Recording Input strokes in editor mode

Hi, I'm trying to create my own level maker as a tool to run in editor mode. However I'm having problems finding a good way to record inputs. I am able to register prefab-creation for undo, but I also want to record occupied positions in a separate datastructure (or in the undo system if possible) to be able to quickly track open/occupied positions on the board.

The code below just spawns prefabs at a given position whenever the shift key is held down. I want to be able to either reliably detect when the key is not being pressed any more (so that I can populate my own datastructure with positions what are occupied, and record the 'strokes' made within the scene) OR register custom data to the undo-system to achieve the same result, if possible.

         void DuringSceneGUI(SceneView sceneview)
     {
         if (Event.current.shift)
         {
             var cell = GetCell();

             if (GridPositionUtilsXZ.IsWithinGrid(cell, xSize, ySize, gridCellSize))
             {
                 var gameObject = TryInstantiateAtCell(cell));
                 currentStroke.Add(gameObject, cell);
             }
         }
         //Something Like This:
         // if (Event.current.type == EventType.KeyUp)
         // {
         //     occupiedCells.Add(currentStroke, strokeCounter++);
         //     currentStroke.Clear();
         // }
     }