Select a file (or multiple) in the right pane
Drag the selected file(s) to another folder in the left pane
While dragging, the file icon(s) shrink and condense into one, and a plus-sign appears
Hover over the target directory, release the mouse
Quite often, PL will leave behind the icon(s) of the moved file(s) in the source directory. Jumping out and back will usually correct that. But this time, I noticed the files were still there, and this was confirmed by terminal, finder, and another attempt at a move. The second attempt actually had PL prompt me “Do you want to replace?”
After playing with it some more, it really seems to happen most often with multiple files selected.
A plus sign means a copy. I did a quick test on Mac which shows that holding the Command key down during the drag removes the plus, making it a move. I would assume on Windows it would be the Control key, though perhaps another modifier.
On windows, if you drag files from one folder to another on the same physical drive the operation is a MOVE. If the drag is from one physical drive to a different physical drive then the operation is a COPY. Not sure if this is true for Mac.
On windows, if you drag using the right mouse button then when you drop the files you will be presented with a menu to select Copy, Move and a couple of other options. So when in doubt, drag with the right mouse button.
Ok I can close this issue now. File drag-and-drop in the PhotoLibrary seems to be a copy by default, but with the Command key it becomes a move.
I don’t know if this is documented anywhere, but it definitely seems to be a change with the advent of PL 4. The key to understanding this was the comment above by zkarj, who pointed out the plus-sign meant copy.