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

> From: Al Haji-Ali <[email protected]>
> Date: Tue, 28 Jul 2026 16:25:33 +0100
> 
> 
> 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.

No, they both assign 'kind' unconditionally.  They then assign the
'arg' member only if tab_bar_arg is non-nil.

> 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.

I prefer that nsterm.m is changed to do the same we do in xterm.c and
w32term.c.  There's no need to change xdisp.c, which is a back-end
independent part of the display engine.

Alan, WDYT?