Hi Arash,

Thanks for your reply. You are right, I just figured out this is not a bug
of auctex, but a bug of polymode instead. L224 in `polymode-compat.el'
messes up with font lock, and causes this problem in LaTeX mode.

Although this is not quite relevant to auctex, a temporary workaround would
be:
(with-eval-after-load "polymode-compat"
  (advice-remove 'font-lock-extend-region-multiline
'pm-check-for-real-change-in-extend-multiline))

Best,
Ruiyang

On Tue, Nov 12, 2019 at 1:11 PM Arash Esbati <arash@gnu.org> wrote:

> Hi Ruiyang,
>
> Ruiyang Wu <ruiyangwu@math.arizona.edu> writes:
>
> > The problem can be reproduced with the `test.tex' file attached below.
> When
> > I tried to add more characters to the `align' environment in `test.tex',
> > the syntax highlighting becomes extremely unstable, switching between
> math
> > and plain text highlighting.
>
> I can't reproduce what you describe.  Your file looks like this for me
> where adding additional characters is no problem:
>
>
> > Emacs  : GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
> >  of 2019-08-29
>
> I'm using Emacs 27 pretest.  Maybe you can try it as well and see if the
> issue persists.  You can fetch it from here:
>
>     https://alpha.gnu.org/gnu/emacs/pretest/windows/emacs-27/
>
> Best, Arash
>


-- 
Ruiyang Wu

Graduate Student
Department of Mathematics
University of Arizona
Tucson, AZ, 85721    USA