Remove the header line after disabling 'which-function-mode'

Previously, the header line would stay around even when after
disabling 'which-function-mode', although it may be empty.  Now
the 'which-function-mode' element is properly removed from
'header-line-format', so the header line will disappear if
there's nothing else in 'header-line-format'.

Also, previously, when we ran (which-function-mode), we would
enable
'which-function-mode' for all buffers even if they didn't support
imenu.  We didn't run the normal logic in 'which-func-ff-hook' to
disable 'which-func-mode' if imenu wasn't present.  Now we do run
that logic, by just calling 'which-func-ff-hook'.  This is
especially important when the header line is enabled, because
otherwise there's a very noticeable header line added to every
buffer, including e.g. *Help* and *Buffer List*.

Also, we now check that 'header-line-format' is a list before trying
to add to it; this makes us work properly when enabling and
disabling 'which-function-mode' for modes which set
'header-line-format' to a string or symbol, such as eww.

* lisp/progmodes/which-func.el (which-func-try-to-enable): Re-add
'which-func-format' to the header line.
(which-func--header-line-remove): New function.
(which-func--disable): Call 'which-func--header-line-remove'.
(which-function-mode): Call 'which-func-ff-hook' and
'which-func--header-line-remove'. (bug#66283)
* test/lisp/progmodes/which-func-tests.el: New test.

2 files changed, 82 insertions, 15 deletions

diff --git a/lisp/progmodes/which-func.el b/lisp/progmodes/which-func.el
index 09d0250515f..0e04bab6ea4 100644
--- a/lisp/progmodes/which-func.el
+++ b/lisp/progmodes/which-func.el
@@ -208,21 +208,28 @@ non-nil.")
 (add-hook 'after-change-major-mode-hook #'which-func-ff-hook t)
 
 (defun which-func-try-to-enable ()
-  (unless (or (not which-function-mode)
-              (local-variable-p 'which-func-mode))
-    (setq which-func-mode (or (eq which-func-modes t)
-                              (member major-mode which-func-modes)))
-    (setq which-func--use-mode-line
-          (member which-func-display '(mode mode-and-header)))
-    (setq which-func--use-header-line
-          (member which-func-display '(header mode-and-header)))
-    (when (and which-func-mode which-func--use-header-line)
+  (when which-function-mode
+    (unless (local-variable-p 'which-func-mode)
+      (setq which-func-mode (or (eq which-func-modes t)
+                                (member major-mode which-func-modes)))
+      (setq which-func--use-mode-line
+            (member which-func-display '(mode mode-and-header)))
+      (setq which-func--use-header-line
+            (member which-func-display '(header mode-and-header))))
+    ;; We might need to re-add which-func-format to the header line,
+    ;; if which-function-mode was toggled off and on.
+    (when (and which-func-mode which-func--use-header-line
+               (listp header-line-format))
       (add-to-list 'header-line-format '("" which-func-format " ")))))
 
-(defun which-func--disable ()
-  (when (and which-func-mode which-func--use-header-line)
+(defun which-func--header-line-remove ()
+  (when (and which-func-mode which-func--use-header-line
+             (listp header-line-format))
     (setq header-line-format
-          (delete '("" which-func-format " ") header-line-format)))
+          (delete '("" which-func-format " ") header-line-format))))
+
+(defun which-func--disable ()
+  (which-func--header-line-remove)
   (setq which-func-mode nil))
 
 (defun which-func-ff-hook ()
@@ -288,9 +295,11 @@ in certain major modes."
   (when which-function-mode
     ;;Turn it on.
     (setq which-func-update-timer
-          (run-with-idle-timer idle-update-delay t #'which-func-update))
-    (dolist (buf (buffer-list))
-      (with-current-buffer buf (which-func-try-to-enable)))))
+          (run-with-idle-timer idle-update-delay t #'which-func-update)))
+  (dolist (buf (buffer-list))
+    (with-current-buffer buf
+      (which-func--header-line-remove)
+      (which-func-ff-hook))))
 
 (defvar which-function-imenu-failed nil
   "Locally t in a buffer if `imenu--make-index-alist' found nothing there.")
diff --git a/test/lisp/progmodes/which-func-tests.el b/test/lisp/progmodes/which-func-tests.el
new file mode 100644
index 00000000000..73709f1c5e5
--- /dev/null
+++ b/test/lisp/progmodes/which-func-tests.el
@@ -0,0 +1,58 @@
+;;; which-func-tests.el --- tests for which-func     -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; Author: Spencer Baugh <sbaugh@catern.com>
+
+;; This file is part of GNU Emacs.
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+(require 'ert)
+(require 'which-func)
+
+(ert-deftest which-func-tests-toggle ()
+  (let ((which-func-display 'mode-and-header) buf-code buf-not)
+    (setq buf-code (find-file-noselect "which-func-tests.el"))
+    (setq buf-not (get-buffer-create "fundamental"))
+    (with-current-buffer buf-code
+      (should-not which-func-mode) (should-not header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))
+    (which-function-mode 1)
+    (with-current-buffer buf-code
+      (should which-func-mode) (should header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))
+    (which-function-mode -1)
+    ;; which-func-mode stays set even when which-function-mode is off.
+    (with-current-buffer buf-code
+      (should which-func-mode) (should-not header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))
+    (kill-buffer buf-code)
+    (kill-buffer buf-not)
+    (which-function-mode 1)
+    (setq buf-code (find-file-noselect "which-func-tests.el"))
+    (setq buf-not (get-buffer-create "fundamental"))
+    (with-current-buffer buf-code
+      (should which-func-mode) (should header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))))
+
+(provide 'which-func-tests)
+;;; which-func-tests.el ends here