Sorta fixed...

scDrawMenuInventory.gml

///@desc draw inventory menu

var _inventoryRows = 5; //***update to dynamically match inventoryCapacity
var _inventoryCols = 4;
var _inventorySlotSprite = spInventorySlot;
var _highlightedInventorySlotSprite = spInventorySlotHighlighted;
var _inventorySlotWidth = sprite_get_width(_inventorySlotSprite);
var _inventorySlotHeight = sprite_get_height(_inventorySlotSprite)
var _itemsInInventory = ds_list_size(oControllerEntity.player.inventory);

var _inventoryItemToShow = 0;
var _currentDrawX = 0;
var _currentDrawY = 0;

var _trueStartX = menuStartX + menuEdgeIndent;
var _trueStartY = menuStartY + menuEdgeIndent;

var _mouseOffsetX  = device_mouse_x_to_gui(0) - _trueStartX ;
var _mouseOffsetY = device_mouse_y_to_gui(0) - _trueStartY ;

var _mouseHighlightCol = floor(_mouseOffsetX / _inventorySlotWidth);
var _mouseHighlightRow = floor(_mouseOffsetY / _inventorySlotHeight);

draw_text(50,10,"menuStartX, menuStartY: " + string(menuStartX) + " " + string(menuStartY));
draw_text(50,20,"_trueStartX, _trueStartY: " + string(_trueStartX) + " " + string(_trueStartY));
draw_text(50,30,"mouse_x, mouse_y: " + string(mouse_x) + " " + string(mouse_y));
draw_text(50,40,"offset_x, offset_y: " + string(_mouseOffsetX) + " " + string(_mouseOffsetY));
draw_text(50,50,"mouseCol, mouseRow: " + string(_mouseHighlightCol) + " " + string(_mouseHighlightRow));

//draw inventory grid by creating slots in position
for (var _i = 1; _i <= _inventoryCols; _i++){
	for (var _j = 1; _j <= _inventoryRows; _j++){
		
		//determine where to draw
		_currentDrawX = _trueStartX + ( _inventorySlotWidth * _j);
		_currentDrawY = _trueStartY + (_inventorySlotHeight * _i)
		
		//draw slot
		if (_mouseHighlightCol == _i and _mouseHighlightRow == _j)
		{
			draw_sprite(_highlightedInventorySlotSprite, -1,_currentDrawX , _currentDrawY );	
		} else {
			draw_sprite(_inventorySlotSprite, -1,_currentDrawX , _currentDrawY );	
		}
		
		//draw_sprite(_inventorySlotSprite, -1,_currentDrawX , _currentDrawY );
		
		
		//draw items, if any left to show
		if _inventoryItemToShow < _itemsInInventory {
			var _entityNumber = oControllerEntity.player.inventory[| _inventoryItemToShow];
			//ds_list_find_value()
			draw_sprite_ext( scGetEntityStat(_entityNumber, actorStat.sprite), scGetEntityStat(_entityNumber, actorStat.subimage), _currentDrawX, _currentDrawY, 1, 1, 0, scGetEntityStat(_entityNumber, actorStat.colour), 1) ;
			
			_inventoryItemToShow++; //increment counter
		}
	}
}