bug#56682: locked narrowing

[Gregory Heytings]

> I think this is just one aspect of the issue.  The loop under this 
> condition "punishes" large buffers with no long lines because we search 
> the entire buffer top to bottom.  To make this scale better, I think we 
> should only search in some predefined vicinity of the window, perhaps 
> the same region to which we narrow the buffer when we do detect long 
> lines.  We can then repeat the search if point moves far away from its 
> last value.
>
> There's no need to search the whole buffer, that will cause delays in 
> very large buffers for no good reason.
>
> We've talked about this a few months ago, and you said fixing this was 
> part of your todo.  I think now is the time.

Yes, it was and still is part of my todo.  At that point of time I was not 
at all expecting that Emacs 29 would be released three months later. 
What I had and still have in mind is too large a change for Emacs 29, so 
at first glance I think for now we'll have to live with the current 
solution.

Note that, apart from Dmitry, nobody reported a problem with that 
detection mechanism, and Dmitry's recipe with which he spotted that 
problem does not reflect actual usage.  And note that with the patch I 
just sent even Dmitry's recipe is fixed.

I'll see if I can design something smaller which would be suitable for 
Emacs 29, however.