bug#73686: 31.0.50; ERC 5.6.1-git: back button gone from describe-face via erc-nicks-list-faces

["J.P." <jp@neverwas.me>]

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The attached PoC patch blindly tries to implement the approach loosely
theorized about in previous posts. While it seems to solve the issue at
hand (in a vacuum), it may well introduce others because I know
basically nothing about the help system and how it's meant to work.


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From 8acedc71dc1344daac90a79378dad894dcd98bab Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <jp@neverwas.me>
Date: Tue, 8 Oct 2024 15:58:49 -0700
Subject: [PATCH] [POC] Overload help-xref-following for non-default help
 buffers

* lisp/help-mode.el (help-xref-following): Mention in doc that value may
be the symbol of a major mode.
(help-buffer): When the current buffer is in `fundamental-mode', check
to see if the value of `help-xref-following' is the symbol of a major
mode deriving from `help-mode' and `help-buffer-under-preparation' is
non-nil.  If so, return the current buffer.
(help-xref-go-back, help-xref-go-forward): Use `help-do-xref' instead of
`apply' to call methods.
(help-do-xref): Bind `help-xref-following' to the value of `major-mode'
instead of t.
* test/lisp/help-mode-tests.el
(help-mode-tests-help-buffer-current-buffer): Add case for
`help-xref-following' being a major-mode rather than t.  (Bug#73686)
---
 lisp/help-mode.el            | 23 +++++++++++++++--------
 test/lisp/help-mode-tests.el | 14 +++++++++++++-
 2 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/lisp/help-mode.el b/lisp/help-mode.el
index 4ee4f4156a1..f704098b2dc 100644
--- a/lisp/help-mode.el
+++ b/lisp/help-mode.el
@@ -516,19 +516,26 @@ help-setup-xref
     (setq help-xref-stack-item item)))
 
 (defvar help-xref-following nil
-  "Non-nil when following a help cross-reference.")
+  "Non-nil when following a help cross-reference.
+May be the value of `major-mode' when an instigating button was clicked.")
 
 ;;;###autoload
 (defun help-buffer ()
   "Return the name of a buffer for inserting help.
-If `help-xref-following' is non-nil and the current buffer is
-derived from `help-mode', this is the name of the current buffer.
-
+If `help-xref-following' is t and the current buffer is derived from
+`help-mode', return the current buffer's name.  As a special case, also
+do so if these three conditions are met: `help-xref-following' is the
+symbol of a major mode deriving from `help-mode', the current buffer is
+in `fundamental-mode', and `help-buffer-under-preparation' is non-nil.
 Otherwise, return \"*Help*\", creating a buffer with that name if
 it does not already exist."
   (buffer-name                         ;for with-output-to-temp-buffer
    (if (and help-xref-following
-            (derived-mode-p 'help-mode))
+            (if (or (eq help-xref-following t)
+                    (not (and help-buffer-under-preparation
+                              (eq major-mode 'fundamental-mode))))
+                (derived-mode-p 'help-mode) ; current buffer
+              (provided-mode-derived-p help-xref-following '(help-mode))))
        (current-buffer)
      (get-buffer-create "*Help*"))))
 
@@ -792,7 +799,7 @@ help-xref-go-back
 	      position (car item)
 	      method (cadr item)
 	      args (cddr item))))
-    (apply method args)
+    (help-do-xref nil method args)
     (with-current-buffer buffer
       (if (get-buffer-window buffer)
 	  (set-window-point (get-buffer-window buffer) position)
@@ -812,7 +819,7 @@ help-xref-go-forward
 	      position (car item)
 	      method (cadr item)
 	      args (cddr item))))
-    (apply method args)
+    (help-do-xref nil method args)
     (with-current-buffer buffer
       (if (get-buffer-window buffer)
 	  (set-window-point (get-buffer-window buffer) position)
@@ -894,7 +901,7 @@ help-do-xref
 Things are set up properly so that the resulting help buffer has
 a proper [back] button."
   ;; There is a reference at point.  Follow it.
-  (let ((help-xref-following t))
+  (let ((help-xref-following major-mode))
     (apply function (if (eq function 'info)
                         (append args (list (generate-new-buffer-name "*info*")))
                       args))))
diff --git a/test/lisp/help-mode-tests.el b/test/lisp/help-mode-tests.el
index 2e64b12732e..968ed7da72e 100644
--- a/test/lisp/help-mode-tests.el
+++ b/test/lisp/help-mode-tests.el
@@ -39,7 +39,19 @@ help-mode-tests-help-buffer-current-buffer
     (help-mode)
     (let ((help-xref-following t))
       (should (equal (buffer-name (current-buffer))
-                     (help-buffer))))))
+                     (help-buffer)))))
+
+  ;; Returns the current `fundamental-mode' buffer when an xref button
+  ;; is clicked and the resulting action function, such as the various
+  ;; `describe-*' commands, binds `help-buffer-under-preparation' to
+  ;; something non-nil.
+  (should (eq major-mode 'fundamental-mode))
+  (let ((help-xref-following 'help-mode)
+        (help-buffer-under-preparation t))
+    (should (equal (buffer-name (current-buffer))
+                   (help-buffer)))
+    (with-current-buffer "*scratch*"
+      (should (equal "*Help*" (help-buffer))))))
 
 (ert-deftest help-mode-tests-make-xrefs ()
   (with-temp-buffer
-- 
2.46.2