Editor drag&drop malfunction

?

In the normal way, the editor allows me to select a string of characters and then to use the mouse pointer to drag them to another place on the page.  For example, suppose I have a series of comma-separated variables in a single declaration:  I might want to select one of them, with its trailing comma, and use the mouse pointer to drag it away from its present position, to a different place in the sequence.  Equally, I might want to select an entire line, then leave it where it is, but drag A COPY OF it into another place on the page.

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.