bug#51590: follow-mode is broken with header-line and tab-line

[Eli Zaretskii]

> Date: Sun, 7 Nov 2021 12:48:22 +0000
> Cc: Eli Zaretskii <eliz@gnu.org>, 51590@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > Indeed, I tested with the header-line, and it has the same problem,
> > so retitled this bug report.
> 
> > After trial and error, I arrived to the following patch that completely
> > fixes these problems.  However, I can't explain how it works.
> 
> I've spent some time on this patch, and I've transformed it into a
> similar patch that I can explain, and I believe is nearly correct[*].
> Could you possibly try it out in your system.  Thanks!

So what's the verdict about posn-at-x-y? does it have a bug when
tab-line is present, or doesn't it?