Re: [Emacs-diffs] emacs-24 d69e9f1: CC Mode: Stop Font Lock forcing fontification from BOL. Fixes debbugs#19669.

[Stefan Monnier]

> +  ;; Prevent `font-lock-default-fontify-region' extending the region it will
> +  ;; fontify to whole lines by removing `font-lock-extend-region-whole-lines'
> +  ;; (and, coincidentally, `font-lock-extend-region-multiline' (which we do
> +  ;; not need)) from `font-lock-extend-region-functions'.  (Emacs only).  
> This
> +  ;; fixes Emacs bug #19669.

My general understanding of font-lock's highlighting is that using
a smaller region is only ever useful for performance reason, rather than
for correctness (and this line of thinking influences the design of the
code).  So I'm really curious to understand how this counter-example
works, since it might hint at further (latent) bugs elsewhere.

Can you walk me through what happens in the scenario of bug#19669?


        Stefan