How can I prevent emacs from opening a new window for a buffer I already have open in this session?

Question

I typically have two emacs frames open at once.

When I run vc-diff a new window is created, regardless of whether a window is displaying that buffer already.

Is there a general way to prevent this occurring?

Answer 1

Setting pop-up-frames to a non-nil value will prevent pop-to-buffer from opening a new window. Instead the *vc-diff* buffer will become the active window in the frame that is displaying it.

(setq pop-up-frames t)

Changing this variable will affect other functions that use pop-to-buffer as well.

Answer 2

I think you need to customize display-buffer-base-action to display-buffer--other-frame-action by adding

(custom-set-variables
 '(display-buffer-base-action display-buffer--other-frame-action))

to your .emacs.el or maybe

(custom-set-variables
 '(display-buffer-base-action 
   '(display-buffer-reuse-window (reusable-frames . t))))

I suggest reading help on all variables ( Ch v ) and functions ( Ch f ) mentioned above.

Good luck!