bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 57207@debbugs.gnu.org,  yantar92@gmail.com
> Date: Sat, 20 Aug 2022 11:57:44 +0200
> 
> I'm not quite understanding why we're narrowing here in the first place.
> Point is not in a long line in this test case -- there's just a long
> line (hidden under a display property) earlier in the buffer.

Once a single long line is detected, the flag is turned on in the
buffer, and all the optimizations/shortcuts, including narrowing, are
applied regardless.

A long line hidden by a display property is indeed a problem: it
doesn't present any issue for the display engine, but is still counted
as a potential problem.