* lisp/tab-line.el: New options, faces, and functions

* lisp/tab-line.el:
(tab-line-tab-face-functions): New option.
(tab-line-tab-inactive-alternate): New face.
(tab-line-tab-special): New face.
(tab-line-tab-face-inactive-alternating): New function.
(tab-line-tab-face-special): New function.
(tab-line-format-template): Use them.

* etc/NEWS: Update.

With thanks to Juri Linkov and Eli Zaretskii for their guidance.

2 files changed, 71 insertions, 6 deletions

diff --git a/etc/NEWS b/etc/NEWS
index dee0a37727b..bbd372c199a 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -383,6 +383,18 @@ value of 'tab-bar-show'.
 If your mouse or trackpad supports it, you can now scroll tabs when
 the mouse pointer is in the tab line by scrolling left or right.
 
+---
+*** New tab-line faces and options
+The face 'tab-line-tab-special' is used for tabs whose buffers are
+special, i.e. not file-backed.  The face
+'tab-line-tab-inactive-alternate' is used to display inactive tabs
+with an alternating background color, making them easier to
+distinguish between, especially if the face 'tab-line-tab' is
+configured to not display with a box; this alternate face is only
+applied when the option 'tab-line-tab-face-functions' is
+so-configured.  That option may also be used to customize tab-line
+faces in other ways.
+
 ** New bindings in occur-mode, 'next-error-no-select' bound to 'n' and
 'previous-error-no-select' bound to 'p'.
 
diff --git a/lisp/tab-line.el b/lisp/tab-line.el
index 46bf89f14eb..c9444718536 100644
--- a/lisp/tab-line.el
+++ b/lisp/tab-line.el
@@ -27,6 +27,7 @@
 
 ;;; Code:
 
+(require 'cl-lib)
 (require 'seq) ; tab-line.el is not pre-loaded so it's safe to use it here
 
 
@@ -35,6 +36,18 @@
   :group 'convenience
   :version "27.1")
 
+(defcustom tab-line-tab-face-functions '(tab-line-tab-face-special)
+  "Functions called to modify tab faces.
+Each function is called with five arguments: the tab, a list of
+all tabs, the face returned by the previously called modifier,
+whether the tab is a buffer, and whether the tab is selected."
+  :type '(repeat
+          (choice (function-item tab-line-tab-face-special)
+                  (function-item tab-line-tab-face-inactive-alternating)
+                  (function :tag "Custom function")))
+  :group 'tab-line
+  :version "28.1")
+
 (defgroup tab-line-faces '((tab-line custom-face)) ; tab-line is defined in faces.el
   "Faces used in the tab line."
   :group 'tab-line
@@ -63,6 +76,25 @@
   :version "27.1"
   :group 'tab-line-faces)
 
+(defface tab-line-tab-inactive-alternate
+  `((t (:inherit tab-line-tab-inactive :background "grey65")))
+  "Alternate face for inactive tab-line tabs.
+Applied to alternating tabs when option
+`tab-line-tab-face-functions' includes function
+`tab-line-tab-face-inactive-alternating'."
+  :version "28.1"
+  :group 'tab-line-faces)
+
+(defface tab-line-tab-special
+  '((default (:weight bold))
+    (((supports :slant italic))
+     (:slant italic :weight normal)))
+  "Face for special (i.e. non-file-backed) tabs.
+Applied when option `tab-line-tab-face-functions' includes
+function `tab-line-tab-face-special'."
+  :version "28.1"
+  :group 'tab-line-faces)
+
 (defface tab-line-tab-current
   '((default
       :inherit tab-line-tab)
@@ -412,7 +444,14 @@ variable `tab-line-tabs-function'."
                                   (cdr (assq 'selected tab))))
                     (name (if buffer-p
                               (funcall tab-line-tab-name-function tab tabs)
-                            (cdr (assq 'name tab)))))
+                            (cdr (assq 'name tab))))
+                    (face (if selected-p
+                              (if (eq (selected-window) (old-selected-window))
+                                  'tab-line-tab-current
+                                'tab-line-tab)
+                            'tab-line-tab-inactive)))
+               (dolist (fn tab-line-tab-face-functions)
+                 (setf face (funcall fn tab tabs face buffer-p selected-p)))
                (concat
                 separator
                 (apply 'propertize
@@ -425,11 +464,7 @@ variable `tab-line-tabs-function'."
                        `(
                          tab ,tab
                          ,@(if selected-p '(selected t))
-                         face ,(if selected-p
-                                   (if (eq (selected-window) (old-selected-window))
-                                       'tab-line-tab-current
-                                     'tab-line-tab)
-                                 'tab-line-tab-inactive)
+                         face ,face
                          mouse-face tab-line-highlight)))))
            tabs))
          (hscroll-data (tab-line-auto-hscroll strings hscroll)))
@@ -453,6 +488,24 @@ variable `tab-line-tabs-function'."
                 tab-line-new-button)
        (list tab-line-new-button)))))
 
+(defun tab-line-tab-face-inactive-alternating (tab tabs face _buffer-p selected-p)
+  "Return FACE for TAB in TABS with alternation.
+When TAB is an inactive buffer and is even-numbered, make FACE
+inherit from `tab-line-tab-inactive-alternate'.  For use in
+`tab-line-tab-face-functions'."
+  (when (and (not selected-p) (cl-evenp (cl-position tab tabs)))
+    (setf face `(:inherit (tab-line-tab-inactive-alternate ,face))))
+  face)
+
+(defun tab-line-tab-face-special (tab _tabs face buffer-p _selected-p)
+  "Return FACE for TAB according to whether it's special.
+When TAB is a non-file-backed buffer, make FACE inherit from
+`tab-line-tab-special'.  For use in
+`tab-line-tab-face-functions'."
+  (when (and buffer-p (not (buffer-file-name tab)))
+    (setf face `(:inherit (tab-line-tab-special ,face))))
+  face)
+
 (defvar tab-line-auto-hscroll)
 
 (defun tab-line-format ()