bug#56682: Fix the long lines font locking related slowdowns

[Stefan Monnier]

>> From: Stefan Monnier <monnier@iro.umontreal.ca>
>> Cc: Gregory Heytings <gregory@heytings.org>,  56682@debbugs.gnu.org
>> Date: Fri, 22 Jul 2022 19:25:47 -0400
>> 
>> For example, the behavior surprises me because I can't see why `M-x`
>> should cause any font-locking at all in the `long-line.xml` file.
>
> Any command that enters the minibuffer causes a thorough redisplay of
> the windows on that frame (because more than one window has to be
> updated).  What does that have to do with font-lock is a separate
> question (but you are the best person to answer it).

AFAIK if the buffer has not been modified (including things like
changing `window-start` or `point`), then a redisplay will just not
run jit-lock (and hence font-lock) at all, no matter how thorough.


        Stefan