bug#41441: 26.3; mhtml misbehaving font-lock extend region can cause Emacs to hang

[Tom Tromey]

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

It's not entirely clear to me that (1) this is a bug, or (2) that it can
be changed.

My recollection is that mhtml mode has to shrink the region in some
cases, because we don't want font-locking to extend beyond the end of a
sub-mode.
           
For example consider things like

<script>some js here;</script><p>more html here</p>

Here, we want to font-lock the body of the script using one set of
rules, and the rest with another set.

Looking at mhtml--submode-fontify-region, though, I wonder if maybe
region extension isn't needed at all, since that function seems to
handle sub-mode region boundaries.  So I guess that is one
experiment that could be done.

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

Maybe this mode could be changed instead.

Tom