GUIOutSideDraggableSample

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

public class GUIOutSideDraggableSample : MonoBehaviour {

    public Vector2 mousePos;
    Vector2 boxSize = new Vector2(100f, 100f);

    GUIStyle whiteBoxStyle;

    private void Awake()
    {
        whiteBoxStyle = new GUIStyle("box");
        whiteBoxStyle.normal.background = Texture2D.whiteTexture;
    }

    void OnGUI()
    {
        using(new GUILayout.HorizontalScope())
        {
            GUILayout.Label("x");
            GUILayout.TextField(mousePos.x.ToString());
        }

        using (new GUILayout.HorizontalScope())
        {
            GUILayout.Label("y");
            GUILayout.TextField(mousePos.y.ToString());
        }

        // 可動範囲のBox
        GUILayout.Box("", GUILayout.Width(boxSize.x), GUILayout.Height(boxSize.y));

        // BoxのRect取得
        var rect = GUILayoutUtility.GetLastRect();


        // コントロールID取得
        var controlID = GUIUtility.GetControlID(FocusType.Passive);

        var ev = Event.current;
        var eventType = ev.GetTypeForControl(controlID);

        // イベントタイプ別処理
        switch(eventType)
        {
            case EventType.MouseDown:
                {
                    if ( rect.Contains(ev.mousePosition))
                    {
                        GUIUtility.hotControl = controlID;
                    }
                }
                break;

            case EventType.MouseUp:
                {
                    if ( GUIUtility.hotControl == controlID)
                    {
                        GUIUtility.hotControl = 0;
                    }
                }
                break;
        }


        // このGUIがコントロール中の処理
        if ( ev.isMouse && (GUIUtility.hotControl == controlID))
        {
            mousePos = Rect.PointToNormalized(rect, ev.mousePosition);

            // 他のGUI処理を無効化
            ev.Use();
        }

        GUI.Box(new Rect(Rect.NormalizedToPoint(rect, mousePos), Vector2.one), "", whiteBoxStyle);
    }
}