DrawPersistentHandle.cs

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

public class DrawHandle : ScriptableObject
{
	[SerializeField] Vector3 m_Position = Vector3.zero;

	[MenuItem("Tools/Draw Handle")]
	static void Init()
	{
		DrawHandle[] existing = Resources.FindObjectsOfTypeAll<DrawHandle>();

		for(int i = 0; i < existing.Length; i++)
			Object.DestroyImmediate(existing[i]);

		// If it was open, close it. Otherwise init.
		if(existing.Length > 0)
			return;
	
		ScriptableObject.CreateInstance<DrawHandle>();
	}

	void OnEnable()
	{
		SceneView.onSceneGUIDelegate += OnSceneGUI;
	}

	void OnDisable()
	{
		SceneView.onSceneGUIDelegate -= OnSceneGUI;
	}

	void OnSceneGUI(SceneView view)
	{
		m_Position = Handles.PositionHandle(m_Position, Quaternion.identity);
	}
}