ibuffer: Display column titles in header line

If the option `ibuffer-use-header-line' is set to `title',
display column titles in the header line.
* lisp/ibuffer.el (ibuffer--format-title)
(ibuffer--format-summary): New functions extracted from
`ibuffer-update-title-and-summary'.
(ibuffer-update-title-and-summary): Use them.
(ibuffer-update): Do not always override `header-line-format'.
(ibuffer-use-header-line): Update docstring and option `:type'.
* lisp/ibuf-macs.el (define-ibuffer-sorter): Add "@" to the
interactive specification for clicks on the header line.

* etc/NEWS: Announce the change.
(Bug#75497)

3 files changed, 95 insertions, 77 deletions

diff --git a/etc/NEWS b/etc/NEWS
index e7c54574688..385e943c997 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -354,6 +354,10 @@ You can now set `asm-comment-char' from 'asm-mode-hook' instead.
 The variable 'ibuffer-formats' configures the Ibuffer formats.  Add
 'recency' to the format to display the column.
 
+*** New value 'title' for the user option 'ibuffer-use-header-line'.
+Display column titles in the header line if 'ibuffer-use-header-line' is
+set to 'title'.
+
 *** New user option 'ibuffer-human-readable-size'.
 When non-nil, buffer sizes are shown in human readable format.
 
diff --git a/lisp/ibuf-macs.el b/lisp/ibuf-macs.el
index ff3dc755c36..7dbc9b4125a 100644
--- a/lisp/ibuf-macs.el
+++ b/lisp/ibuf-macs.el
@@ -146,7 +146,7 @@ value if and only if `a' is \"less than\" `b'.
   `(progn
      (defun ,(intern (concat "ibuffer-do-sort-by-" (symbol-name name))) ()
        ,(or documentation "No :documentation specified for this sorting method.")
-       (interactive)
+       (interactive "@")
        (setq ibuffer-sorting-mode ',name)
        (when (eq ibuffer-sorting-mode ibuffer-last-sorting-mode)
          (setq ibuffer-sorting-reversep (not ibuffer-sorting-reversep)))
diff --git a/lisp/ibuffer.el b/lisp/ibuffer.el
index 11340d4bd9e..a94d9fddade 100644
--- a/lisp/ibuffer.el
+++ b/lisp/ibuffer.el
@@ -310,8 +310,10 @@ in completion lists of the `ibuffer-jump-to-buffer' command."
   :type 'boolean)
 
 (defcustom ibuffer-use-header-line t
-  "If non-nil, display a header line containing current filters."
-  :type 'boolean)
+  "If non-nil, display a header line.
+If the variable's value is t, the header line displays the current
+filters.  For the value `title', display the column titles."
+  :type '(choice boolean (const :tag "Column titles" :value title)))
 
 (defcustom ibuffer-default-directory nil
   "The default directory to use for a new Ibuffer buffer.
@@ -2048,59 +2050,90 @@ the value of point at the beginning of the line for that buffer."
   (ibuffer-update-format)
   (ibuffer-redisplay t))
 
+(defun ibuffer--format-title (element &optional header-line)
+  (if (stringp element)
+      element
+    (pcase-let ((`(,sym ,min ,_max ,align) element))
+      ;; Ignore negative MIN, since the titles are left-aligned.
+      (when (cl-minusp min)
+        (setq min (- min)))
+      (let* ((name (or (get sym 'ibuffer-column-name)
+                       (error "Unknown column %s in ibuffer-formats" sym)))
+             (len (length name))
+             (hmap (get sym 'header-mouse-map))
+             (strname (if (< len min)
+                          (ibuffer-format-column name
+                                                 (- min len)
+                                                 align)
+                        name)))
+        (when hmap
+          (setq
+           strname
+           (propertize strname 'mouse-face 'highlight 'keymap
+                       (if header-line
+                           (define-keymap "<header-line>" hmap)
+                         hmap))))
+        strname))))
+
+(defun ibuffer--format-summary (element)
+  (if (stringp element)
+      (make-string (length element) ?\s)
+    (pcase-let ((`(,sym ,min ,_max ,align) element))
+      ;; Ignore negative MIN, since the summaries are left-aligned.
+      (when (cl-minusp min)
+        (setq min (- min)))
+      (let* ((summary
+              (if (get sym 'ibuffer-column-summarizer)
+                  (funcall (get sym 'ibuffer-column-summarizer)
+                           (get sym 'ibuffer-column-summary))
+                (make-string
+                 (length (get sym 'ibuffer-column-name))
+                 ?\s)))
+             (len (length summary)))
+        (if (< len min)
+            (ibuffer-format-column summary
+                                   (- min len)
+                                   align)
+          summary)))))
+
 (defun ibuffer-update-title-and-summary (format)
   (ibuffer-assert-ibuffer-mode)
   ;; Don't do funky font-lock stuff here
   (let ((inhibit-modification-hooks t))
-    (if (get-text-property (point-min) 'ibuffer-title)
-        (delete-region (point-min)
-                       (next-single-property-change
-                        (point-min) 'ibuffer-title)))
-    (goto-char (point-min))
-    (add-text-properties
-     (point)
-     (progn
-       (let ((opos (point)))
-         ;; Insert the title names.
-         (dolist (element format)
-           (insert
-            (if (stringp element)
-                element
-              (pcase-let ((`(,sym ,min ,_max ,align) element))
-                ;; Ignore a negative min when we're inserting the title
-                (when (cl-minusp min)
-                  (setq min (- min)))
-                (let* ((name (or (get sym 'ibuffer-column-name)
-                                 (error "Unknown column %s in ibuffer-formats" sym)))
-                       (len (length name))
-                       (hmap (get sym 'header-mouse-map))
-                       (strname (if (< len min)
-                                    (ibuffer-format-column name
-                                                           (- min len)
-                                                           align)
-                                  name)))
-                  (when hmap
-                    (setq
-                     strname
-                     (propertize strname 'mouse-face 'highlight 'keymap hmap)))
-                  strname)))))
-         (add-text-properties opos (point) '(ibuffer-title-header t))
-         (insert "\n")
-         ;; Add the underlines
-         (let ((str (save-excursion
-                      (forward-line -1)
-                      (beginning-of-line)
-                      (buffer-substring (point) (line-end-position)))))
-           (apply #'insert (mapcar
-                            (lambda (c)
-                              (if (not (or (eq c ?\s)
-                                           (eq c ?\n)))
-                                  ?-
-                                ?\s))
-                            str)))
-         (insert "\n"))
-       (point))
-     `(ibuffer-title t font-lock-face ,ibuffer-title-face))
+    ;; Insert the title names.
+    (if (eq ibuffer-use-header-line 'title)
+        (setq header-line-format
+              `("" header-line-indent
+                ,(propertize " " 'display
+                             '(space :align-to header-line-indent-width))
+                ,@(mapcar (lambda (e) (ibuffer--format-title e t)) format)))
+      (if (get-text-property (point-min) 'ibuffer-title)
+          (delete-region (point-min)
+                         (next-single-property-change
+                          (point-min) 'ibuffer-title)))
+      (goto-char (point-min))
+      (add-text-properties
+       (point)
+       (progn
+         (let ((opos (point)))
+           (apply #'insert (mapcar #'ibuffer--format-title format))
+           (add-text-properties opos (point) '(ibuffer-title-header t))
+           (insert "\n")
+           ;; Add the underlines
+           (let ((str (save-excursion
+                        (forward-line -1)
+                        (beginning-of-line)
+                        (buffer-substring (point) (line-end-position)))))
+             (apply #'insert (mapcar
+                              (lambda (c)
+                                (if (not (or (eq c ?\s)
+                                             (eq c ?\n)))
+                                    ?-
+                                  ?\s))
+                              str)))
+           (insert "\n"))
+         (point))
+       `(ibuffer-title t font-lock-face ,ibuffer-title-face)))
     ;; Now, insert the summary columns.
     (goto-char (point-max))
     (if (get-text-property (1- (point-max)) 'ibuffer-summary)
@@ -2112,27 +2145,7 @@ the value of point at the beginning of the line for that buffer."
          (point)
          (progn
            (insert "\n")
-           (dolist (element format)
-             (insert
-              (if (stringp element)
-                  (make-string (length element) ?\s)
-                (pcase-let ((`(,sym ,min ,_max ,align) element))
-                  ;; Ignore a negative min when we're inserting the title.
-                  (when (cl-minusp min)
-                    (setq min (- min)))
-                  (let* ((summary
-                          (if (get sym 'ibuffer-column-summarizer)
-                              (funcall (get sym 'ibuffer-column-summarizer)
-                                       (get sym 'ibuffer-column-summary))
-                            (make-string
-                             (length (get sym 'ibuffer-column-name))
-                             ?\s)))
-                         (len (length summary)))
-                    (if (< len min)
-                        (ibuffer-format-column summary
-                                               (- min len)
-                                               align)
-                      summary))))))
+           (apply #'insert (mapcar #'ibuffer--format-summary format))
            (point))
          '(ibuffer-summary t)))))
 
@@ -2197,10 +2210,11 @@ If optional arg SILENT is non-nil, do not display progress messages."
   ;; I tried to update this automatically from the mode-line-process format,
   ;; but changing nil-ness of header-line-format while computing
   ;; mode-line-format is asking a bit too much it seems.  --Stef
-  (setq header-line-format
-        (and ibuffer-use-header-line
-             ibuffer-filtering-qualifiers
-             ibuffer-header-line-format)))
+  (unless (eq ibuffer-use-header-line 'title)
+    (setq header-line-format
+          (and ibuffer-use-header-line
+               ibuffer-filtering-qualifiers
+               ibuffer-header-line-format))))
 
 (defun ibuffer-sort-bufferlist (bmarklist)
   (unless ibuffer-sorting-functions-alist