disabled option still allows an element to move

I see with `disabled` option the element is not draggable anymore but still moves when I drag another element over it. How can I make it "fixed" position and not let it change the order.
In my case, I need to set the first item fixed, so it should not be draggable and unsortable.

Start by tracing how the `disabled` option is handled during sorting and how an item changes order when another item is dragged over it. Reproduce the case with the first item disabled, then verify that it remains fixed, cannot be dragged, and does not change position when the other items are reordered.

Written by the indexing model from the issue text.