakingData = true;
			var sceneCam = lastScene.camera;
			dummySceneInDebugMode.camera.transform.SetPositionAndRotation(sceneCam.transform.position, sceneCam.transform.rotation);
			dummySceneInDebugMode.camera.fieldOfView = sceneCam.fieldOfView;
		}
	}
}