bug#17905: 24.3.50; writing with a giant font triggers RTL text entry

[Eli Zaretskii <eliz@gnu.org>]

> From: Joe Corneli <holtzermann17@gmail.com>
> Date: Wed, 02 Jul 2014 23:30:33 +0100
> 
> My eyesight is very bad without my glasses, and I was experimenting with
> a giant font on my laptop that I could comfortably read without them.
> 
> To replicate my set-up: shift+click and select "Increase Buffer Text
> Size" several times, to get letters that are about an inch tall!  The
> buffer is in fundamental mode.  Emacs is compiled with Xft support.
> 
> I was able to replicate the following weird behavior by following the
> above recipe with emacs -q --no-site-file, using the font that is the
> default there.
> 
> Here's a sample text generated by simply typing with a giant-sized font:
> 
> SAMPLE:
> 
> Maybe it's when the lines are particularly long, like if I keep writing
> after a ?seog rosruc eht erehw fo yltnednepedni ,elihw
> 
> (DECRYPTED):
> 
> Maybe it's when the lines are particularly long, like if I keep writing
> after a while, independently of where the cursor goes?
> 
> It seems that quite reliably, with this giant font, the lines will flip
> to RTL after writing for a while.  If I write in the same buffer in a
> window that doesn't have the giant font set, it doesn't trigger RTL.
> Here's a text I wrote, starting in window in which the buffer is
> normal-sized, and then switching, back to the giant sized buffer after a
> while:
> 
> SAMPLE:
> 
> Is it true even if the font size is a bit smaller, I wonder, or does it
> only happen with my glasses off?  Is there a particular point when it
> hits, or does it only happen if I get going with some particular word in
> the way?  ...ezis tnof yb dereggirt eb ot yletinifed smees tI
> 
> Finally, here's a text written in the giant-sized buffer that can be
> used to estimate where it switches:
> 
> SAMPLE:
> 
> a b c d e f g h i j k l m n o p q r s t u v w x y z a b c d e f g h i j k l m n u t s r q p o
> 
> ... hm, it seems to flip at EXACTLY AT 80 CHARACTERS!  (That matches the
> first sample text above, too.)

Could you please provide a complete self-contained recipe for
reproducing this?  Something like this:

  - start with "emacs -Q"
  - enlarge the font by shift-mouse and selecting "Increase Buffer
    Text Size" N times (please tell what is the value of N)
  - type the following text: ....

etc.

Given your description, I have hard time understanding what to do to
reproduce this.

Thanks.