bug#57669: 29.0.50; C-n, C-p off under long lines

[Gregory Heytings]

> > I'm not, I asked him a few pointers, and he replied with the hash of a 
> > commit which disables what exists on master.
>
> The code is there, you can read it.

I confess I don't have the patience to do that.  I already don't have as 
much free time as I'd like to devote to Emacs, so I'd better spend it 
wisely.

> In a nutshell, his changes disable too many features of the GUI display 
> (the ones that make the Emacs display slow with long lines), and they 
> disable those features _always_, not just when the lines are long.

Okay, that's not the right way to do it, indeed.

Dick, if you feel the above doesn't do justice to your code, feel free to 
elaborate on your chosen strategy, either here or privately.