bug#45898: 27.1; wedged in redisplay again

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> and then tried to move around in the buffer, search with Isearch, and
>> insert a few chars.
>> I don't notice any difference with or without the patch :-(
>
> If you start moving with C-n or C-v, Emacs becomes unusable
> approximately around the same buffer position with and without the
> patch?

Yup.  And moving at the end of the file with C-p and M-v is also "about
as bad".

> If so, I guess the expensive part is somewhere else.

Unsuprisingly so: none of `C-n/C-p/C-v/...` involve font-lock or
jit-lock either during their operation or during the
subsequent redisplay phase in the current code: the one-line is all
fontified once and for all when you open the file and after that
font-lock is not involved any more (until you make an edit, that is).

The only operation where I can see an improvement is when editing the
file near the beginning where the `master` code take a couple seconds
per edit (the time to refontify the whole line/buffer) whereas with the
patch it takes a fraction of a second (definitely not immediate, tho).

It also does open up the file a bit faster, but it pays for it by taking
more time to jump to the end:

- open file:  master=5s  patch=3s
- M->      :  master=10s patch=14s

[ Note: these time measurements are *very* approximate. ]

So while we do get some benefit occasionally by chunking, the chunking
itself slows us down.

> I'd install this patch regardless, because it sounds like TRT not to
> let syntactic fontifications run amok this way.  We do similar stuff
> in bidi.c for similar reasons.

OK, I pushed it to `master`.  The improvement is far from convincing,
but I tend to agree that it's "The Right Thing".