Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

I'd just like to give some details about some errors which I reported a few moments ago.

WinSCP version: 6.5 and 6.5.6.
System: Window 10
Transfer Protocl: SFTP
Reproduction steps:
(I don't know which of the steps below are really relevant for the issue)

1. Set WinSCP to commander mode (not sure if this is needed).
   
2. In the Login dialogue create a new folder (not sure if this is needed).
   
3. Save 7 SFTP sessions to similar IPv4-Adresses in that folder, with password stored.
   
4. Right click the new folder and select open, to open the 7 tabs for the 7 sessions.
   
5. Click at each tab and let WinSCP connect to the server and display the folder.
   
6. Hover with the mouse over the close-button (x) of the left-most tab. Now left-click very fast at least 7 time at the close-button.
   
7. Error window appears: "List index out of bounds (-1)", Content:
   Stack trace:
   (00B8C039) _winscp_query
   (0008822F) ntdll.dll
   (00074711) ntdll.dll.KiUserExceptionDispatcher
   (00C9D7C1) TSecureShell::EventSelectLoop
   (00C9C0FC) TSecureShell::Close
   (00CF1F87) TSFTPFileSystem::Close
   (000CA38F) TTerminalManager::FreeActiveTerminal
   (00BB2D62) TThemePageControl::Dispatch
   (003A356D) Vcl::Controls::TWinControl::WndProc
   (003A2A78) Vcl::Controls::TWinControl::MainWndProc
   (00204C90) System::Classes::_18364
   (00040C09) USER32.dll
   (000372C5) USER32.dll
   (000369F1) USER32.dll.GetSystemMetricsForDpi
   (000351F6) USER32.dll.CallWindowProcW
   (00539473) Dragdrop::TDragDrop::WndMethod
   (00204C90) System::Classes::_18364
   (00040C09) USER32.dll
   (000372C5) USER32.dll
   (00035B6B) USER32.dll
   (0003569B) USER32.dll.DispatchMessageW
   
   (001EADF3) System::Classes::TList::Get
   (001EADF3) System::Classes::TList::Get
   (000CA38F) TTerminalManager::FreeActiveTerminal
   (00BB2D62) TThemePageControl::Dispatch
   (003A356D) Vcl::Controls::TWinControl::WndProc
   (003A2A78) Vcl::Controls::TWinControl::MainWndProc
   (00204C90) System::Classes::_18364
   (00040C09) USER32.dll
   (000372C5) USER32.dll
   (000369F1) USER32.dll.GetSystemMetricsForDpi
   (000351F6) USER32.dll.CallWindowProcW
   (00539473) Dragdrop::TDragDrop::WndMethod
   (00204C90) System::Classes::_18364
   (00040C09) USER32.dll
   (000372C5) USER32.dll
   (00035B6B) USER32.dll
   (0003569B) USER32.dll.DispatchMessageW