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

From: Gregory Heytings <[email-masked]>
Date: Wed, 10 Aug 2022 08:59:42 +0000

>
> When I visit addons.json (from my Firefox directory), which is a single 
> line of (only) 19540 characters, and type `C-n' and hold down both keys, 
> the cursor stops moving almost immediately (on the third visual line) 
> and after letting go of the keys and waiting several seconds, point 
> jumps down the buffer to its actual position; likewise with `C-p'. 
> This is on master with -Q, and is no different from Emacs 28 (the only 
> difference I notice is that in master the buffer is wrongly fontified 
> from position 8235 to the end, while in 28 the entire buffer is 
> correctly fontified).  Is this expected?  (When I enable so-long-mode in 
> the buffer, holding down `C-n' or `C-p' produces no delay, both in 
> master and 28.)
>

That's not expected, no, and rather surprising because it would contradict 
what we've seen so far.  Can you post that file somewhere?  The only 
possible reason I can see ATM is that the file contains Arabic characters, 
which are (and will remain) slow.