bug#74246: [PATCH] Reuse display windows in image-dired

[martin rudalics]

>> When using image-dired, one can use SPC (image-dired-display-next)
>> repeatedly to display successive images in a dedicated buffer.  However,
>> it seems to choose a different window each time I press SPC.
>>
>> I can reproduce this in "emacs -Q" when I have 4 windows.
>>
>> The following patch works on my system.
>
> Thanks.
>
> Martin and Stefan, any comments or suggestions?

It's a problem we've seen in other contexts before: For historical
reasons 'display-buffer' by default tries to use the least recently used
window which changes whenever we display an image in it.  Juri invented
the 'some-window' buffer display option and I quote the corresponding
entry from the Elisp manual here:

     The above describes the behavior when the ‘some-window’ ALIST entry
     is ‘lru’ or ‘nil’ which is the default.  Another possible value is
     ‘mru’.  If, for example, ‘display-buffer-base-action’ is customized
     to ‘(nil . ((some-window . mru)))’, then this function will prefer
     the most recently used window.  This will try to display several
     buffers from consecutive calls of ‘display-buffer’ in the same
     window.  Consider a configuration of three or more windows where a
     user wants to consult, in a non-selected window, one after the
     other, the results of a query spread among several buffers.  With
     the ‘lru’ strategy, Emacs may continuously choose another window
     because the least recently used window changes with every call of
     ‘display-buffer-use-some-window’.  With the ‘mru’ strategy, the
     window chosen would always remain the same, resulting in a
     predictable user experience.

So I think that Morgan should try to use this approach first.  But if
the image is always displayed in 'image-dired-image-display' mode then
the appropriate action function 'image-dired' should use is
'display-buffer-reuse-mode-window'.

+  (let* ((buf (get-buffer image-dired-display-image-buffer))
+         (windows (and buf
+                       (get-buffer-window-list buf t t)))
+         (cur-win (selected-window)))
     (when buf
       (kill-buffer buf))
     (when-let ((buf (find-file-noselect file nil t)))
+      (dolist (window windows)
+        (set-window-buffer window buf))

Is this supposed to show the same image in all windows that previously
displayed 'image-dired-display-image-buffer'?

FWIW, I would rewrite 'image-dired-display-image' as the (largely
untested)

(defun image-dired-display-image (file &optional _ignored)
  "Display image FILE in the image buffer window.
If FILE is an image, the window will use `image-dired-image-mode'
which is based on `image-mode'."
  (declare (advertised-calling-convention (file) "29.1"))
  (setq file (expand-file-name file))
  (when (not (file-exists-p file))
    (error "No such file: %s" file))
  (let* ((buffer (get-buffer-create image-dired-display-image-buffer))
         (window (get-buffer-window buffer)))
    (find-file-noselect-1 buffer file nil t nil nil)
    (with-current-buffer buffer
      (if (string-match (image-file-name-regexp) file)
          (image-dired-image-mode)
        ;; Support visiting PDF files.
        (normal-mode)))

    (unless window
        (display-buffer buffer))))

instead of doing all that 'kill-buffer' (which could delete the selected
window as a side-effect so the final 'select-window' will throw an
error) 'rename-buffer' rigmarole.

martin