bug#62216: Narrowed the issue to one line

[Eli Zaretskii]

> Date: Tue, 22 Aug 2023 09:10:10 -0500
> Cc: Eli Zaretskii <eliz@gnu.org>, luangruo@yahoo.com, 62216@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
> On 8/22/23 08:08, Jonathon McKitrick wrote:
> > Other than comparing the different lengths being calculated and reporting
> > them, is there any other logging I should run locally that could shed
> > light on this mystery?
> 
> Possibly the problem has to do with markers pointing into the buffer, so 
> it might be helpful to log their contents before and after the insert in 
> question.

Can you elaborate?  What kind of problems could the markers cause in
this case? disrupt the character-to-byte position conversion?
something else?