How can you specify the z-index for a sortable list so that the first
list element is the highest z-index and the last list element is the
lowest. I would also like the z-index to update itself as the user
resorts each list item. So if the bottom list element is moved to the
top, it's z-index would update to be the highest number and so on.

Is this possible?

I have a dropdown menu in each list item so when list items are moved up the dropdown will fall behind the list item below it.