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, monnier <at> iro.umontreal.ca, dgutov <at> yandex.ru
Subject: bug#56682: Fix the long lines font locking related slowdowns
Date: Fri, 05 Aug 2022 11:50:56 +0000

>
> There's (at least) one more aspect of this, as long as Text mode is 
> being used: Text mode doesn't force bidi-paragraph-direction to be 
> left-to-right, whereas all descendants of prog-mode, including js-mode, 
> do.  Leaving bidi-paragraph-direction at nil means Emacs needs to 
> determine the base paragraph direction each time it's about to redisplay 
> a window, and that might be expensive, especially in a large buffer 
> without any paragraph breaks (by default, an empty line), because that 
> is determined by the first strong directional character of the 
> paragraph.  So for a more fair comparison with Text mode, you should set 
> bidi-paragraph-direction to the value left-to-right in text-mode 
> buffers.
>

Indeed, that seems to be the culprit here, I didn't know that text-mode 
was an exception here.  If I set bidi-paragraph-direction to 
'left-to-right after visiting the arabic-small.txt file, Emacs (mis) 
behaves like it does for the other Arabic files: it becomes slow, and C-n 
C-p do not work correctly anymore.

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