This is WinSCP 5.9.3 build 7136. The isuse has existed since 5.9.2 when I upgraded from a seriously old version of WinSCP and encountered this effect.
This is installed on Windows 7x64.
When it happens, there is no error message.
Steps to recreate:
1) Open WinSCP and get to the Login window. The main window is behind the Login window.
2) Left click on the Login window title bar and attempt to move the window.
3) At the slightest movement of the Login window, the background main window files off the screen in the general direction that the Login Window was moved. Example, attempt to move left, the main window flies off the screen to the left.
4) After this, the main window is no longer accessible and must be process killed from Task Manager.
5) If the login window is not moved, one is able to log in and use WinSCP.