bug#56393: Auto narrowing autside of redisplay

[Gregory Heytings]

> Okay, I understand the internal buffer argument.  But what if it is not 
> such a buffer?  Say a large XML or JS file with background 
> fontification.
>
> WDYT about marking buffers that don't want auto narrowing?  Or the 
> opssite, mark buffers that want it?

One of the explicit aims when designing that feature was that it should be 
as transparent as possible.  And it is: only a few selected places in 
xdisp.c see its effect.

That being said, you can do what you want nonetheless: if you (setq-local 
long-line-threshold nil) the optimizations will never be used (in that 
buffer), if you (setq-local long-line-threshold 0) the optimizations will 
always be used (in that buffer).

Adding code to expose the feature further to Lisp would be (IMO) a 
mistake.