bug#49562: 27.2; Crash with specific mode-line-format in BiDi processing

[Eli Zaretskii]

> From: Martin Brüstel <martin.bruestel@tu-dresden.de>
> CC: <49562@debbugs.gnu.org>
> Date: Thu, 15 Jul 2021 11:30:27 +0200
> 
> > The "evil" mode-line string includes invalid use of bidi formatting
> > controls: you have there a U+202A LEFT-TO-RIGHT EMBEDDING without a
> > matching U+202C POP DIRECTIONAL FORMATTING.  Removing the former or
> > adding the latter avoids the crash.
> 
> Thanks for finding the cause!  It turns out, the control chars are
> actually already in the window title of the browser window.  The
> imbalance results from truncating the name to a certain length.

This tricky bug is now fixed on the master branch.  It actually
revealed a design flaw in the original code.

Thanks.