I'd like to propose that `tab-bar' allow optionally showing tabs for inactive groups when `tab-bar-format-tabs-groups' is in force rather than the always-on policy of collapsing inactive groups.

The proposed custom variable `tab-bar-show-inactive-group-tabs' is backward compatible for existing users and has no visible changes for people who do not set it to t. I will occasionally set the value locally to alter frame by frame behavior. I've also used a key binding to toggle the behavior to nice effect.

I've been using these changes (via advice :override) under Emacs 29.3.

Thanks, again, for this package.

The proposed implementation is below with new/changed code in bold.

(defcustom tab-bar-show-inactive-group-tabs nil
  "Show tabs which are in inactive groups."
  :type 'boolean
  :initialize 'custom-initialize-default
  :set (lambda (sym val)
         (set-default sym val)
         (force-mode-line-update))
  :group 'tab-bar
  :version "30.x")

(defun tab-bar-format-tabs-groups ()
  "Produce tabs for the tab bar grouped according to their groups."
  (let* ((tabs (funcall tab-bar-tabs-function))
         (current-group (funcall tab-bar-tab-group-function
                                 (tab-bar--current-tab-find tabs)))
         (previous-group nil)
         (i 0))
    (mapcan
     (lambda (tab)
       (let ((tab-group (funcall tab-bar-tab-group-function tab)))
         (setq i (1+ i))
         (prog1 (cond
                 ;; Show current group tabs and ungrouped tabs
                 ((or (equal tab-group current-group) (not tab-group))
                  (append
                   ;; Prepend current group name before first tab
                   (when (and (not (equal previous-group tab-group)) tab-group)
                     (tab-bar--format-tab-group tab i t))
                   ;; Override default tab faces to use group faces
                   (let ((tab-bar-tab-face-function tab-bar-tab-group-face-function))
                     (tab-bar--format-tab tab i))))
                 ;; Show first tab of other groups with a group name
                 ((not (equal previous-group tab-group))
                  (append
                   (tab-bar--format-tab-group tab i)
                   (when tab-bar-show-inactive-group-tabs
                     (let ((tab-bar-tab-face-function tab-bar-tab-group-face-function))
                       (tab-bar--format-tab tab i)))))
                 ;; Hide other group tabs
                 (t (when tab-bar-show-inactive-group-tabs
                      (let ((tab-bar-tab-face-function tab-bar-tab-group-face-function))
                        (tab-bar--format-tab tab i)))))
           (setq previous-group tab-group))))
     tabs)))

-Stephane