Well-spotted: The behaviour as described does indeed concern making a copy of a file within the same folder. I should have mentioned this above. Within the same folder, dragging and pressing Ctrl to copy a file is simply ignored by Thunar. Just to be on the safe side, I have tested both dragging and then pressing and holding Ctrl, and first pressing and holding Ctrl and then dragging. It shouldn't make a difference, but neither works.
Dear Lionel
Well-spotted: The behaviour as described does indeed concern making a copy of a file within the same folder. I should have mentioned this above. Within the same folder, dragging and pressing Ctrl to copy a file is simply ignored by Thunar. Just to be on the safe side, I have tested both dragging and then pressing and holding Ctrl, and first pressing and holding Ctrl and then dragging. It shouldn't make a difference, but neither works.