Make *Buffer List* always appear in horizontal split

Question

I know Emacs tries to be intellectual and opens its helper buffers depending on which dimension of the window is bigger, so it may appear in vertical split window if current width is bigger than height, and in horizontal split otherwise.

But I’d prefer it to open that list always in horizontal split, because there are long paths I can’t see when the buffer is placed in vertical split. How can I do this?

Answer 1

I believe you've got the horizontal/vertical split terminology back to front (I can never remember which is which either), but as your goal was to retain the original window's width, I'm forcing a vertical split.

See C-hf split-window-sensibly RET. It tells you what to do:

You can enforce this function to not split WINDOW horizontally,
by setting (or binding) the variable `split-width-threshold' to
nil.  If, in addition, you set `split-height-threshold' to zero,
chances increase that this function does split WINDOW vertically.

So as a permanent setting:

(setq split-width-threshold nil)
(setq split-height-threshold 0)

For just a specific function, you can advise that function (but see Edit 2 below!):

(defadvice list-buffers (around list-buffers-split-vertically)
  "Always split vertically when displaying the buffer list.
See `split-window-sensibly'."
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
    ad-do-it))
(ad-activate 'list-buffers)

Edit: Actually, in this instance I suspect you're only concerned with the interactive case, in which case it's preferable to define a function and remap the bindings:

(defun my-list-buffers-vertical-split ()
  "`list-buffers', but forcing a vertical split.
    See `split-window-sensibly'."
  (interactive)
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
    (call-interactively 'list-buffers)))

(global-set-key [remap list-buffers] 'my-list-buffers-vertical-split)

Edit 2: And Stefan points out that display-buffer-alist facilitates such things without advising functions (and of course avoiding unnecessary advice is always a good thing). I believe we still need a custom action, so:

(defun my-display-buffer-pop-up-same-width-window (buffer alist)
  "A `display-buffer' ACTION forcing a vertical window split.
    See `split-window-sensibly' and `display-buffer-pop-up-window'."
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
    (display-buffer-pop-up-window buffer alist)))

(add-to-list 'display-buffer-alist
             '("\\*Buffer List\\*" my-display-buffer-pop-up-same-width-window))