defrain-shooter-unity/Assets/Mirror/Editor/EnterPlayModeSettingsCheck.cs

// Unity 2019.3 has an experimental 'disable domain reload on play'
// feature. keeping any global state between sessions will break
// Mirror and most of our user's projects. don't allow it for now.
// https://blogs.unity3d.com/2019/11/05/enter-play-mode-faster-in-unity-2019-3/
using UnityEditor;
using UnityEngine;

namespace Mirror
{
    public class EnterPlayModeSettingsCheck : MonoBehaviour
    {
        [InitializeOnLoadMethod]
        static void OnInitializeOnLoad()
        {
#if UNITY_2019_3_OR_NEWER
            // We can't support experimental "Enter Play Mode Options" mode
            // Check immediately on load, and before entering play mode, and warn the user
            CheckPlayModeOptions();
#endif

            // Hook this event to see if we have a good weave every time
            // user attempts to enter play mode or tries to do a build
            EditorApplication.playModeStateChanged += OnPlayModeStateChanged;
        }

        static void OnPlayModeStateChanged(PlayModeStateChange state)
        {
            // Per Unity docs, this fires "when exiting edit mode before the Editor is in play mode".
            // This doesn't fire when closing the editor.
            if (state == PlayModeStateChange.ExitingEditMode)
            {
#if UNITY_2019_3_OR_NEWER
                // We can't support experimental "Enter Play Mode Options" mode
                // Check and prevent entering play mode if enabled
                CheckPlayModeOptions();
#endif
            }
        }

#if UNITY_2019_3_OR_NEWER
        static void CheckPlayModeOptions()
        {
            // enabling the checkbox is enough. it controls all the other settings.
            if (EditorSettings.enterPlayModeOptionsEnabled)
            {
                Debug.LogError("Enter Play Mode Options are not supported by Mirror. Please disable 'ProjectSettings -> Editor -> Enter Play Mode Settings (Experimental)'.");
                EditorApplication.isPlaying = false;
            }
        }
#endif
    }
}