bug#52881: setting tab-line-separator to any value breaks clicking tabs with mouse

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Thu, 30 Dec 2021 10:05:24 +0200
> Cc: 52881@debbugs.gnu.org
> 
> > This is using Emacs 27.2 on a CentOS8 machine at work. I use Emacs
> > exclusively with -nw in a terminal with mouse support enabled.
> >
> > Using something like
> >
> > (setq tab-line-separator " | ")
> >
> > breaks the ability to click on a tab to select it. Leaving it at the
> > default value avoids the bug.
> 
> Sorry, tab-line had hard-coded assumption on separators
> no longer than 1 character wide.
> 
> Eli, is it ok to push this patch to the release branch
> to support longer separators:

Yes, but can you make it a bit cleaner?  Like having a single function
to do that, and also not calling next-single-property-change twice an
a row?

Thanks.