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