bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> > I don't know yet if it's necessary to add another similar recomputation 
> > inside init_iterator.
>
> I'll play with other callers of init_iterator and start_display, and see 
> if they can do similar things.

In case this helps, it is easy to enter the following conditional, for 
example with C-s, or with your previous M-g c 20000 and M-: 
(vertical-motion 200) RET recipe (in both steps).  It's not clear to me if 
updating the narrowing bounds there has an actual impact.  At least 
applying that change does not seem to have negative effects.

diff --git a/src/xdisp.c b/src/xdisp.c
index b1ee7889d4..e415320a52 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3429,6 +3429,12 @@ init_iterator (struct it *it, struct window *w,
     {
       it->narrowed_begv = get_narrowed_begv (w, window_point (w));
       it->narrowed_zv = get_narrowed_zv (w, window_point (w));
+      if (charpos >= 0
+         && (charpos < it->narrowed_begv || charpos > it->narrowed_zv))
+       {
+         it->narrowed_begv = get_narrowed_begv (w, charpos);
+         it->narrowed_zv = get_narrowed_zv (w, charpos);
+       }
     }

   /* If a buffer position was specified, set the iterator there,