The FolderSizes main window detail view supports drag and drop operations for files and folders.
File and folders can be dragged from within the detail view, and then dropped onto itself (e.g. into a detail view folder), the folder browser panel, or the scan data panel.
By default, objects that are dragged and dropped will be moved. If you're prefer to copy the object(s) instead, hold the CTRL key down during the drag and drop operation.