bug#59351: 29.0.50; [PATCH] Fix mouse click position to menu bar entry

[Eli Zaretskii <eliz@gnu.org>]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: Manuel Giraud <manuel@ledu-giraud.fr>,  luangruo@yahoo.com,
>   59351@debbugs.gnu.org
> Date: Mon, 21 Nov 2022 15:46:51 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Manuel Giraud <manuel@ledu-giraud.fr>
> >> Cc: Manuel Giraud <manuel@ledu-giraud.fr>,  luangruo@yahoo.com,
> >>   59351@debbugs.gnu.org
> >> Date: Mon, 21 Nov 2022 14:40:16 +0100
> >> 
> >> I'm trying to debug this from "M-x gdb".  I've put a breakpoint at
> >> display_menu_bar but whenever I'm doing a 'next' at the init_iterator
> >> call I get the following message:
> >> 
> >> --8<---------------cut here---------------start------------->8---
> >> Thread 1 received signal SIGTRAP, Trace/breakpoint trap.
> >> _thread_sys_poll () at /tmp/-:3
> >> 3	/tmp/-: No such file or directory.
> >> --8<---------------cut here---------------end--------------->8---
> >
> > What does "bt" show in this case?
> 
> Here it is:
> --8<---------------cut here---------------start------------->8---
> #0  _thread_sys_poll () at /tmp/-:3
> #1  0x00000a4800b10c4e in _libc_poll_cancel (fds=0x7f7ffffc52a8, nfds=1, timeout=-1) at /usr/src/lib/libc/sys/w_poll.c:27
> #2  0x00000a4798da5532 in _xcb_conn_wait (c=0xa47977e9000, cond=<optimized out>, vector=0x0, count=0x0) at /usr/xenocara/lib/libxcb/libxcb/../../../dist/libxcb/src/xcb_conn.c:508
> #3  0x00000a4798db7ad4 in wait_for_reply (c=0xa47977e9000, request=815, e=0x7f7ffffc53b8) at /usr/xenocara/lib/libxcb/libxcb/../../../dist/libxcb/src/xcb_in.c:522

Hmm... that's strange.  Po Lu, any idea why we get SIGTRAP there?

Anyway, instead of stepping with "next", try this, after the breakpoint in
display_menu_bar breaks:

 (gdb) tbreak 26300
 (gdb) continue

If this works, and Emacs is stopped at line 26300, I'd suggest stepping
directly into compute_line_metrics, which is called at the end of
display_menu_bar:

 (gdb) tbreak compute_line_metrics
 (gdb) continue

This should stop inside compute_line_metrics, and then step through it and
take note of the various metrics of the glyph row that it uses to compute
the height.  You should see the metrics that correspond to the new font.