Underline and Strikethrough are not visible for small text sizes

[robojeb]

Bevy version and features

Version 0.19.0

What you did

Added the Underline component to some 12px sized text.

What went wrong

I expected underlined text, instead the underline was not visible.

Additional information

This occurs because the derived underline_size() returned by parley is less than 1px in height.

Adding a .max(Vec::new(0.0, 1.0)) to the ExtractedUINode created in extract_text_decorations for underlines resolves this issue, though I don't know enough to know if that will interact poorly with window and ui-scaling.
The alternative to allow customized underline thickness is mentioned in #21676 but doesn't yet have a tracking issue.

This issue also affects the Strikethrough component, but due to the different position of the strike-through element it only affects some small font sizes but not all.

Reproduction Example

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, scene.spawn())
        .run();
}

fn scene() -> impl SceneList {
    bsn_list!(Camera2d, ex())
}

fn ex() -> impl Scene {
    let text = [8, 10, 12, 14, 16, 20, 24, 32, 48]
        .iter()
        .map(|size| {
            bsn! {
                    TextFont {
                        font_size: FontSize::Px({*size as f32})
                    }
                    Text::new("Underlined")
                    Underline
                    Strikethrough
            }
        })
        .collect::<Vec<_>>();

    bsn! {
        Node {
            width: percent(100),
            height: percent(100),
            flex_direction: FlexDirection::Column,
            justify_content: JustifyContent::Center,
            align_items: AlignItems::Center,
        }
        Children [{text}]
    }
}

[robojeb]

After reviewing the code-path, the scaling factor is computed prior to assigning the Font size, so both window scaling and UiScale should be accounted for already at the point the Rect is calculated for the underline.

Some simple testing shows that the "clamp to one px" option works for both underline and strikethrough across UiScale from 0.1 to 2.0 for font sizes between 1 and 24 px.