Re: master 7362554: Widen around c-font-lock-fontify-region. This fixes bug #38049.

[Dmitry Gutov <dgutov@yandex.ru>]

On 14.11.2019 15:42, Stefan Monnier wrote:
>>   ;; Return number of screen lines between START and END.
>>   (defun repos-count-screen-lines (start end)
>> +  ;; Make sure jit-lock doesn't have to run in a narrowed context.
>> +  (font-lock-ensure start end)
>>     (save-excursion
>>       (save-restriction
>>         (narrow-to-region start end)
> 
> Hmm... the buffer might have font-lock disabled, in which case we'd
> be fontifying where the user didn't want fontification.

We could wrap it in (when font-lock-mode ...), then? Or something to 
that effect.

> But I haven't followed the thread enough to know what we're talking
> about, really.  E.g. I have no idea what's the connection between
> CC-mode, font-lock-dont-widen, and the above hunk.  More specifically,
> when looking at the above patch my first reaction is "if it's needed
> here, why isn't it needed almost everywhere else as well?"

The hope is that calling vertical-motion (or font-lock-ensure directly) 
inside narrowing is a rare occasion, and it doesn't happen in a lot of 
functions. At least that has been my experience.