Undocumented menu feature?

[Sergey Organov]

Hello,

I've asked this in help-gnu-emacs, but nobody seems to have a clue
there. Hopefully some developer(s) knwow.

Here is the reference:

http://lists.gnu.org/archive/html/help-gnu-emacs/2014-10/msg00007.html

And here is my original question:

I suddenly found that my emacs (GNU Emacs 23.2.1, running on
Debian/Gnome) is capable of "detaching" some sub-menus and putting them
into separate frames. For example, if I click "File" in the main menu,
submenu appears containing dashed line at the top, and if I click on
that dashed line, entire "File" submenu appears in a separate frame.

I can't find anything about the feature by googling for it. Could somebody
please point me to relevant documentation?

Thanks.

-- 
Sergey.