Hi!

The font-lock system provides a system for major and minor modes to extend
the region that is being highlighted.

In Mhtml mode, the function `mhtml--extend-font-lock-region` sometimes
reduce the size of the region. If this is combined with, say, a minor mode
that extends the region then Emacs could hang.

Steps to repeat:

Create a file, test.html, with the following content:

---------
alpha

beta
---------

Evaluate:

(let ((font-lock-beg 7)
      (font-lock-end 13))
  (mhtml--extend-font-lock-region)
  (cons font-lock-beg font-lock-end))

The 7 and 13 corresponds to a region that includes the last two lines. The
expression returns (8 . 13) representing a region that only include the
last line. If this is combined with another function that extends the
region to include line 2, Emacs hangs.

I suggest that:

* Modify the documentation of the variable
`font-lock-extend-region-functions` so that it's clear that the region
should not shrink.

* Add a check in the function `font-lock-default-fontify-region` to prevent
the region from shrinking even if a function tries to do so. (By ensuring
that the region only extends and never shrinks, the function will always
terminate.)

* Fix the problem in `mhtml--extend-font-lock-region`.

One minor mode that, when enabled, would cause Emacs to hang with the above
buffer is https://github.com/Lindydancer/char-font-lock (this package
highlights incorrect whitespace).

    -- Anders Lindgren




1) The function `mhtml--extend-font-lock-region should be modified so that
it never shrinks the region.


In GNU Emacs 26.3 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17
Version 10.10.5 (Build 14F2511))
 of 2019-09-02 built on builder10-10.porkrind.org
Windowing system distributor 'Apple', version 10.3.1348
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
(New file)
funcall-interactively: End of buffer
Saving file /Users/anders/emacs/src/foreach/foo.html...
Wrote /Users/anders/emacs/src/foreach/foo.html
Char: C-j (10, #o12, #xa) point=7 of 12 (50%) column=0
Quit
Entering debugger...
Back to top level
(8 . 13) [2 times]
Configured using:
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Configured features:
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Important settings:
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Memory information:
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