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: 55970@debbugs.gnu.org,  lumarzeli30@gmail.com
> Date: Wed, 15 Jun 2022 11:29:23 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > 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?
> 
> The mouse click code that expects an up or down modifier is only used
> for scroll bar events when USE_TOOLKIT_SCROLL_BARS isn't defined.  The
> code actually used with toolkit scroll bars starts at line 6359 of
> keyboard.c.

Then how come the code which aborted got executed contrary to what you
say?  And given that it did, would it do any harm to install the fix I
proposed?

More importantly, does it really make sense to "fix" events in
keyboard.c, rather than generate them correctly in low-level
toolkit-specific code to begin with?  I mean this part:

        /* Always treat scroll bar events as clicks.  */
        event->modifiers |= click_modifier;
        event->modifiers &= ~up_modifier;

Why do we override the modifiers of the event, and why in keyboard.c
of all places?  Can we do this cleaner, or at least in
toolkit-specific code?