I would normally expect the drag and drop action simply to move the selected string to its new position.  I would also expect, if I hold down the Ctrl key (or in MacOS maybe the Cmd key) for the drag and drop operation to leave the selected string where it is but to move a new copy of it also to the drop-off point.  Some software attaches a '+' symbol to the mouse pointer during the drag-and-drop action, when the Ctrl key is being held down, to signify the addition of a COPY of the text being dragged.

These actions seem to be partially-performed by the SF editor but not as described above. 

Drag-and-drop without the Ctrl key attaches a plus sign, when it ought not to, and drags a copy, when it should drag the selected string itself. 

Ctrl-Drag-and-drop fails to attach a plus sign, when it would make sense to have one, but does also drag a copy.

I can't find a way to drag the string itself rather than a new copy of it.
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I hope this is clearer.
John.

Would you be able to explain this behavior in detail a bit more?  I'm trying to sort out what exact buttons you're pressing each time on macOS to perform these operations, including keyboard and mouse operations.  I'm not sure I understand exactly where the issue is.

• Drag&drop        : MOVEs a selected string from its origin to its destination.

• Ctrl-Drag&drop : DUPLICATEs a ditto from/to ditto.

SF's editor in MacOS seemed at first to have these actions reversed.  But on further investigation, I now think:

- It is DUPLICATING in both cases, which seems wrong, AND
- It shows a red '+' symbol during the plain Drag&Drop action, which also seems wrong.

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John