Zero-width-joiner-joined emojis break everything

[kokolem]

Hi,

there is a problem with zero-width-joiner-joined emojis in kitty, watch this:

Full quality video here.

Deleting part of an emoji made up of two emojis joined by a zero width joiner doesn't change the emoji displayed. For example, when pasting the Man Mechanic 👨‍🔧 (👨 U+1F468, Zero Width Joiner U+200D, 🔧 U+1F527) emoji into kitty, it correctly joins the characters into a single emoji followed by two invisible characters (the Zero Width Joiner U+200D and the Wrench emoji 🔧 U+1F527). However, after deleting the last character (🔧 U+1F527), kitty still displays the Man Mechanic, even though only 👨 U+1F468 and Zero Width Joiner U+200D should be displayed (they don't form any special emoji on their own). After pressing Return to get a new prompt and then pressing upwards arrow to use the previous command, kitty displays the emoji sequence differently (correctly) even though it's still the same sequence.

Pasting a more complicated zero-width-joiner-joined emoji into kitty breaks it even more. For example the Man Mechanic: Dark Skin Tone 👨🏿‍🔧 (👨 U+1F468, 🏿 U+1F3FF, Zero Width Joiner U+200D, 🔧 U+1F527) causes kitty to make the cursor jump over the character before the emoji and then makes deleting the PS1 prompt possible while duplicating the character after the emoji.

Configuration:
kitty: 0.17.2
kitty config: default
OS: Arch Linux
Kernel: 5.6.3
Shell: bash 5.0.16

[kovidgoyal]

dup of #1978

[kokolem]

Hi,

I didn't find anyone mentioning ligatures not updating properly, cursor position problems, being able to delete the PS1 prompt, nor duplicating characters in #1978. Is this issue really a duplicate?

[kovidgoyal]

yeah it is. kitty has no support for ZWJ and frankly might never support
it as its a really bad idea that has serious performance implications to
support.

[trygveaa]

I can reproduce the issue of being able to delete PS1 with just 👨🏿, which doesn't have any ZWJ though (by moving the cursor before the emoji and pressing backspace).

But I guess that might be an issue with the shell, not with kitty, even though it happens in both bash and zsh? I don't know of any other terminal emulators that support skin tones, so I can't test if it also happens in other terminal emulators.

[kovidgoyal]

Try it with cat, this is almost certainly a shell issue

[trygveaa]

I can't test exactly the same with cat, since I did that by moving the cursor before the emoji. But I see that even if I don't move the cursor, two characters from PS1 can be deleted per emoji typed. I tried with echo -n test && cat, and yeah, test is never removed in that case.

@kokolem: You could try reporting the issue about deleting PS1 to bash or zsh.