commit https://github.com/vim/vim/commit/8b877ac38e96424a08a8b8eb713ef4b3cf0064be Author: Bram Moolenaar <Bram@vim.org> Date: Mon Mar 28 19:16:20 2016 +0200 patch 7.4.1669 Problem: When writing buffer lines to a pipe Vim may block. Solution: Avoid blocking, write more lines later.

" Settings for test script execution
" Always use "COMMAND.COM", don't use the value of "$SHELL".
set shell=c:\COMMAND.COM shellquote= shellxquote= shellcmdflag=/c shellredir=>
" This is used only when the +eval feature is available.
if executable("cmd.exe")
   set shell=cmd.exe
endif