Try which-func-mode' for Org' and `pdf-tools'

README.org

@@ -4712,6 +4712,67 @@ it for this buffer is by typing {{{kbd(C-c C-e t U)}}} to export the it to a
   (global-set-key (kbd "C-c g") #'writegood-mode))
 #+end_src
 
+** [[info:emacs#Which Function][Which-function-mode (info)]]
+:PROPERTIES:
+:CUSTOM_ID: sec:which-function-mode
+:END:
+
+The code in listing [[lst:setup-which-function-mode]] has stolen ideas from the
+following links:
+- [[https://list.orgmode.org/20240205.141235.268481480563517065.teika@gmx.com/][Show current org-mode headline in frame header]].
+- [[https://emacs.stackexchange.com/questions/30894/][Show current org-mode headline in modeline]].
+
+#+caption[Setup ~which-function-mode~]:
+#+caption: Setup ~which-function-mode~.
+#+name: lst:setup-which-function-mode
+#+begin_src emacs-lisp -n :results silent
+;; https://emacs.stackexchange.com/questions/30894/
+;; https://list.orgmode.org/20240205.141235.268481480563517065.teika@gmx.com/
+
+(defvar which-func-functions nil)
+
+(defun which--org-function ()
+  "Return level and title of the current headline.
+Return the document title when point is above the first headline."
+  (interactive) ;; Keep this function interactive for debugging.
+  (when (eq major-mode 'org-mode)
+    (let (text)
+      (unless (org-at-heading-p)
+        (save-excursion
+          (org-previous-visible-heading 1)
+          (let ((eap (org-element-at-point)))
+            (when (org-element-type-p eap 'keyword)
+              (setq text (format "0|%s" (org-element-property :value eap)))))))
+      (unless text
+        (let* ((chain (org-get-outline-path t))
+               (count (length chain)))
+          (setq text (format "%s|%s" count (nth (1- count) chain)))))
+      text)))
+
+(add-to-list 'which-func-functions 'which--org-function)
+
+(defun which--pdf-function ()
+  "Return the title of the current headline.
+Return \"Front Matter\" when point is above the first headline."
+  (interactive) ;; Keep this function interactive for debugging.
+  (when (eq major-mode 'pdf-view-mode)
+    (let* ((current-page (pdf-view-current-page))
+           (outline (pdf-info-outline (current-buffer)))
+           (hl-count (length outline))
+           (hl-index 0)
+           (hl-page 0)
+           (old-title "Front Matter")
+           (new-title old-title))
+      (while (and (< hl-index hl-count) (< hl-page current-page))
+        (setq old-title new-title)
+        (setq hl-page (alist-get 'page (nth hl-index outline)))
+        (setq new-title (alist-get 'title (nth hl-index outline)))
+        (cl-incf hl-index))
+      (if (< current-page hl-page) old-title new-title))))
+
+(add-to-list 'which-func-functions 'which--pdf-function)
+#+end_src
+
 * [[info:emacs#Saving Emacs Sessions][Saving Emacs sessions (info)]]
 :PROPERTIES:
 :CUSTOM_ID: sec:desktop