bug#55970: 29.0.50; Emacs crashes when dragging the scroll bar with "--with-pgtk" and "--without-toolkit-scroll-bars" configure flags

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: समीर सिंह Sameer Singh
>  <lumarzeli30@gmail.com>,  55970@debbugs.gnu.org
> Date: Wed, 15 Jun 2022 08:59:20 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Thanks for testing, I will let it to Po Lu to decide whether that's
> > the proper fix in this case.
> 
> Non-toolkit scroll bars use different kinds of events compared to
> toolkit scroll bars.  PGTK only implements the latter kind, so turning
> toolkit scroll bars off should trigger this piece of configure.ac:
> 
>   elif test "${window_system}" != "x11" && "${window_system}" != "none"; then
>     AC_MSG_ERROR(Non-toolkit scroll bars are not implemented for your system)
>   fi
> 
> Perhaps Sameer's configure script is out of date and needs to be
> regenerated.

OK, but please explain how come the code in pgtk_send_scroll_bar_event
leaves the modifiers unset, and that is still OK, notwithstanding the
code in keyboard.c that expects either up or down modifier for any
mouse click event?  Isn't that a bug regardless?