special-display-function

special-display-function is a variable defined in `window.el'.
Its value is

special-display-popup-frame

• This variable is obsolete since 24.3;
  use `display-buffer-alist' instead.
• This variable may be risky if used as a file-local variable.

Documentation:
Function to call for displaying special buffers.
This function is called with two arguments - the buffer and,
optionally, a list - and should return a window displaying that
buffer. The default value usually makes a separate frame for the
buffer using `special-display-frame-alist' to specify the frame
parameters. See the definition of `special-display-popup-frame'
for how to specify such a function.

A buffer is special when its name is either listed in
`special-display-buffer-names' or matches a regexp in
`special-display-regexps'.

The specified function should call `display-buffer-record-window'
with corresponding arguments to set up the quit-restore parameter
of the window used.

You can customize this variable.