I have 2 project windows open. I have them arranged so that I can drag and drop files from the project panel in one window to appropriate directory of the second window.
I hold the control key down (to get a copy) and click on the source file, hold and drag to second project. The new target will be highlighted with a red box. When I release the mouse one of three things happens
1. A confirmation pops up for copying the file from the project source to the new target directory, and the file is copied.
2. No confirmation pop up -- Nothing is copied.
3. A confirmation pop up showing the source to be the whole .idea directory from the source directory. Confirming this will actually drop into the target directory the whole .idea directory from the source project.
If the file to copy is already selected then doing a control-click deselects the file. Doing a second control-click on the file will provide a "plus" arrow to drag the file.
I don't know how you spec'ed this but I would prefer control-click of selected or unselected file will yield a "plus" arrow to drag.