bug#6204: vc-dir always splits the frame

[Drew Adams]

>  > The downside to adding such parameters in a case where you 
>  > modify existing functions (esp. if used heavily in
>  > existing code) is that it can make it harder
>  > for 3rd-party code to take advantage of the new feature 
>  > and still work with older versions that do not have the
>  > new parameters.
> 
> I suppose that re-using the `other-window' argument of 
> `display-buffer' as Juri proposed will handle the problem
> you describe.

Dunno. As I say, I haven't really been following this.

As a user and someone who has code that must work with multiple versions, I
would like existing calls to `display-buffer' (and to functions that call it) to
do the same thing across versions.

Any additional behavior that is specific to Emacs 24+ I would like to be able to
choose/determine/set in some other way than via the traditional `display-buffer'
signature.

A global variable is useful for that. The default value of the variable would be
such as to obtain the pre-24 behavior (IOW, that would be the default behavior).

If you instead add optional parameters at the end of the list, that's OK too -
same effect, as long as the default behavior (the behavior for the traditional
signature) is the same as (or very close to) the pre-24 behavior.

That way, someone who makes an ordinary, pre-24-style call to the function gets
the traditional behavior. It is only by making a non-pre-24 call or setting a
new variable to a non-default value that you get the new feature (whatever it
might be).

To me, that is the ideal way to handle this kind of thing. You might have some
specific needs and reasons that conflict with that ideal - dunno. For such a
case *especially*, I would encourage you to take the discussion to the wider dev
forum, emacs-devel. That's what it's for.

HTH.