I am trying to test the demo, but it asks me about Emacs C sources? Does it mean 
I have to download source code to check this demo out?

I also get error if window is too small:

Debugger entered--Lisp error: (error "Window #<window 7 on quick-edit-demo.el> too small for splitting")
  signal(error ("Window #<window 7 on quick-edit-demo.el> too small for splitting"))
  error("Window %s too small for splitting" #<window 7 on quick-edit-demo.el>)
  byte-code("\204\222

Would be cool to test it :).

> From: deng@randomsample.de
> To: eliz@gnu.org
> Subject: Re: "Adobe Brackets like" editing in emacs
> Date: Thu, 20 Mar 2014 17:59:50 +0100
> CC: emacs-devel@gnu.org
> 
> Eli Zaretskii writes:
> > I suggested to have this feature first based on the existing
> > infrastructure, i.e. in another window.  This should be easy to
> > implement, and will allow collecting user experience which we
> > currently lack.  Then decision of whether we need a new UI, and which
> > one, will be based on something, rather than on thin air.
> 
> OK, I've hacked up a little demonstrator. Load the attached file and
> eval-buffer it (preferably with 'emacs -Q', that's how I tested it).
> Then move point over an Emacs function; hitting 'C-c e' will open the
> quick-edit window with the function's definition, and 'C-c c' will close
> it.
> 
> Problems I've noticed so far:
> 
> - mode-line-format is buffer-local, so you cannot deactivate it only for
>   the upper window. That means, while in quick-edit, you will have no
>   mode-line at all.
> 
> - You cannot set a different background color for a complete window. You
>   can only do that with an overlay, but that will only cover the parts
>   with text.
> 
> - Of course, you can easily mess things up be leaving/closing the window
>   otherwise.
> 
> In a nutshell, it'd be nice to have something like 'window-local'
> variables for this.
> 
> -David
>