bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > It doesn't, see again the long-line-excerpt.xml file I sent you.  It is 
> > only 30K long, which would be a typical size for a restriction, and 
> > editing that file is slow when font locking is on and fast when font 
> > locking is off.
>
> I guess it's because we leave ZV at its original value, and font-lock 
> attempts to fontify the entire long line till the end?  Does it help to 
> make the value of syntax-wholeline-max smaller?

Sorry, I wasn't clear enough: I meant opening that file with Emacs from 
master.  Given that it's only 30K long, which would be a typical size for 
a temporarily narrowed buffer, and that editing it is slow, I don't see 
how applying a narrowing similar to the size of that file before calling 
fontification-functions could possibly help.

> > But fontification-functions are not the only problem here.  What I also 
> > observe is that, for example, moving in a fontified buffer takes (much) 
> > longer than moving in a non-fontified buffer.  For example, in 
> > long-line.xml, vertical-motion takes about 40 ms backward and 10 ms 
> > forward in a non-fontified buffer, and about 180 ms backward and 40 ms 
> > forward in a fontified buffer.
>
> I guess that's because vertical-motion calls the display code, and that 
> calls fontification-functions.

No, fontification-functions are not called when moving around in an 
already fontified portion of the buffer.  So the slowdown of C-n and C-p 
(and others) in that case is not caused by fontification-functions.