bug#81510: 31.0.90; NS (macOS): clicking a disabled (:enable nil) tab-bar item crases Emacs

Clicking a tab-bar menu-item that has an explicit `:enable nil' property
(i.e. a disabled item) crashes Emacs outright on the NS (macOS) port.

Minimal recipe:

    emacs -Q -l repro.el

where repro.el contains:

    (defun repro-tab-bar-format ()
      '((disabled-item menu-item "disabled item -- click me" ignore
         :enable nil)))
    (setq tab-bar-format '(repro-tab-bar-format))
    (tab-bar-mode 1)

Then click on the "disabled item -- click me" text in the tab bar and
note the crash.

Backtrace (macOS crash report, Thread 0):

    0   libsystem_kernel.dylib   __pthread_kill + 8
    1   libsystem_pthread.dylib  pthread_kill + 296
    2   libsystem_c.dylib        raise + 32
    3   Emacs                    terminate_due_to_signal + 228
    4   Emacs                    emacs_abort + 20
    5   Emacs                    kbd_buffer_store_buffered_event + 652
    6   Emacs                    ns_read_socket_1 + 112
    7   Emacs                    gobble_input + 264
    ...
    -[EmacsView mouseDown:]

Root cause, as far as I can tell from reading src/xdisp.c,
src/keyboard.c and src/nsterm.m:

1. keyboard.c's parse_tab_bar_item defaults TAB_BAR_ITEM_ENABLED_P to t,
   and an explicit `:enable FORM' property overrides it.

2. xdisp.c's handle_tab_bar_click returns nil on a disabled item, rather
   than (Fcons (Qtab_bar, Qnil)) like a missed clicked.

3. nsterm.m's -[EmacsView mouseDown:] does:

       if (!(tab_bar_p && NILP (tab_bar_arg)))
         emacs_event->kind = MOUSE_CLICK_EVENT;
       ...
       EV_TRAILER (theEvent);

   For a disabled item, emacs_event->kind is never assigned -- it's left
   at its default, NO_EVENT. EV_TRAILER then calls
   kbd_buffer_store_event_hold.

4. keyboard.c's kbd_buffer_store_buffered_event opens with:

       if (event->kind == NO_EVENT)
         emacs_abort ();

I have not tested on X11/w32, so I am not sure if they have the same
issue.

Possible minimal fix: make handle_tab_bar_click's disabled-item
branch return (Fcons (Qtab_bar, Qnil)) instead of bare Qnil, matching
the "click missed" case, which nsterm.m (and presumably the other
ports) already handle correctly as a harmless plain click. I haven't
verified this doesn't have side effects for other callers of
handle_tab_bar_click across the X11/w32 ports.

------------------

In GNU Emacs 31.0.90 (build 1, aarch64-apple-darwin25.5.0, NS
 appkit-2685.60 Version 26.5.1 (Build 25F80)) of 2026-07-12 built on
 HW-R9XXWKPJ4D
Windowing system distributor 'Apple', version 10.3.2685
System Description:  macOS 26.5.2

Configured using:
 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/opt/homebrew/share/emacs/site-lisp
 --infodir=/opt/homebrew/Cellar/emacs-plus@31/31.0.90/share/info/emacs
 --prefix=/opt/homebrew/Cellar/emacs-plus@31/31.0.90
 --with-native-compilation=aot --with-xml2 --with-gnutls
 --without-compress-install 'CFLAGS=-DFD_SETSIZE=10000
 -DDARWIN_UNLIMITED_SELECT -I/opt/homebrew/opt/sqlite/include
 -I/opt/homebrew/opt/gcc/include -I/opt/homebrew/opt/libgccjit/include'
 --without-dbus --without-imagemagick --with-modules --with-rsvg
 --with-webp --with-ns --disable-ns-self-contained
 'LDFLAGS=-L/opt/homebrew/opt/sqlite/lib -L/opt/homebrew/lib/gcc/16
 -Wl,-rpath,/opt/homebrew/lib/gcc/16''

Configured features:
ACL GIF GLIB GMP GNUTLS JPEG LCMS2 LIBXML2 MODULES NATIVE_COMP NOTIFY
KQUEUE NS PDUMPER PNG RSVG SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP XIM ZLIB

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  minibuffer-nonselected-mode: t
  minibuffer-regexp-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr comp-run bytecomp byte-compile comp-common rx
emacsbug lisp-mnt message mailcap yank-media puny dired dired-loaddefs
rfc822 mml mml-sec password-cache epa derived epg rfc6068 epg-config
gnus-util text-property-search time-date subr-x mm-decode mm-bodies
mm-encode mailabbrev gmm-utils mailheader cl-loaddefs cl-lib sendmail
mail-parse rfc2231 rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils rmc iso-transl tooltip cconv eldoc paren electric uniquify
ediff-hook vc-hooks lisp-float-type elisp-mode mwheel term/ns-win ns-win
ucs-normalize mule-util term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads kqueue cocoa ns lcms2
multi-tty make-network-process tty-child-frames native-compile emacs)

Memory information:
((conses 16 56258 9223) (symbols 48 6141 0) (strings 32 15274 1404)
 (string-bytes 1 530642) (vectors 16 9719)
 (vector-slots 8 134388 10017) (floats 8 22 2) (intervals 56 266 0)
 (buffers 1064 10))