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VIM: runtime/menu.vim

A general prerequisite for using menu.vim is that FEAT_MENU is defined.
It is sourced from the function gui_init() in gui.c.
$VIMRUNTIME/menu.vim is (also?) sourced if :filetype on is executed in vim with a GUI.
This file is not sourced if M is included in guioptions. Apparently, otherwise, even if using -U NONE -u NONE, menu.vim would be sourced.
The path of menu.vim is stored in the macro SYS_MENU_FILE which is defined in one of the os_*.h file, for example in os_dos.h: 
#ifdef FEAT_GUI
  …
# ifndef SYS_MENU_FILE
#  define SYS_MENU_FILE		"$VIMRUNTIME\\menu.vim"
# endif
#endif

See also

The FEAT_MENU macro.

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