bug#26586: 25.1; header-line-format spins cpu

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Fri, 21 Apr 2017 11:01:59 -0700
> Cc: 26586@debbugs.gnu.org
> 
> On Fri, Apr 21, 2017 at 10:31 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> > You need to replace % with %%, and then put the property on the
> > result.  Or make the replacement string have the property.  I think.
> 
> Right, the tricky bit is matching the properties that the % had.

Doesn't text-properties-at fit the bill?