bug#39967: 27.0.90; how to debug point moving erratically?

[Eli Zaretskii]

> From: William Rankin <william@bydasein.com>
> Date: Sat, 7 Mar 2020 19:10:09 +1000
> Cc: 39967@debbugs.gnu.org
> 
> > Find the feature you load/activate in your init files which causes
> > this?
> 
> Hmm given the way it moves point, I would put money on this not being my
> init. I'm not smart enough to write something that would work in such a
> sporadic way.

I didn't mean something you wrote, I meant some feature you turn on
that is not turned on by default, a feature that is part of Emacs or
some third-party package you are using.

> Given the impending release of 27.1 maybe there are other avenues I can
> check too?

You could try running under GDB with a breakpoint in set_point_both
and temp_set_point_both, with breakpoint commands that show the C and
Lisp backtrace and then immediately continue the program.  Let me know
if you need more detailed instructions for setting this up.

Can you describe the issue in more detail?  Are you saying this
happens when Emacs is idle, i.e. you are not typing any commands and
no sub-processes should be running?  And what exactly do you see when
this happens?  Also, do you see any error messages in *Messages*? when
that happens?

FWIW, I don't see anything like this on my system in Emacs 27.0.90, so
I have hard time believing that something as basic as JIT font-lock
could do this, at least not by default.

Thanks.