But I'm not sure what to do when you have two files selected. If one is "A; B" and the other is "B; A", and you add "C" (and optionally drag "C" to a position), what should the new order be of the two files?
Would it be possible to display "<varies>" for each position of the list where the items in that position are not the same? And then allow the same add behaviour—that is, insert the new item to a particular position in the list?
In most cases, only one record is going to be edited at a time. Or if multiple records are selected, the intent would likely be to make the list the same for all. So seeing "varies" is going to be a clue the selection is wrong, or that one record should be edited and the results copied to the rest. I don't see how you could program a more "intelligent" response—the user has to decide.
An alternative would be to simply not allow an edit in this case, but I'm sure some have a use-case for this sort of edit. I suppose the obvious one is you need to add one item to a bunch of records with varying lists. You accept it cannot be added in the "correct" position of each list in one edit, but you're willing to add it to the same position in every list.
What about
changes for multiple files? Will it be possible to select an existing item in the list (including "<varies>" and change it?
BTW, is this going to apply to all list fields? If so, where there be a convenient option to sort the items alphabetically, if it's not an
ordered list?