bug#45652: so-long mode not triggered despite big file with very long lines

[積丹尼 Dan Jacobson]

Let's make a deal: have so-long mode be the default (for those who have
opted in) for all "big" files in the first place.

I recall for this .har, it starts out well behaved, but then the
whopping lines are closer to the bottom of the file.

I would say that opening any large file is fraught with danger (of
locking up emacs, requiring kill -1.)

And, if the user is sure his file is safe, he can always toggle so-long
mode off for that file.

Even having emacs scan a file for big lines sounds like it might be
risky. So it would be great to give the scanner a vacation in such large
file cases.

PS> for `so-long-detected-long-line-p' for details.
Not yet in 27.1
PS> https://git.savannah.nongnu.org/cgit/so-long.git/plain/so-long.el?h=wip
That's OK, I'm not really following this that closely. Just giving suggestions.