Arabic isn't properly rendered in Neovim until a newline is inserted

[pro-anon]

The letters here aren't properly rendered

Only after inserting a new line they get fixed

This issue doesn't happen when typing directly into the terminal

[arakiken]

Sorry for replying late.
Thanks for your report, but I couldn't reproduce this problem in my environment. (neovim 0.8.2 with empty ~/.config/nvim)
Will you tell me your neovim version and configuration?

[pro-anon]

v0.8.2
I've tried it without any config, the issue is still the same

[pro-anon]

I'm running it on arch. Installed mlterm from here https://aur.archlinux.org/packages/mlterm

[arakiken]

I tested plain mlterm(3.9.2) and plain neovim(0.8.2) without any configuration, but I haven't reproduced it yet.
I'm not sure, but does ./configure without --enable-optimize-redrawing option in building mlterm improve it?

[pro-anon]

Nope, still the same. Maybe it's an issue with the font? What font do you use?

[pro-anon]

I've noticed that, in addition to inserting a new line, inserting ﻻ re-renders the line properly

[arakiken]

I use "Not Sans Arabic" font to show arabic characters, but I think it doesn't caused by font configuration.
Sorry but I haven't found out the cause yet.

[holytrousers]

I don't think the bug is in mlterm.
Try to run nvim with the --clean option.
Looks like it's happening when you have set cursorline and the cursor is highlighting the arabic text. This can happen if you are using variable width fonts.
Make sure you have monospace fonts with Arabic glyphs installed and you have configured mlterm to use them (Try Dejavu Sans Mono)
if you are using xft, you have to specify which fonts to use for Arabic glyphs. I have tried to set all Arabic Unicode ranges to use Deja Vu Sans Mono and managed to make it work :

DEFAULT = Fantasque Sans Mono, 12
ISO10646_UCS4_1_FULLWIDTH = Noto Sans Mono CJK JP, 10
U+10a60-10a7f=DejaVu Sans Mono, 10
U+10a80-10a9f=DejaVu Sans Mono, 10
U+600-6ff=    DejaVu Sans Mono, 10
U+750-77f=    DejaVu Sans Mono, 10
U+8a0-8ff=    DejaVu Sans Mono, 10
U+1ee00-1eeff=DejaVu Sans Mono, 10 
U+10e60-10e7f=DejaVu Sans Mono, 10
U+fe70-feff=  DejaVu Sans Mono, 10
U+fb50-fdff=  DejaVu Sans Mono, 10

[pro-anon]

@holytrousers
I've tried the --clean flag with neovim and DejaVu Sans Mono, but the issue got worse. Now I have to insert a new line, then delete it so that the line before it would get rendered correctly.

[holytrousers]

have you tried the mlterm-git package ?
the one you mentioned has been flagged as out of date

[holytrousers]

@pro-anon I have reproduced your problem by using the :set arabic option and actually writing the text. Previously i pasted arabic text from yamli.com into vim that's why i wasn't able to reproduce the problem.
Writing in arabic (using the "ara" keymap) inside the shell produces the same behaviour though. لا fixes it.
The same problem happens when writing in arabic inside the shell in other terminals xterm, alacritty.
Looks like Kitty implements bicon properly out of the box.
Running bicon in xterm and alacritty and writing in arabic inside the shell fixes the problem.
Writing arabic with the :set arabic option in vim works fine in xterm and alacritty without running bicon.
Running bicon in mlterm doesn't fix the problem though.

As a temporary workaround try pressing Control+L in normal mode inside vim, that will refresh the screen and render the ligatures properly (inoremap to refresh every time you switch to normal mode )
Another trick i've found fixes ligatures : try to insert a letter, then move backward one letter, so that the cursor is inserting text before that letter. ligatures seem to work then.
Looks like it's a problem with the bicon implementation in mlterm. @arakiken might give an insight on this

[holytrousers]

ps. add inoremap <Esc> <Esc><C-L> to your vim configuration

[holytrousers]

NB. The trick with prepending a letter before the inserted text works only partially though. Baseline Ligatures work then but the letters at the end of the word don't change their shape.

[holytrousers]

I think we have narrowed down the problem, try :

• Start vim
• :set arabic
• Begin typing in latin by pressing ctrl-^ (from left-to-right)
• Switch back to arabic by pressing again ctrl-^ and write in arabic

Ligatures and end of word letters render properly.
Which means that the bug occurs only when the whole line is written from right-to-left, ie. the beginning of the line is on the right side of the window.

[holytrousers]

@pro-anon : your prompt is styled to align the text to the left side of the screen, that's why when you type arabic text in the shell it displays properly.
This confirms the previous conclusion : the problem occurs only when the whole line is RTL.