bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> I did try this, and it does happen.  Here:
>
>  (with-temp-buffer
>    (insert-file-contents "long-line.xml")
>    (point-max))
>
> (where long-line.xml is the file whose URL I've shown in my message). 
> This gives me 30001, whereas the correct value is 306666.

That's of course not what I meant with "in practice".  What I meant is: 
are there actual uses of "(with-temp-buffer (insert-file-contents ..." 
which insert actual files with sufficiently long lines?  Such files are 
not so common.  As I said, if that's the case, it's easy to add a "(let 
((auto-narrow-long-line-threshold nil))" around the code, in this case 
either in the with-temp-buffer macro or in the code above.

> I think it will happen quite a lot in the wild.

Let's see if it does before reaching a conclusion.

> That means the list will be very long and users will have to constantly 
> update it.  E.g., whenever they install a new package or even a new 
> version of a package.

Packages can take care of this by themselves, can't they?  When the 
package is loaded, (add-to-list auto-narrow-widen-automatically 
the-package-command).

> > > Also, the automatic restriction is very visible and causes surprising 
> > > effects: (point-min) and (point-max) evaluate to the narrowed region 
> > > (so it's basically very hard to know the real size of the buffer), and 
> > > scrolling through the buffer causes the scroll-bar thumb move in a 
> > > non-monotonic manner: I scroll towards the end of the buffer, but the 
> > > thumb sometimes jumps back up, as if I moved towards the beginning.
> >
> > Indeed.  Users will see the "Auto-Narrow" indication in the modeline 
> > with which they can easily find an explanation for that behavior.  If 
> > this behavior is problematic, it could be fixed by using the dimensions 
> > of the whole buffer when updating the scroll bar (and perhaps also the 
> > modeline), when auto-narrow-mode is active.
>
> I think it should be fixed, yes.

Okay, I'll see what I can do.