GNU bug report logs - #56682
Fix the long lines font locking related slowdowns

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From: Gregory Heytings <gregory <at> heytings.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: gerd.moellmann <at> gmail.com, 56682 <at> debbugs.gnu.org, larsi <at> gnus.org, monnier <at> iro.umontreal.ca
Subject: bug#56682: Fix the long lines font locking related slowdowns
Date: Sat, 30 Jul 2022 13:31:42 +0000

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>
> And I've found the culprit: we weren't restoring point after lifting the 
> locked narrowing.  narrow-to-region can move point if the new 
> restriction puts point outside of the region.  So what was happening is 
> that isearch-update was calling pos-visible-in-window-group-p to see 
> whether the match is visible, and that call would move point from under 
> the feet of isearch-update, because pos-visible-in-window-p calls 
> display routines.  So any subsequent uses of point would use a 
> completely wrong value of point.
>
> I've now made narrow-to-region preserve point across locked narrowing, 
> and the problem went away.
>
> Ugh! this one was a bitch to debug!
>

😉  Thanks, that's even better!

So the only remaining question is whether it is necessary to recompute 
narrowed_begv and narrowed_zv in init_iterator:

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);
+       }
    }

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