bug#71384: treesit-font-lock-fontify-region unnecessarily re-unfontify's

[JD Smith]

> On Jun 6, 2024, at 1:34 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: JD Smith <jdtsmith@gmail.com>
>> Date: Wed, 5 Jun 2024 18:06:54 -0400
>> 
>> The function treesit-font-lock-fontify-region in treesit.el calls 
>> (font-lock-unfontify-region start end) right away (see below).  But this 
>> function is called as the value of font-lock-fontify-syntactically-function 
>> from font-lock-default-fontify-region, which has just itself made the exact 
>> same call to font-lock-unfontify-region. I commented out that superfluous 
>> call in treesit-font-lock-fontify-region and noticed no ill effects.
> 
> Why is that duplicate call a problem?  

I don't know if it's ever really a problem, other than the small amount of time 
spent removing properties that have just been removed.

> We don't say that treesit-font-lock-fontify-region _must_ be called _only_ 
> from
> font-lock-default-fontify-region, do we?


I presume the same logic would apply to font-lock-fontify-keywords-region, but 
that does not re-run unfontify (and is obviously much older code).   Maybe 
treesit reuses treesit-font-lock-fontify-region in non-font-lock contexts?  
Feel free to close if Yuan concurs.