bug#75120: 31.0.50; which-key crashes on xt if the scroll bar is activated.

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Madhu <enometh@meer.net>,  75120@debbugs.gnu.org
> Date: Wed, 08 Jan 2025 14:11:57 +0100
> 
> >>>>> On Sat, 28 Dec 2024 14:10:35 +0200, Eli Zaretskii <eliz@gnu.org> said:
> 
>     >> From: Madhu <enometh@meer.net>
>     >> Date: Thu, 26 Dec 2024 20:56:01 +0530
>     >> 
>     >> Hello, Having heard about which-key mode I decided to try it.
>     >> 
>     >> gdb --args ../build-xt-native/src/emacs  -Q  -eval '(which-key-mode 1)' -eval '(setq window-min-height 8)'
>     >> 
>     >> 1. type C-x, wait 5 seconds, type C-g.
>     >> 2. type C-x, wait 2 seconds for the 8 line frame with scrollbar, click the "up
>     >> arrow" on the top of the scrollbar.
> 
>     Eli> When I try this, Emacs signals an error, displaying "C-x
>     Eli> <vertical-scroll-bar> <mouse-1> is undefined" in the echo area.  But
>     Eli> I'm not on X.
> 
>     Eli> Can someone else reproduce the crash?
> 
> Yes. In xt_action_hook:
> 
>       x_send_scroll_bar_event (window_being_scrolled,
> 			       scroll_bar_end_scroll, 0, 0, false);
>       w = XWINDOW (window_being_scrolled);
>       bar = XSCROLL_BAR (w->vertical_scroll_bar); <-
> 			       w->vertical_scroll_bar == Qnil here
> 
> Although I have no clue what to do about it: I suspect the scrollbars
> have been condemned by condemn_scroll_bars_hook, but then not
> redeemed. Or maybe whatever is undoing or preventing the global
> setting of "<vertical-scroll-bar> <mouse-1>" is responsible, but I
> canʼt find where that is.

Maybe we should simply do what xt_horizontal_action_hook does for the
horizontal scroll bar, i.e. do nothing if w->vertical_scroll_bar is
nil?

Martin, WDYT?