Subject: Re: bug#68081: 30.0.50; derived-mode and display-buffer-alist

From: martin rudalics <[email-masked]>
Date: Sat, 30 Dec 2023 10:30:40 +0100

> Thanks.  I tend to document this subtlety, and otherwise leave it
 > alone.

This was not an issue when 'display-buffer' was rewritten in 2011.  It
became an issue when it started to use 'buffer-match-p' in 2022.

A fairly safe fix would be

diff --git a/lisp/info.el b/lisp/info.el
index 51e9eb72edf..e0d35591ee5 100644
--- a/lisp/info.el
+++ b/lisp/info.el
@@ -768,6 +768,12 @@ info
      ;; of names that might have been wrapped (in emails, etc.).
      (setq file-or-node
            (string-replace "\n" " " file-or-node)))
+
+  (unless (or buffer (derived-mode-p 'Info-mode))
+    (setq buffer (get-buffer-create "*info*"))
+    (with-current-buffer buffer
+      (Info-mode)))
+
    (info-setup file-or-node
  	      (pop-to-buffer-same-window (or buffer "*info*"))))

A still conservative but more advanced fix (that should DTRT in the case
no window can be found) would be

diff --git a/lisp/info.el b/lisp/info.el
index 51e9eb72edf..11e228b9bf8 100644
--- a/lisp/info.el
+++ b/lisp/info.el
@@ -768,8 +768,16 @@ info
      ;; of names that might have been wrapped (in emails, etc.).
      (setq file-or-node
            (string-replace "\n" " " file-or-node)))
-  (info-setup file-or-node
-	      (pop-to-buffer-same-window (or buffer "*info*"))))
+
+  (unless buffer
+    (if (derived-mode-p 'Info-mode)
+	(setq buffer "*info*")
+      (setq buffer (get-buffer-create "*info*"))
+      (with-current-buffer buffer
+	(Info-mode))))
+
+  (pop-to-buffer-same-window buffer)
+  (info-setup file-or-node buffer))

  (defun info-setup (file-or-node buffer)
    "Display Info node FILE-OR-NODE in BUFFER."

Otherwise, you could suggest using

(setq display-buffer-alist '(((derived-mode . Info-mode)
                               (display-buffer-in-side-window))
			     ("*info*" (display-buffer-in-side-window))))

as a fallback.

martin