As using a scroll wheel on a mouse causes the sizes of the thumbnails to become bigger or smaller, there are no means to scroll within the thumbnail window. Working with large numbers of pictures this is very annoying to have to cumbersomely try to drag the scroll handle or manage to click above or below the scroll handle. Thus a simple solution would be to introduce keyboard shortcuts for moving a page up or down. Many keyboards offer such dedicated keys. Offering at least a menu command would also already help all Mac users as it is imple to add a keyboard shortcut to such menu commands via macOS System Settings...