>> I'd much prefer a longer `font-lock-fontify-diwm` which tries to 
>> reproduce more or less the same behavior as your favorite, but by 
>> explicitly testing the different circumstances you care about.
>
> Sure.  It could, for instance, refontify the region (if selected) 
> instead of the entire buffer.  And probably a lot of other DWIM-ish 
> niceness.
>

Here's a patch to do this, and to fix the problem when "too large" buffers 
are refontified (xdisp.c for example).