Observe the scene selection rect events without taking over the event with .Use()/HotControl

SelectionHandler1.cs

 public static class SelectionHandler<T> where T : IHandleSelectable, IEquatable<T>
 {
     private static int _selectionHandleHash = nameof(SelectionHandler<T>).GetHashCode();
     public static bool IsSelecting { get; private set; }
     public static Vector2 StartPosition { get; private set; }
     public static Vector2 CurrentPosition{ get; private set; }
     public static HashSet<T> Selections { get; } = new HashSet<T>();

     public delegate void SelectionEvent();

     public static HashSet<T> Scan(IEnumerable<T> selectables)
     {
         var id = GUIUtility.GetControlID(_selectionHandleHash, FocusType.Passive);
         var eventType = Event.current.GetTypeForControl(id);
         if (EditorWindow.focusedWindow.GetInstanceID() != SceneView.currentDrawingSceneView.GetInstanceID())
             return Selections;

         if (eventType == EventType.MouseDown && Event.current.button == 0 && !IsSelecting)
         {
             StartPosition = Event.current.mousePosition;
             CurrentPosition = StartPosition;
             IsSelecting = true;
             Selections.Clear();
         }

         if (!IsSelecting)
             return Selections;
         
        var isMouseUp = GUIUtility.hotControl == 0;
        if (Event.current.isMouse && isMouseUp)
        {
            IsSelecting = false;
        }
        else
        {
            CurrentPosition += new Vector2(Event.current.delta.x, Event.current.delta.y);

            var screenDelta = CurrentPosition - StartPosition;
            var rect = new Rect(StartPosition, screenDelta);

            for (var i = 0; i < SceneView.sceneViews.Count; i++)
            {
                ((SceneView)SceneView.sceneViews[i]).Repaint();
            }

            if (rect.height < 0)
            {
                rect.height = -rect.height;
                rect.y -= rect.height;
            }
            if (rect.width < 0)
            {
                rect.width = -rect.width;
                rect.x -= rect.width;
            }
            
            Select(selectables, rect);
        }

        return Selections;

     }

     private static void Select(IEnumerable<T> selectables, Rect rect)
     {
         foreach (var item in selectables)
         {
             var screenPoint = HandleUtility.WorldToGUIPoint(item.WorldPosition);

             if (rect.Contains(screenPoint))
             {
                 if (!Selections.Contains(item))
                     Selections.Add(item);
             }
             else if (Selections.Contains(item))
             {
                 Selections.Remove(item);
             }
         }
     }
     
     private static void DrawColoredScreenRect(Vector2 screenPoint, Vector2 size, Color color)
     {
         var rect = new Rect(screenPoint, size);
         Handles.BeginGUI();
         GUI.color = color;
         GUI.Box(rect, GUIContent.none);
         GUI.color = Color.white;
         Handles.EndGUI();
     }
 }

SelectionHandler2.cs

// And the normal version

 public static class SelectionHandler<T> where T : IHandleSelectable, IEquatable<T>
 {
     private static int _selectionHandleHash = nameof(SelectionHandler<T>).GetHashCode();
     
     private static Rect _rect;
     public static bool IsSelecting { get; private set; }
     public static Vector2 StartPosition { get; private set; }
     public static Vector2 CurrentPosition{ get; private set; }
     public static HashSet<T> Selections { get; } = new HashSet<T>();

     public delegate void SelectionEvent();

     public static HashSet<T> Scan(GameObject source, IEnumerable<T> selectables)
     {
         if (Selection.activeGameObject != source)
             return Selections;
         
         var id = GUIUtility.GetControlID(_selectionHandleHash, FocusType.Passive);
         var eventType = Event.current.GetTypeForControl(id);

         switch (eventType)
         {
             case EventType.MouseDown:

                 if (!IsSelecting && (Event.current.button == 0))
                 {
                     GUIUtility.hotControl = id;
                     Event.current.Use();
                     StartPosition = Event.current.mousePosition;
                     CurrentPosition = StartPosition;
                     IsSelecting = true;
                     Selections.Clear();
                 }
                 break;
             
             case EventType.MouseUp:

                 if (GUIUtility.hotControl == id && (Event.current.button == 0))
                 {
                     GUIUtility.hotControl = 0;
                     Event.current.Use();
                     EditorGUIUtility.SetWantsMouseJumping(0);
                     IsSelecting = false;
                     _rect = default;
                 }
                 break;
             
             case EventType.MouseDrag:
                 
                 if (IsSelecting)
                 {
                     CurrentPosition += new Vector2(Event.current.delta.x, Event.current.delta.y);

                     var screenDelta = CurrentPosition - StartPosition;
                     _rect = new Rect(StartPosition, screenDelta);
                     
                     if (_rect.height < 0)
                     {
                         _rect.height = -_rect.height;
                         _rect.y -= _rect.height;
                     }
                     if (_rect.width < 0)
                     {
                         _rect.width = -_rect.width;
                         _rect.x -= _rect.width;
                     }
            
                     Select(selectables, _rect);
                     
                     RefreshSceneViews();
                 }

                 break;
             
             case EventType.Repaint:

                 if (IsSelecting)
                 {
                     Handles.BeginGUI();
                     var prev = GUI.color;
                     GUI.color = Color.green;
                     GUI.Box(_rect, GUIContent.none);
                     GUI.color = prev;
                     Handles.EndGUI();
                     RefreshSceneViews();
                 }

                 break;
         }
         



         /*
         if (eventType == EventType.MouseDown && Event.current.button == 0 && !IsSelecting)
         {
             GUILayout
             StartPosition = Event.current.mousePosition;
             CurrentPosition = StartPosition;
             IsSelecting = true;
             Selections.Clear();
         }

         if (!IsSelecting)
             return Selections;
         
        var isMouseUp = GUIUtility.hotControl == 0;
        if (Event.current.isMouse && isMouseUp)
        {
            IsSelecting = false;
        }
        else
        {
            CurrentPosition += new Vector2(Event.current.delta.x, Event.current.delta.y);

            var screenDelta = CurrentPosition - StartPosition;
            var rect = new Rect(StartPosition, screenDelta);

            for (var i = 0; i < SceneView.sceneViews.Count; i++)
            {
                ((SceneView)SceneView.sceneViews[i]).Repaint();
            }

            if (rect.height < 0)
            {
                rect.height = -rect.height;
                rect.y -= rect.height;
            }
            if (rect.width < 0)
            {
                rect.width = -rect.width;
                rect.x -= rect.width;
            }
            
            Select(selectables, rect);
        }*/

        return Selections;

     }

     private static void RefreshSceneViews()
     {
         //var currentId = EditorWindow.focusedWindow.GetInstanceID();
         for (var i = 0; i < SceneView.sceneViews.Count; i++)
         {
             var thisScene = (SceneView) SceneView.sceneViews[i];
             //if (thisScene.GetInstanceID() != currentId)
                 thisScene.Repaint();
         }
     }

     private static void Select(IEnumerable<T> selectables, Rect rect)
     {
         foreach (var item in selectables)
         {
             var screenPoint = HandleUtility.WorldToGUIPoint(item.WorldPosition);

             if (rect.Contains(screenPoint))
             {
                 if (!Selections.Contains(item))
                     Selections.Add(item);
             }
             else if (Selections.Contains(item))
             {
                 Selections.Remove(item);
             }
         }
     }
     
     private static void DrawColoredScreenRect(Vector2 screenPoint, Vector2 size, Color color)
     {
         var rect = new Rect(screenPoint, size);
         Handles.BeginGUI();
         GUI.color = color;
         GUI.Box(rect, GUIContent.none);
         GUI.color = Color.white;
         Handles.EndGUI();
     }
     
     private static void DrawColoredScreenRect(Rect rect, Color color)
     {
         Handles.BeginGUI();
         GUI.color = color;
         GUI.Box(rect, GUIContent.none);
         GUI.color = Color.white;
         Handles.EndGUI();
     }
 }