Since 5.14, it is always possible to use drag&drop to move remote files to external applications, while copy is always the default operation.

Thanks for your report.

This is unfortunately true. There's nothing we can do about it.

Though:
1) You should be using WinSCP drag&drop shell extension. When using it, this cannot happen.
https://winscp.net/eng/docs/dragext

2) If you do not want/cannot use the extension, you should not be moving files with drag&drop. It is disabled by default (you must have enabled it previously), also for this reason.
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Hello,
please, follow this procedure to reproduce this bug:

1. drag file (use a big one) from WinSCP
2. drop into Windows Explorer folder pressing SHIFT key to move it (it happens with copy too, but with move is even worse)
3. WinSCP starts to copy the file in tmp folder
4. While WinSCP copies file to tmp, change folder in Windows Explorer, or close Windows Explorer window
5. WinSCP finishes the copy of file in tmp folder
6. The file IS NOT copied from tmp folder to destination Windows Explorer folder
7. The file is deleted from tmp folder
8. The file is lost :'(

Installations where I reproduced the bug:
Windows 7 64bit + WinSCP 5.1.0 (Build 2625)
Windows 7 32bit + WinSCP 4.3.6 (Build 1655)
Windows 7 32bit + WinSCP 5.1.1 (Build 2735)

WinSCP settings:
Interface: Explorer
Drag & Drop using temporary folder
File protocol: SFTP

Thanks.