draw_in_sceneview.cs

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

namespace EngineTools
{
    [InitializeOnLoad]
    public class ShowFlagsWindow : Editor
    {
        static ShowFlagsWindow()
        {
            SceneView.onSceneGUIDelegate -= OnSceneGUI;
            SceneView.onSceneGUIDelegate += OnSceneGUI;
        }

        private static void OnSceneGUI(SceneView sceneView)
        {
            DrawToolMenu(sceneView);
        }

        private static void DrawToolMenu(SceneView sceneView)
        {
            Handles.BeginGUI();
            GUILayout.BeginArea(new Rect(0, sceneView.position.height - 35, sceneView.position.width, 20), EditorStyles.toolbar);
            GUILayout.BeginHorizontal();

            if (GUILayout.Button("ShowFlags", EditorStyles.toolbarButton, GUILayout.Width(100)))
            {
                Debug.Log("test");
            }

            GUILayout.EndHorizontal();
            GUILayout.EndArea();
            Handles.EndGUI();
        }
    }
}