gpui/text_system/

line.rs

use crate::{
    App, Bounds, DevicePixels, Half, Hsla, LineLayout, Pixels, Point, RenderGlyphParams, Result,
    ShapedGlyph, ShapedRun, SharedString, StrikethroughStyle, TextAlign, UnderlineStyle, Window,
    WrapBoundary, WrappedLineLayout, black, fill, point, px, size,
};
use derive_more::{Deref, DerefMut};
use smallvec::SmallVec;
use std::sync::Arc;

/// Pre-computed glyph data for efficient painting without per-glyph cache lookups.
///
/// This is produced by `ShapedLine::compute_glyph_raster_data` during prepaint
/// and consumed by `ShapedLine::paint_with_raster_data` during paint.
#[derive(Clone, Debug)]
pub struct GlyphRasterData {
    /// The raster bounds for each glyph, in paint order.
    pub bounds: Vec<Bounds<DevicePixels>>,
    /// The render params for each glyph (needed for sprite atlas lookup).
    pub params: Vec<RenderGlyphParams>,
}

/// Set the text decoration for a run of text.
#[derive(Debug, Clone)]
pub struct DecorationRun {
    /// The length of the run in utf-8 bytes.
    pub len: u32,

    /// The color for this run
    pub color: Hsla,

    /// The background color for this run
    pub background_color: Option<Hsla>,

    /// The underline style for this run
    pub underline: Option<UnderlineStyle>,

    /// The strikethrough style for this run
    pub strikethrough: Option<StrikethroughStyle>,
}

/// A line of text that has been shaped and decorated.
#[derive(Clone, Default, Debug, Deref, DerefMut)]
pub struct ShapedLine {
    #[deref]
    #[deref_mut]
    pub(crate) layout: Arc<LineLayout>,
    /// The text that was shaped for this line.
    pub text: SharedString,
    pub(crate) decoration_runs: SmallVec<[DecorationRun; 32]>,
}

impl ShapedLine {
    /// The length of the line in utf-8 bytes.
    #[allow(clippy::len_without_is_empty)]
    pub fn len(&self) -> usize {
        self.layout.len
    }

    /// The width of the shaped line in pixels.
    ///
    /// This is the glyph advance width computed by the text shaping system and is useful for
    /// incrementally advancing a "pen" when painting multiple fragments on the same row.
    pub fn width(&self) -> Pixels {
        self.layout.width
    }

    /// Override the len, useful if you're rendering text a
    /// as text b (e.g. rendering invisibles).
    pub fn with_len(mut self, len: usize) -> Self {
        let layout = self.layout.as_ref();
        self.layout = Arc::new(LineLayout {
            font_size: layout.font_size,
            width: layout.width,
            ascent: layout.ascent,
            descent: layout.descent,
            runs: layout.runs.clone(),
            len,
        });
        self
    }

    /// Paint the line of text to the window.
    pub fn paint(
        &self,
        origin: Point<Pixels>,
        line_height: Pixels,
        align: TextAlign,
        align_width: Option<Pixels>,
        window: &mut Window,
        cx: &mut App,
    ) -> Result<()> {
        paint_line(
            origin,
            &self.layout,
            line_height,
            align,
            align_width,
            &self.decoration_runs,
            &[],
            window,
            cx,
        )?;

        Ok(())
    }

    /// Paint the background of the line to the window.
    pub fn paint_background(
        &self,
        origin: Point<Pixels>,
        line_height: Pixels,
        align: TextAlign,
        align_width: Option<Pixels>,
        window: &mut Window,
        cx: &mut App,
    ) -> Result<()> {
        paint_line_background(
            origin,
            &self.layout,
            line_height,
            align,
            align_width,
            &self.decoration_runs,
            &[],
            window,
            cx,
        )?;

        Ok(())
    }

    /// Split this shaped line at a byte index, returning `(prefix, suffix)`.
    ///
    /// - `prefix` contains glyphs for bytes `[0, byte_index)` with original positions.
    ///   Its width equals the x-advance up to the split point.
    /// - `suffix` contains glyphs for bytes `[byte_index, len)` with positions
    ///   shifted left so the first glyph starts at x=0, and byte indices rebased to 0.
    /// - Decoration runs are partitioned at the boundary; a run that straddles it is
    ///   split into two with adjusted lengths.
    /// - `font_size`, `ascent`, and `descent` are copied to both halves.
    pub fn split_at(&self, byte_index: usize) -> (ShapedLine, ShapedLine) {
        let x_offset = self.layout.x_for_index(byte_index);

        // Partition glyph runs. A single run may contribute glyphs to both halves.
        let mut left_runs = Vec::new();
        let mut right_runs = Vec::new();

        for run in &self.layout.runs {
            let split_pos = run.glyphs.partition_point(|g| g.index < byte_index);

            if split_pos > 0 {
                left_runs.push(ShapedRun {
                    font_id: run.font_id,
                    glyphs: run.glyphs[..split_pos].to_vec(),
                });
            }

            if split_pos < run.glyphs.len() {
                let right_glyphs = run.glyphs[split_pos..]
                    .iter()
                    .map(|g| ShapedGlyph {
                        id: g.id,
                        position: point(g.position.x - x_offset, g.position.y),
                        index: g.index - byte_index,
                        is_emoji: g.is_emoji,
                    })
                    .collect();
                right_runs.push(ShapedRun {
                    font_id: run.font_id,
                    glyphs: right_glyphs,
                });
            }
        }

        // Partition decoration runs. A run straddling the boundary is split into two.
        let mut left_decorations = SmallVec::new();
        let mut right_decorations = SmallVec::new();
        let mut decoration_offset = 0u32;
        let split_point = byte_index as u32;

        for decoration in &self.decoration_runs {
            let run_end = decoration_offset + decoration.len;

            if run_end <= split_point {
                left_decorations.push(decoration.clone());
            } else if decoration_offset >= split_point {
                right_decorations.push(decoration.clone());
            } else {
                let left_len = split_point - decoration_offset;
                let right_len = run_end - split_point;
                left_decorations.push(DecorationRun {
                    len: left_len,
                    color: decoration.color,
                    background_color: decoration.background_color,
                    underline: decoration.underline,
                    strikethrough: decoration.strikethrough,
                });
                right_decorations.push(DecorationRun {
                    len: right_len,
                    color: decoration.color,
                    background_color: decoration.background_color,
                    underline: decoration.underline,
                    strikethrough: decoration.strikethrough,
                });
            }

            decoration_offset = run_end;
        }

        // Split text
        let left_text = if byte_index == self.text.len() {
            self.text.clone()
        } else {
            SharedString::new(&self.text[..byte_index])
        };
        let right_text = if byte_index == 0 {
            self.text.clone()
        } else {
            SharedString::new(&self.text[byte_index..])
        };

        let left_width = x_offset;
        let right_width = self.layout.width - left_width;

        let left = ShapedLine {
            layout: Arc::new(LineLayout {
                font_size: self.layout.font_size,
                width: left_width,
                ascent: self.layout.ascent,
                descent: self.layout.descent,
                runs: left_runs,
                len: byte_index,
            }),
            text: left_text,
            decoration_runs: left_decorations,
        };

        let right = ShapedLine {
            layout: Arc::new(LineLayout {
                font_size: self.layout.font_size,
                width: right_width,
                ascent: self.layout.ascent,
                descent: self.layout.descent,
                runs: right_runs,
                len: self.layout.len - byte_index,
            }),
            text: right_text,
            decoration_runs: right_decorations,
        };

        (left, right)
    }
}

/// A line of text that has been shaped, decorated, and wrapped by the text layout system.
#[derive(Default, Debug, Deref, DerefMut)]
pub struct WrappedLine {
    #[deref]
    #[deref_mut]
    pub(crate) layout: Arc<WrappedLineLayout>,
    /// The text that was shaped for this line.
    pub text: SharedString,
    pub(crate) decoration_runs: Vec<DecorationRun>,
}

impl WrappedLine {
    /// The length of the underlying, unwrapped layout, in utf-8 bytes.
    #[allow(clippy::len_without_is_empty)]
    pub fn len(&self) -> usize {
        self.layout.len()
    }

    /// Paint this line of text to the window.
    pub fn paint(
        &self,
        origin: Point<Pixels>,
        line_height: Pixels,
        align: TextAlign,
        bounds: Option<Bounds<Pixels>>,
        window: &mut Window,
        cx: &mut App,
    ) -> Result<()> {
        let align_width = match bounds {
            Some(bounds) => Some(bounds.size.width),
            None => self.layout.wrap_width,
        };

        paint_line(
            origin,
            &self.layout.unwrapped_layout,
            line_height,
            align,
            align_width,
            &self.decoration_runs,
            &self.wrap_boundaries,
            window,
            cx,
        )?;

        Ok(())
    }

    /// Paint the background of line of text to the window.
    pub fn paint_background(
        &self,
        origin: Point<Pixels>,
        line_height: Pixels,
        align: TextAlign,
        bounds: Option<Bounds<Pixels>>,
        window: &mut Window,
        cx: &mut App,
    ) -> Result<()> {
        let align_width = match bounds {
            Some(bounds) => Some(bounds.size.width),
            None => self.layout.wrap_width,
        };

        paint_line_background(
            origin,
            &self.layout.unwrapped_layout,
            line_height,
            align,
            align_width,
            &self.decoration_runs,
            &self.wrap_boundaries,
            window,
            cx,
        )?;

        Ok(())
    }
}

fn paint_line(
    origin: Point<Pixels>,
    layout: &LineLayout,
    line_height: Pixels,
    align: TextAlign,
    align_width: Option<Pixels>,
    decoration_runs: &[DecorationRun],
    wrap_boundaries: &[WrapBoundary],
    window: &mut Window,
    cx: &mut App,
) -> Result<()> {
    let line_bounds = Bounds::new(
        origin,
        size(
            layout.width,
            line_height * (wrap_boundaries.len() as f32 + 1.),
        ),
    );
    window.paint_layer(line_bounds, |window| {
        let padding_top = (line_height - layout.ascent - layout.descent) / 2.;
        let baseline_offset = point(px(0.), padding_top + layout.ascent);
        let mut decoration_runs = decoration_runs.iter();
        let mut wraps = wrap_boundaries.iter().peekable();
        let mut run_end = 0;
        let mut color = black();
        let mut current_underline: Option<(Point<Pixels>, UnderlineStyle)> = None;
        let mut current_strikethrough: Option<(Point<Pixels>, StrikethroughStyle)> = None;
        let text_system = cx.text_system().clone();
        let mut glyph_origin = point(
            aligned_origin_x(
                origin,
                align_width.unwrap_or(layout.width),
                px(0.0),
                &align,
                layout,
                wraps.peek(),
            ),
            origin.y,
        );
        let mut prev_glyph_position = Point::default();
        let mut max_glyph_size = size(px(0.), px(0.));
        let mut first_glyph_x = origin.x;
        for (run_ix, run) in layout.runs.iter().enumerate() {
            max_glyph_size = text_system.bounding_box(run.font_id, layout.font_size).size;

            for (glyph_ix, glyph) in run.glyphs.iter().enumerate() {
                glyph_origin.x += glyph.position.x - prev_glyph_position.x;
                if glyph_ix == 0 && run_ix == 0 {
                    first_glyph_x = glyph_origin.x;
                }

                if wraps.peek() == Some(&&WrapBoundary { run_ix, glyph_ix }) {
                    wraps.next();
                    if let Some((underline_origin, underline_style)) = current_underline.as_mut() {
                        if glyph_origin.x == underline_origin.x {
                            underline_origin.x -= max_glyph_size.width.half();
                        };
                        window.paint_underline(
                            *underline_origin,
                            glyph_origin.x - underline_origin.x,
                            underline_style,
                        );
                        if glyph.index < run_end {
                            underline_origin.x = origin.x;
                            underline_origin.y += line_height;
                        } else {
                            current_underline = None;
                        }
                    }
                    if let Some((strikethrough_origin, strikethrough_style)) =
                        current_strikethrough.as_mut()
                    {
                        if glyph_origin.x == strikethrough_origin.x {
                            strikethrough_origin.x -= max_glyph_size.width.half();
                        };
                        window.paint_strikethrough(
                            *strikethrough_origin,
                            glyph_origin.x - strikethrough_origin.x,
                            strikethrough_style,
                        );
                        if glyph.index < run_end {
                            strikethrough_origin.x = origin.x;
                            strikethrough_origin.y += line_height;
                        } else {
                            current_strikethrough = None;
                        }
                    }

                    glyph_origin.x = aligned_origin_x(
                        origin,
                        align_width.unwrap_or(layout.width),
                        glyph.position.x,
                        &align,
                        layout,
                        wraps.peek(),
                    );
                    glyph_origin.y += line_height;
                }
                prev_glyph_position = glyph.position;

                let mut finished_underline: Option<(Point<Pixels>, UnderlineStyle)> = None;
                let mut finished_strikethrough: Option<(Point<Pixels>, StrikethroughStyle)> = None;
                if glyph.index >= run_end {
                    let mut style_run = decoration_runs.next();

                    // ignore style runs that apply to a partial glyph
                    while let Some(run) = style_run {
                        if glyph.index < run_end + (run.len as usize) {
                            break;
                        }
                        run_end += run.len as usize;
                        style_run = decoration_runs.next();
                    }

                    if let Some(style_run) = style_run {
                        if let Some((_, underline_style)) = &mut current_underline
                            && style_run.underline.as_ref() != Some(underline_style)
                        {
                            finished_underline = current_underline.take();
                        }
                        if let Some(run_underline) = style_run.underline.as_ref() {
                            current_underline.get_or_insert((
                                point(
                                    glyph_origin.x,
                                    glyph_origin.y + baseline_offset.y + (layout.descent * 0.618),
                                ),
                                UnderlineStyle {
                                    color: Some(run_underline.color.unwrap_or(style_run.color)),
                                    thickness: run_underline.thickness,
                                    wavy: run_underline.wavy,
                                },
                            ));
                        }
                        if let Some((_, strikethrough_style)) = &mut current_strikethrough
                            && style_run.strikethrough.as_ref() != Some(strikethrough_style)
                        {
                            finished_strikethrough = current_strikethrough.take();
                        }
                        if let Some(run_strikethrough) = style_run.strikethrough.as_ref() {
                            current_strikethrough.get_or_insert((
                                point(
                                    glyph_origin.x,
                                    glyph_origin.y
                                        + (((layout.ascent * 0.5) + baseline_offset.y) * 0.5),
                                ),
                                StrikethroughStyle {
                                    color: Some(run_strikethrough.color.unwrap_or(style_run.color)),
                                    thickness: run_strikethrough.thickness,
                                },
                            ));
                        }

                        run_end += style_run.len as usize;
                        color = style_run.color;
                    } else {
                        run_end = layout.len;
                        finished_underline = current_underline.take();
                        finished_strikethrough = current_strikethrough.take();
                    }
                }

                if let Some((mut underline_origin, underline_style)) = finished_underline {
                    if underline_origin.x == glyph_origin.x {
                        underline_origin.x -= max_glyph_size.width.half();
                    };
                    window.paint_underline(
                        underline_origin,
                        glyph_origin.x - underline_origin.x,
                        &underline_style,
                    );
                }

                if let Some((mut strikethrough_origin, strikethrough_style)) =
                    finished_strikethrough
                {
                    if strikethrough_origin.x == glyph_origin.x {
                        strikethrough_origin.x -= max_glyph_size.width.half();
                    };
                    window.paint_strikethrough(
                        strikethrough_origin,
                        glyph_origin.x - strikethrough_origin.x,
                        &strikethrough_style,
                    );
                }

                let max_glyph_bounds = Bounds {
                    origin: glyph_origin,
                    size: max_glyph_size,
                };

                let content_mask = window.content_mask();
                if max_glyph_bounds.intersects(&content_mask.bounds) {
                    let vertical_offset = point(px(0.0), glyph.position.y);
                    if glyph.is_emoji {
                        window.paint_emoji(
                            glyph_origin + baseline_offset + vertical_offset,
                            run.font_id,
                            glyph.id,
                            layout.font_size,
                        )?;
                    } else {
                        window.paint_glyph(
                            glyph_origin + baseline_offset + vertical_offset,
                            run.font_id,
                            glyph.id,
                            layout.font_size,
                            color,
                        )?;
                    }
                }
            }
        }

        let mut last_line_end_x = first_glyph_x + layout.width;
        if let Some(boundary) = wrap_boundaries.last() {
            let run = &layout.runs[boundary.run_ix];
            let glyph = &run.glyphs[boundary.glyph_ix];
            last_line_end_x -= glyph.position.x;
        }

        if let Some((mut underline_start, underline_style)) = current_underline.take() {
            if last_line_end_x == underline_start.x {
                underline_start.x -= max_glyph_size.width.half()
            };
            window.paint_underline(
                underline_start,
                last_line_end_x - underline_start.x,
                &underline_style,
            );
        }

        if let Some((mut strikethrough_start, strikethrough_style)) = current_strikethrough.take() {
            if last_line_end_x == strikethrough_start.x {
                strikethrough_start.x -= max_glyph_size.width.half()
            };
            window.paint_strikethrough(
                strikethrough_start,
                last_line_end_x - strikethrough_start.x,
                &strikethrough_style,
            );
        }

        Ok(())
    })
}

fn paint_line_background(
    origin: Point<Pixels>,
    layout: &LineLayout,
    line_height: Pixels,
    align: TextAlign,
    align_width: Option<Pixels>,
    decoration_runs: &[DecorationRun],
    wrap_boundaries: &[WrapBoundary],
    window: &mut Window,
    cx: &mut App,
) -> Result<()> {
    let line_bounds = Bounds::new(
        origin,
        size(
            layout.width,
            line_height * (wrap_boundaries.len() as f32 + 1.),
        ),
    );
    window.paint_layer(line_bounds, |window| {
        let mut decoration_runs = decoration_runs.iter();
        let mut wraps = wrap_boundaries.iter().peekable();
        let mut run_end = 0;
        let mut current_background: Option<(Point<Pixels>, Hsla)> = None;
        let text_system = cx.text_system().clone();
        let mut glyph_origin = point(
            aligned_origin_x(
                origin,
                align_width.unwrap_or(layout.width),
                px(0.0),
                &align,
                layout,
                wraps.peek(),
            ),
            origin.y,
        );
        let mut prev_glyph_position = Point::default();
        let mut max_glyph_size = size(px(0.), px(0.));
        for (run_ix, run) in layout.runs.iter().enumerate() {
            max_glyph_size = text_system.bounding_box(run.font_id, layout.font_size).size;

            for (glyph_ix, glyph) in run.glyphs.iter().enumerate() {
                glyph_origin.x += glyph.position.x - prev_glyph_position.x;

                if wraps.peek() == Some(&&WrapBoundary { run_ix, glyph_ix }) {
                    wraps.next();
                    if let Some((background_origin, background_color)) = current_background.as_mut()
                    {
                        if glyph_origin.x == background_origin.x {
                            background_origin.x -= max_glyph_size.width.half()
                        }
                        window.paint_quad(fill(
                            Bounds {
                                origin: *background_origin,
                                size: size(glyph_origin.x - background_origin.x, line_height),
                            },
                            *background_color,
                        ));
                        if glyph.index < run_end {
                            background_origin.x = origin.x;
                            background_origin.y += line_height;
                        } else {
                            current_background = None;
                        }
                    }

                    glyph_origin.x = aligned_origin_x(
                        origin,
                        align_width.unwrap_or(layout.width),
                        glyph.position.x,
                        &align,
                        layout,
                        wraps.peek(),
                    );
                    glyph_origin.y += line_height;
                }
                prev_glyph_position = glyph.position;

                let mut finished_background: Option<(Point<Pixels>, Hsla)> = None;
                if glyph.index >= run_end {
                    let mut style_run = decoration_runs.next();

                    // ignore style runs that apply to a partial glyph
                    while let Some(run) = style_run {
                        if glyph.index < run_end + (run.len as usize) {
                            break;
                        }
                        run_end += run.len as usize;
                        style_run = decoration_runs.next();
                    }

                    if let Some(style_run) = style_run {
                        if let Some((_, background_color)) = &mut current_background
                            && style_run.background_color.as_ref() != Some(background_color)
                        {
                            finished_background = current_background.take();
                        }
                        if let Some(run_background) = style_run.background_color {
                            current_background.get_or_insert((
                                point(glyph_origin.x, glyph_origin.y),
                                run_background,
                            ));
                        }
                        run_end += style_run.len as usize;
                    } else {
                        run_end = layout.len;
                        finished_background = current_background.take();
                    }
                }

                if let Some((mut background_origin, background_color)) = finished_background {
                    let mut width = glyph_origin.x - background_origin.x;
                    if background_origin.x == glyph_origin.x {
                        background_origin.x -= max_glyph_size.width.half();
                    };
                    window.paint_quad(fill(
                        Bounds {
                            origin: background_origin,
                            size: size(width, line_height),
                        },
                        background_color,
                    ));
                }
            }
        }

        let mut last_line_end_x = origin.x + layout.width;
        if let Some(boundary) = wrap_boundaries.last() {
            let run = &layout.runs[boundary.run_ix];
            let glyph = &run.glyphs[boundary.glyph_ix];
            last_line_end_x -= glyph.position.x;
        }

        if let Some((mut background_origin, background_color)) = current_background.take() {
            if last_line_end_x == background_origin.x {
                background_origin.x -= max_glyph_size.width.half()
            };
            window.paint_quad(fill(
                Bounds {
                    origin: background_origin,
                    size: size(last_line_end_x - background_origin.x, line_height),
                },
                background_color,
            ));
        }

        Ok(())
    })
}

fn aligned_origin_x(
    origin: Point<Pixels>,
    align_width: Pixels,
    last_glyph_x: Pixels,
    align: &TextAlign,
    layout: &LineLayout,
    wrap_boundary: Option<&&WrapBoundary>,
) -> Pixels {
    let end_of_line = if let Some(WrapBoundary { run_ix, glyph_ix }) = wrap_boundary {
        layout.runs[*run_ix].glyphs[*glyph_ix].position.x
    } else {
        layout.width
    };

    let line_width = end_of_line - last_glyph_x;

    match align {
        TextAlign::Left => origin.x,
        TextAlign::Center => (origin.x * 2.0 + align_width - line_width) / 2.0,
        TextAlign::Right => origin.x + align_width - line_width,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{FontId, GlyphId};

    /// Helper: build a ShapedLine from glyph descriptors without the platform text system.
    /// Each glyph is described as (byte_index, x_position).
    fn make_shaped_line(
        text: &str,
        glyphs: &[(usize, f32)],
        width: f32,
        decorations: &[DecorationRun],
    ) -> ShapedLine {
        let shaped_glyphs: Vec<ShapedGlyph> = glyphs
            .iter()
            .map(|&(index, x)| ShapedGlyph {
                id: GlyphId(0),
                position: point(px(x), px(0.0)),
                index,
                is_emoji: false,
            })
            .collect();

        ShapedLine {
            layout: Arc::new(LineLayout {
                font_size: px(16.0),
                width: px(width),
                ascent: px(12.0),
                descent: px(4.0),
                runs: vec![ShapedRun {
                    font_id: FontId(0),
                    glyphs: shaped_glyphs,
                }],
                len: text.len(),
            }),
            text: SharedString::new(text),
            decoration_runs: SmallVec::from(decorations.to_vec()),
        }
    }

    #[test]
    fn test_split_at_invariants() {
        // Split "abcdef" at every possible byte index and verify structural invariants.
        let line = make_shaped_line(
            "abcdef",
            &[
                (0, 0.0),
                (1, 10.0),
                (2, 20.0),
                (3, 30.0),
                (4, 40.0),
                (5, 50.0),
            ],
            60.0,
            &[],
        );

        for i in 0..=6 {
            let (left, right) = line.split_at(i);

            assert_eq!(
                left.width() + right.width(),
                line.width(),
                "widths must sum at split={i}"
            );
            assert_eq!(
                left.len() + right.len(),
                line.len(),
                "lengths must sum at split={i}"
            );
            assert_eq!(
                format!("{}{}", left.text.as_ref(), right.text.as_ref()),
                "abcdef",
                "text must concatenate at split={i}"
            );
            assert_eq!(left.font_size, line.font_size, "font_size at split={i}");
            assert_eq!(right.ascent, line.ascent, "ascent at split={i}");
            assert_eq!(right.descent, line.descent, "descent at split={i}");
        }

        // Edge: split at 0 produces no left runs, full content on right
        let (left, right) = line.split_at(0);
        assert_eq!(left.runs.len(), 0);
        assert_eq!(right.runs[0].glyphs.len(), 6);

        // Edge: split at end produces full content on left, no right runs
        let (left, right) = line.split_at(6);
        assert_eq!(left.runs[0].glyphs.len(), 6);
        assert_eq!(right.runs.len(), 0);
    }

    #[test]
    fn test_split_at_glyph_rebasing() {
        // Two font runs (simulating a font fallback boundary at byte 3):
        //   run A (FontId 0): glyphs at bytes 0,1,2  positions 0,10,20
        //   run B (FontId 1): glyphs at bytes 3,4,5  positions 30,40,50
        // Successive splits simulate the incremental splitting done during wrap.
        let line = ShapedLine {
            layout: Arc::new(LineLayout {
                font_size: px(16.0),
                width: px(60.0),
                ascent: px(12.0),
                descent: px(4.0),
                runs: vec![
                    ShapedRun {
                        font_id: FontId(0),
                        glyphs: vec![
                            ShapedGlyph {
                                id: GlyphId(0),
                                position: point(px(0.0), px(0.0)),
                                index: 0,
                                is_emoji: false,
                            },
                            ShapedGlyph {
                                id: GlyphId(0),
                                position: point(px(10.0), px(0.0)),
                                index: 1,
                                is_emoji: false,
                            },
                            ShapedGlyph {
                                id: GlyphId(0),
                                position: point(px(20.0), px(0.0)),
                                index: 2,
                                is_emoji: false,
                            },
                        ],
                    },
                    ShapedRun {
                        font_id: FontId(1),
                        glyphs: vec![
                            ShapedGlyph {
                                id: GlyphId(0),
                                position: point(px(30.0), px(0.0)),
                                index: 3,
                                is_emoji: false,
                            },
                            ShapedGlyph {
                                id: GlyphId(0),
                                position: point(px(40.0), px(0.0)),
                                index: 4,
                                is_emoji: false,
                            },
                            ShapedGlyph {
                                id: GlyphId(0),
                                position: point(px(50.0), px(0.0)),
                                index: 5,
                                is_emoji: false,
                            },
                        ],
                    },
                ],
                len: 6,
            }),
            text: "abcdef".into(),
            decoration_runs: SmallVec::new(),
        };

        // First split at byte 2 — mid-run in run A
        let (first, remainder) = line.split_at(2);
        assert_eq!(first.text.as_ref(), "ab");
        assert_eq!(first.runs.len(), 1);
        assert_eq!(first.runs[0].font_id, FontId(0));

        // Remainder "cdef" should have two runs: tail of A (1 glyph) + all of B (3 glyphs)
        assert_eq!(remainder.text.as_ref(), "cdef");
        assert_eq!(remainder.runs.len(), 2);
        assert_eq!(remainder.runs[0].font_id, FontId(0));
        assert_eq!(remainder.runs[0].glyphs.len(), 1);
        assert_eq!(remainder.runs[0].glyphs[0].index, 0);
        assert_eq!(remainder.runs[0].glyphs[0].position.x, px(0.0));
        assert_eq!(remainder.runs[1].font_id, FontId(1));
        assert_eq!(remainder.runs[1].glyphs[0].index, 1);
        assert_eq!(remainder.runs[1].glyphs[0].position.x, px(10.0));

        // Second split at byte 2 within remainder — crosses the run boundary
        let (second, final_part) = remainder.split_at(2);
        assert_eq!(second.text.as_ref(), "cd");
        assert_eq!(final_part.text.as_ref(), "ef");
        assert_eq!(final_part.runs[0].glyphs[0].index, 0);
        assert_eq!(final_part.runs[0].glyphs[0].position.x, px(0.0));

        // Widths must sum across all three pieces
        assert_eq!(
            first.width() + second.width() + final_part.width(),
            line.width()
        );
    }

    #[test]
    fn test_split_at_decorations() {
        // Three decoration runs: red [0..2), green [2..5), blue [5..6).
        // Split at byte 3 — red goes entirely left, green straddles, blue goes entirely right.
        let red = Hsla {
            h: 0.0,
            s: 1.0,
            l: 0.5,
            a: 1.0,
        };
        let green = Hsla {
            h: 0.3,
            s: 1.0,
            l: 0.5,
            a: 1.0,
        };
        let blue = Hsla {
            h: 0.6,
            s: 1.0,
            l: 0.5,
            a: 1.0,
        };

        let line = make_shaped_line(
            "abcdef",
            &[
                (0, 0.0),
                (1, 10.0),
                (2, 20.0),
                (3, 30.0),
                (4, 40.0),
                (5, 50.0),
            ],
            60.0,
            &[
                DecorationRun {
                    len: 2,
                    color: red,
                    background_color: None,
                    underline: None,
                    strikethrough: None,
                },
                DecorationRun {
                    len: 3,
                    color: green,
                    background_color: None,
                    underline: None,
                    strikethrough: None,
                },
                DecorationRun {
                    len: 1,
                    color: blue,
                    background_color: None,
                    underline: None,
                    strikethrough: None,
                },
            ],
        );

        let (left, right) = line.split_at(3);

        // Left: red(2) + green(1) — green straddled, left portion has len 1
        assert_eq!(left.decoration_runs.len(), 2);
        assert_eq!(left.decoration_runs[0].len, 2);
        assert_eq!(left.decoration_runs[0].color, red);
        assert_eq!(left.decoration_runs[1].len, 1);
        assert_eq!(left.decoration_runs[1].color, green);

        // Right: green(2) + blue(1) — green straddled, right portion has len 2
        assert_eq!(right.decoration_runs.len(), 2);
        assert_eq!(right.decoration_runs[0].len, 2);
        assert_eq!(right.decoration_runs[0].color, green);
        assert_eq!(right.decoration_runs[1].len, 1);
        assert_eq!(right.decoration_runs[1].color, blue);
    }
}