From 3bacf74adb4223e3dded46d63d5743554afd7c94 Mon Sep 17 00:00:00 2001
From: Juri Linkov <juri@linkov.net>
Date: Wed, 24 Mar 2021 22:14:52 +0200
Subject: [PATCH] * lisp/tab-bar.el (tab-bar-tab-post-change-group-functions):
 New hook.

(tab-bar-change-tab-group): Run it.
(tab-bar-move-tab-to-group): New command for new hook.
---
 etc/NEWS        |  4 +++-
 lisp/tab-bar.el | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/etc/NEWS b/etc/NEWS
index 49a4bb8106f..5dbd61ff9ae 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -562,7 +562,9 @@ It also supports a negative argument.
 *** 'C-x t G' assigns a group name to the tab.
 'tab-close-group' can close all tabs that belong to the selected group.
 The user option 'tab-bar-new-tab-group' defines the default group of a
-new tab.
+new tab.  After customizing 'tab-bar-tab-post-change-group-functions'
+to 'tab-bar-move-tab-to-group', changing the tab group will also move it
+closer to other tabs in the same group.
 
 ---
 *** New user option 'tab-bar-tab-name-format-function'.
diff --git a/lisp/tab-bar.el b/lisp/tab-bar.el
index 63769673b93..2e27b293c5e 100644
--- a/lisp/tab-bar.el
+++ b/lisp/tab-bar.el
@@ -1412,6 +1412,42 @@ function `tab-bar-tab-name-function'."
 
 ;;; Tab groups
 
+(defun tab-bar-move-tab-to-group (&optional tab)
+  "Relocate TAB (or the current tab) closer to its group."
+  (interactive)
+  (let* ((tabs (funcall tab-bar-tabs-function))
+         (tab (or tab (tab-bar--current-tab-find tabs)))
+         (tab-index (tab-bar--tab-index tab))
+         (group (alist-get 'group tab))
+         ;; Beginning position of the same group
+         (beg (seq-position tabs group
+                            (lambda (tb gr)
+                              (and (not (eq tb tab))
+                                   (equal (alist-get 'group tb) gr)))))
+         ;; Size of the same group
+         (len (when beg
+                (seq-position (nthcdr beg tabs) group
+                              (lambda (tb gr)
+                                (not (equal (alist-get 'group tb) gr))))))
+         (pos (when beg
+                (cond
+                 ;; Don't move tab when it's already inside group bounds
+                 ((and len (>= tab-index beg) (<= tab-index (+ beg len))) nil)
+                 ;; Move tab from the right to the group end
+                 ((and len (> tab-index (+ beg len))) (+ beg len 1))
+                 ;; Move tab from the left to the group beginning
+                 ((< tab-index beg) beg)))))
+    (when pos
+      (tab-bar-move-tab-to pos (1+ tab-index)))))
+
+(defcustom tab-bar-tab-post-change-group-functions nil
+  "List of functions to call after changing a tab group.
+The current tab is supplied as an argument."
+  :type 'hook
+  :options '(tab-bar-move-tab-to-group)
+  :group 'tab-bar
+  :version "28.1")
+
 (defun tab-bar-change-tab-group (group-name &optional arg)
   "Add the tab specified by its absolute position ARG to GROUP-NAME.
 If no ARG is specified, then set the GROUP-NAME for the current tab.
@@ -1445,6 +1481,8 @@ While using this command, you might also want to replace
         (setcdr group group-new-name)
       (nconc tab `((group . ,group-new-name))))
 
+    (run-hook-with-args 'tab-bar-tab-post-change-group-functions tab)
+
     (force-mode-line-update)
     (unless tab-bar-mode
       (message "Set tab group to '%s'" group-new-name))))
-- 
2.39.2