While dragging and dropping resources between directories in an FTP site should cause those resources to move, dragging from an FTP site to a local directory should cause those resources to be copied instead. This would be consistent with both Mac OS and Windows. My team has often inadvertently removed files from our FTP sites by dragging them onto their desktop.
Please read FAQ:
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Additionally, having the ability to right-click drag like in Windows would be most welcome. In that way, a user may explicitly define whether those resources should be moved or copied as a result of the drag-and-drop operation by selecting an option from the right-click context menu.
Right-click drag is always supported, when dragging between WinSCP panels. When dragging to Windows Explorer, it is supported when WinSCP drag&drop shell extension is installed/enabled: