How do I minimize ambiguity with Drag-N'Drop for vertical lists?

Question

Say I have two lists with List A positioned directly above List B (Requirement). Both lists allow Drag-N'-Drop re-ordering, initiated by "long-touching" an item. Auto-scroll functionality also triggers for re-ordering long lists. Finally, items in List B can be dropped into List A.

Here's the problem: The auto-scrolling and List B to List A interactions are too ambiguous (i.e., when to trigger scroll, do I bring-to-front the dragged item, what if the item is half here and half there, etc.).

I thought of mitigating this with a "tap-first-then-hold-down" gesture, but such a gesture would be too close to "tap" & "double-tap" (which are also being used), no? And an "Edit" button would just increase clutter on the screen (minimizing clutter is a Requirement).

Does anyone have any suggestions on how to solve this?

Answer 1

The common fix is to build in some delays. E.g. auto-scrolling on Macs only works in the lower half line or so, and only after you've hovered over that area for a while. That way, a reasonably fast drag from a list A location into list B won't trigger auto-scroll.

Also, make sure there is a reasonable amount of "drop-dead" whitespace between the two lists, so a user who is trying to auto-scroll but overshoots a bit doesn't end up dropping an item in list B by accident.

And finally, engineer forgiveness into your UI. Make sure the most common task is the easiest. Your problem domain and window title decide whether the main action on this screen is moving items between lists or re-arranging inside a list (so, when in doubt, bias your tracking code towards the more common operation).

But auto-scrolling is really just a shortcut for "move to list" and "scroll and move within list repeatedly". So when in doubt, bias against autoscrolling. If it takes a tad too long when someone hovers over the autoscroll area to start scrolling and the user thinks you don't support autoscrolling, they can work around it. If you autoscroll too quickly and users can't re-arrange items because the list keeps jerking up and down, they have no chance to work around that themselves.

Answer 2

If both lists allow re-ordering, why not remove the drag-n-drop ambiguity by disallowing drag-n-drop between lists?

Make moving an item from ListB to ListA a separate action (with a button or whatever). The re-ordering drag-n-drop in ListA can then be used to position the item where it is wanted.