* lisp/tab-bar.el (tab-bar-show): Support the 0 value (bug#80713).

Add explicit choice for the 0 value to always show the tab bar.
Also add a choice for other non-negative numbers.
In the setter enable 'tab-bar-mode' only for numbers,
but not for the t value that should not enable the tab bar
after customization, only after creating a new tab.

lisp/tab-bar.el

@@ -638,6 +638,7 @@ on different frames: the tab bar can be shown on some frames and
 hidden on others, depending on how many tab-bar tabs are on that
 frame, and whether that number is greater than the numerical value
 of this variable.
+If 0, always keep the tab bar shown.
 If nil, always keep the tab bar hidden.  In this case it's still
 possible to use persistent named window configurations by relying on
 keyboard commands `tab-new', `tab-close', `tab-next', `tab-switcher', etc.
@@ -648,13 +649,15 @@ update the tab bar on all frames according to the new value.
 
 To enable or disable the tab bar individually on each frame,
 you can use the command `toggle-frame-tab-bar'."
-  :type '(choice (const :tag "Always" t)
+  :type '(choice (const :tag "On creating a new tab" t)
+                 (const :tag "Always" 0)
                  (const :tag "When more than one tab" 1)
+                 (natnum :tag "When more than this number")
                  (const :tag "Never" nil))
   :initialize #'custom-initialize-default
   :set (lambda (sym val)
          (set-default sym val)
-         (if val
+         (if (natnump val)
              (tab-bar-mode 1)
            (tab-bar--update-tab-bar-lines t)))
   :group 'tab-bar