Dropzone flickering

We are closing alpha4 soon

I spent some time to dig into Sortable.js library and found that Sortable.js has a feature to detect the nearest empty sortable drop zone when dragging item https://github.com/SortableJS/Sortable/blob/3696da44b57c81dbe441aa48af28...,
I believe this is the reason for flickering issue. I think this feature might not be helpful with nested dropzone, so I think we can disable it by setting emptyInsertThreshold as 0

Added to merge train, thanks.