VIM scroll bar is missing scroll box

Thanks for reading.
I am using Terminal VIM and it works very well.
And the scroll bar for Terminal works fine.
But when I run VIM (from Terminal or batch file),
the scroll bar has no "scroll box".
The "scroll box" is that small rectangle indicating
the location of the current window.

Batch file:
@echo off
chdir C:/cygwin64/bin
start mintty.exe -p 0,0 /usr/bin/vim.exe %1

How can I get the scroll bar working for Terminal VIM?

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