Subject: Re: bug#57476: 29.0.50; Mouse wheel event ignored on Lucid build

From: Stephen Berman <[email-masked]>
Date: Tue, 30 Aug 2022 10:20:06 +0200

On Tue, 30 Aug 2022 09:25:45 +0800 Po Lu <[email-masked]> wrote:

> Stephen Berman <[email-masked]> writes:
>
>> 0. emacs -Q
>> 1. Display a buffer with enough lines to scroll, e.g.:
>>    C-x d /path/to/emacs/sources RET
>> 2. Scroll mouse wheel
>> => The buffers fails to scroll.
>> 3. Type `C-h k' and then scroll the mouse wheel
>> => The echo area shows no input, only the prompt "Describe the following
>>    key, mouse click, or menu item: "
>>
>> I see this only on the Lucid build.  Also on Lucis, if I hold down
>> mouse-1 or mouse-3 and simultaneously scroll the wheel, then the buffer
>> does scroll.  After releasing mouse-1 or mouse-3, scrolling the wheel
>> scrolls the buffer once by one line (either up or down).  Further
>> scrolling of the wheel fails to scroll the buffer.
>
> To be clear here, you are talking about wheel-up and wheel-down events,
> correct?

Yes.

On Tue, 30 Aug 2022 09:29:34 +0800 Po Lu <[email-masked]> wrote:

> Unfortunately, I cannot reproduce the bug on any build of Emacs.  Please
> instrument the following code around line 21053 of xterm.c as follows:
>
>
> 		      /* See the comment on top of x_cache_xi_devices
> 			 for more details on how scroll wheel movement
> 			 is reported on XInput 2.  */
> 		      delta = x_get_scroll_valuator_delta (dpyinfo, device,
> 							   i, *values, &val);
> 		      values++;
>
> 		      if (!val)
> 			{
> 			  other_valuators_found = true;
> 			  continue;
> 			}
>
> 		      printf ("%g\n", delta); <=== add this instrumentation
>
> 		      if (delta != DBL_MAX)
> 			{
>
> and show what is printed when you try to scroll using the mouse.

1.79769e+308

(This is printed in the shell I invoked emacs -Q from each time I scroll
the mouse wheel up or down.)

Steve Berman