Multiple fonts in a single `Text` do not work unless there is a space character

[rparrett]

Bevy version and features

0.18.0-rc.1 and
current main (bisected to #21699 and further upstream, see pop-os/cosmic-text#445)

Relevant system information

SystemInfo { os: "macOS 15.6.1 Sequoia", kernel: "24.6.0", cpu: "Apple M4 Max", core_count: "16", memory: "64.0 GiB" }
AdapterInfo { name: "Apple M4 Max", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }

What you did

Place the following code in examples and cargo run --example 22190.

22190.rs

//! #22190

use bevy::prelude::*;

fn main() {
    let mut app = App::new();

    app.add_plugins(DefaultPlugins).add_systems(Startup, setup);

    app.run();
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    let garamond = TextFont {
        font: asset_server.load("fonts/EBGaramond12-Regular.otf"),
        ..default()
    };

    let sans_bold = TextFont {
        font: asset_server.load("fonts/FiraSans-Bold.ttf"),
        ..default()
    };

    commands.spawn(Camera2d);

    commands.spawn((
        Text::new("default"),
        Node {
            position_type: PositionType::Absolute,
            top: px(100),
            left: px(100),
            ..default()
        },
        children![
            (TextSpan::new("garamond"), garamond.clone()),
            (TextSpan::new("sans_bold"), sans_bold.clone())
        ],
    ));

    commands.spawn((
        Text::new("default "),
        Node {
            position_type: PositionType::Absolute,
            top: px(140),
            left: px(100),
            ..default()
        },
        children![
            (TextSpan::new("garamond "), garamond.clone()),
            (TextSpan::new("sans_bold "), sans_bold.clone())
        ],
    ));
}

What went wrong

Each line of text should show 3 separate fonts. But on the top line of text where there are no space characters, there is only one font.

Additional information

Discovered while migrating bevy_simple_text_input.

[ickshonpe]

Specifically, the font doesn't change until the text reaches a space, even mid-text-section:

Each of the font names has its own text section. In the second to last line, "Mona Sans" is a single text section using the MonaSans font, but it renders "Mona" in FiraSans and only "Sans" in MonaSans.

Font weights don't appear to be affected.

[ickshonpe]

Made an upstream patch that seems to work:

https://github.com/ickshonpe/cosmic-text/tree/ascii-fast-path-fix