Zero-width joined emoji incorrectly displayed

[Nogesma]

Description

When trying to display a zero-width joined emoji like 👩‍🔬 ("\u{1f469}\u{200d}\u{1f52c}"), on kitty terminal, kitty will only display 👩 ("\u{1f469}").

This happens because when ratatui calculates the diff of the two terminal buffers, it will not skip the correct amount of cells. It seems to only skip 1 cell instead of the the 3 needed for this emoji. This causes it to overwrite the other cells, which deletes part of the emoji.

I am not exactly sure how the diff is calculated, but it looks like the to_skip value gets overwritten on each loop iteration, which makes it skip only 1 cell, instead of the 3 returned by current.symbol().width().saturating_sub(1).

This issue is only visible on terminals that correctly display this emoji over 4 cells, like kitty.

Expected behaviour

Here's a test that should pass:

#[test]
fn buffer_diffing_zero_width_joined() {
    let mut prev = Buffer::empty(Rect::new(0, 0, 5, 1));
    prev.set_string(0, 0, "aaaaa", Style::default());

    let mut next = Buffer::empty(Rect::new(0, 0, 5, 1));
    next.set_string(0, 0, "\u{1f469}\u{200d}\u{1f52c}", Style::default());

    let diff = prev.diff(&next);

    assert_eq!(
        diff,
        vec![
            (0, 0, &cell("\u{1f469}\u{200d}\u{1f52c}")),
            (4, 0, &cell(" ")),
        ]
    );
}

We instead get

vec![
    (0, 0, &cell("\u{1f469}\u{200d}\u{1f52c}")),
    (2, 0, &cell(" ")),
    (3, 0, &cell(" ")),
    (4, 0, &cell(" ")),
]

[kovidgoyal]

Not that kitty's behavior with ZWJ is incorrect and subject to change in the future, see the open issue about it in the kitty bug tracker.

[joshka]

Also note #75 which I have no idea how to resolve in a way that satisfies all parties.

[kdheepak]

Relevant issue for Kitty: kovidgoyal/kitty#3810

PS: @kovidgoyal thank you for making and maintaining Kitty (and Calibre)!

[Nogesma]

Okay, I thought kitty had the right behavior, and that ratatui was not respecting the 4 cell width given by the unicode-width crate.

Feel free to close this if you consider it a duplicate of #75.