bug#66664: On modified lines containing 🌐︎ and Unicode VS15 in text mode emacs does not show the real content of the lines

[Eli Zaretskii <eliz@gnu.org>]

> From: Дилян Палаузов
>  <dilyan.palauzov@aegee.org>
> Date: Sat, 21 Oct 2023 14:07:52 +0200
> 
> Hello,
> 
> I have a text unix file u.txt where I insert this line:
> 
> 
> t.add('🌐︎ ' + 'abcd')
> 
> That is a globe U+1F310 followed by Unicode Variation Selector 15 to remove the colour of the globe.
> 
> $ hexdump -C  u.txt
> 00000000  74 2e 61 64 64 28 27 f0  9f 8c 90 ef b8 8e 20 27  |t.add('....... '|
> 00000010  20 2b 20 27 61 62 63 64  27 29 0a                 | + 'abcd').|
> 0000001b
> 
> I open the file in text mode, emacs-fedora-28.3 -nw, and press the right arrow repeatedly to go over c.  Then I press x.  emacs shows that c is deleted and on its place appears x. I press Ctrl+L to refresh.  Now c appears before x.
> 
> I close emacs, repeat the above steps and get consistent results.  I try this on a different system with self-compiled emacs 29.1, same results.

What is the terminal emulator you are using, and on what OS?

Basically, what the above means is that the text terminal you are
using doesn't support the sequence of U+1F310 followed by VS-15 as
Emacs expects it.  On such a terminal you need to disable
auto-composition-mode to be able to edit such text.