Inline `FontFeatures` changes in `Buffer::set_rich_text` are ignored within a word

[Byunk]

When adjacent rich-text spans differ only in FontFeatures, the given feature is ignore if it occurs within a word.

For example, 'HNO3' where '3' has 'subs' feature produce the normal '3' glyph, while shaping '3' separately with the same attributes correctly produces subscript.

Environment

• cosmic-text: 0.19.0
• Font: Carlito Regular

Reproduction

Cargo.toml:

[package]
name = "example"
version = "0.1.0"
edition = "2024"

[dependencies]
cosmic-text = "0.19"

main.rs:

use cosmic_text::{
    Attrs, Buffer, Family, FeatureTag, FontFeatures, FontSystem, Metrics, Shaping, fontdb,
};

fn main() {
    let mut database = fontdb::Database::new();
    database.load_font_data(include_bytes!("../Carlito/Carlito-Regular.ttf").to_vec());
    let mut font_system = FontSystem::new_with_locale_and_db("en-US".into(), database);

    let normal = Attrs::new().family(Family::Name("Carlito"));
    let mut features = FontFeatures::new();
    features.enable(FeatureTag::new(b"subs"));
    let subscript = normal.clone().font_features(features);

    // glyph id for "3" without features
    let normal_id = shape_text(&mut font_system, "3", &normal)[0];
    // glyph id for "3" with "subs" feature enabled
    let subscript_id = shape_text(&mut font_system, "3", &subscript)[0];

    let mut buffer = Buffer::new(&mut font_system, Metrics::new(32.0, 40.0));
    buffer.set_rich_text(
        [("HNO", normal.clone()), ("3", subscript)],
        &normal,
        Shaping::Advanced,
        None,
    );
    // glyph id for "3" within shaped text
    let rich_text_id = glyph_ids(&mut font_system, &mut buffer)[3];

    println!("normal: {normal_id}");
    println!("subscript: {subscript_id}");
    println!("rich text: {rich_text_id}");

    assert_ne!(normal_id, subscript_id); // must be different when font supports subscript

    // FAIL: currently panic here
    assert_eq!(rich_text_id, subscript_id);
}

fn shape_text(font_system: &mut FontSystem, text: &str, attrs: &Attrs<'_>) -> Vec<u16> {
    let mut buffer = Buffer::new(font_system, Metrics::new(32.0, 40.0));
    buffer.set_text(text, attrs, Shaping::Advanced, None);
    glyph_ids(font_system, &mut buffer)
}

fn glyph_ids(font_system: &mut FontSystem, buffer: &mut Buffer) -> Vec<u16> {
    buffer.shape_until_scroll(font_system, false);
    buffer
        .layout_runs()
        .flat_map(|run| run.glyphs.iter().map(|glyph| glyph.glyph_id))
        .collect()
}

Output with Carlito Regular:

normal '3' glyph:     397
standalone subs '3':  405
rich-text subs '3':   397

which leads panic on the last assert_eq

[Byunk]

From quick investigation, it seems that Attrs::compatible returns true when font_features differs, though i don't have any deeper context on this.