gpui_engine_default/

line_layout.rs

use collections::FxHashMap;
use gpui_engine::{
    FontRun, LineLayout, LineLayoutIndex, PlatformTextSystem, WrapBoundary, WrappedLineLayout,
};
use gpui_shared_string::SharedString;
use gpui_types::{Pixels, px};
use parking_lot::{Mutex, RwLock, RwLockUpgradableReadGuard};
use smallvec::SmallVec;
use std::{
    borrow::Borrow,
    hash::{Hash, Hasher},
    ops::Range,
    sync::{
        Arc,
        atomic::{AtomicUsize, Ordering},
    },
};

use gpui_engine::LineWrapper;

pub(crate) fn compute_wrap_boundaries(
    layout: &LineLayout,
    text: &str,
    wrap_width: Pixels,
    max_lines: Option<usize>,
) -> SmallVec<[WrapBoundary; 1]> {
    let mut boundaries = SmallVec::new();
    let mut first_non_whitespace_ix = None;
    let mut last_candidate_ix = None;
    let mut last_candidate_x = px(0.);
    let mut last_boundary = WrapBoundary {
        run_ix: 0,
        glyph_ix: 0,
    };
    let mut last_boundary_x = px(0.);
    let mut prev_ch = '\0';
    let mut glyphs = layout
        .runs
        .iter()
        .enumerate()
        .flat_map(move |(run_ix, run)| {
            run.glyphs.iter().enumerate().map(move |(glyph_ix, glyph)| {
                let character = text[glyph.index..].chars().next().unwrap();
                (
                    WrapBoundary { run_ix, glyph_ix },
                    character,
                    glyph.position.x,
                )
            })
        })
        .peekable();

    while let Some((boundary, ch, x)) = glyphs.next() {
        if ch == '\n' {
            continue;
        }

        // Here is very similar to `LineWrapper::wrap_line` to determine text wrapping,
        // but there are some differences, so we have to duplicate the code here.
        if LineWrapper::is_word_char(ch) {
            if prev_ch == ' ' && ch != ' ' && first_non_whitespace_ix.is_some() {
                last_candidate_ix = Some(boundary);
                last_candidate_x = x;
            }
        } else {
            if ch != ' ' && first_non_whitespace_ix.is_some() {
                last_candidate_ix = Some(boundary);
                last_candidate_x = x;
            }
        }

        if ch != ' ' && first_non_whitespace_ix.is_none() {
            first_non_whitespace_ix = Some(boundary);
        }

        let next_x = glyphs.peek().map_or(layout.width, |(_, _, x)| *x);
        let width = next_x - last_boundary_x;

        if width > wrap_width && boundary > last_boundary {
            // When used line_clamp, we should limit the number of lines.
            if let Some(max_lines) = max_lines
                && boundaries.len() >= max_lines.saturating_sub(1)
            {
                break;
            }

            if let Some(last_candidate_ix) = last_candidate_ix.take() {
                last_boundary = last_candidate_ix;
                last_boundary_x = last_candidate_x;
            } else {
                last_boundary = boundary;
                last_boundary_x = x;
            }
            boundaries.push(last_boundary);
        }
        prev_ch = ch;
    }

    boundaries
}

/// A per-window cache of shaped and wrapped line layouts, retaining the
/// previous frame so reused layouts survive a frame boundary.
pub struct LineLayoutCache {
    previous_frame: Mutex<FrameCache>,
    current_frame: RwLock<FrameCache>,
    platform_text_system: Arc<dyn PlatformTextSystem>,
    /// Advances when [`TextSystem::add_fonts`] successfully changes the font database.
    font_generation: Arc<AtomicUsize>,
    /// Records the generation represented by both frame caches.
    cached_font_generation: AtomicUsize,
}

#[derive(Default)]
struct FrameCache {
    lines: FxHashMap<Arc<CacheKey>, Arc<LineLayout>>,
    wrapped_lines: FxHashMap<Arc<CacheKey>, Arc<WrappedLineLayout>>,
    used_lines: Vec<Arc<CacheKey>>,
    used_wrapped_lines: Vec<Arc<CacheKey>>,

    // Content-addressable caches keyed by caller-provided text hash + layout params.
    // These allow cache hits without materializing a contiguous `SharedString`.
    //
    // IMPORTANT: To support allocation-free lookups, we store these maps using a key type
    // (`HashedCacheKeyRef`) that can be computed without building a contiguous `&str`/`SharedString`.
    // On miss, we allocate once and store under an owned `HashedCacheKey`.
    lines_by_hash: FxHashMap<Arc<HashedCacheKey>, Arc<LineLayout>>,
    wrapped_lines_by_hash: FxHashMap<Arc<HashedCacheKey>, Arc<WrappedLineLayout>>,
    used_lines_by_hash: Vec<Arc<HashedCacheKey>>,
    used_wrapped_lines_by_hash: Vec<Arc<HashedCacheKey>>,
}

impl LineLayoutCache {
    /// Creates an empty cache backed by `platform_text_system`.
    pub fn new(
        platform_text_system: Arc<dyn PlatformTextSystem>,
        font_generation: Arc<AtomicUsize>,
    ) -> Self {
        let cached_font_generation = font_generation.load(Ordering::Acquire);
        Self {
            previous_frame: Mutex::default(),
            current_frame: RwLock::default(),
            platform_text_system,
            font_generation,
            cached_font_generation: AtomicUsize::new(cached_font_generation),
        }
    }

    /// Saves the current cache position so it can be reused or truncated later.
    pub fn layout_index(&self) -> LineLayoutIndex {
        let font_generation = self.clear_if_font_generation_changed();
        let frame = self.current_frame.read();
        LineLayoutIndex {
            font_generation,
            lines_index: frame.used_lines.len(),
            wrapped_lines_index: frame.used_wrapped_lines.len(),
            lines_by_hash_index: frame.used_lines_by_hash.len(),
            wrapped_lines_by_hash_index: frame.used_wrapped_lines_by_hash.len(),
        }
    }

    /// Re-inserts layouts from the previous frame created before `range`.
    pub fn reuse_layouts(&self, range: Range<LineLayoutIndex>) {
        let font_generation = self.clear_if_font_generation_changed();
        if range.start.font_generation != font_generation
            || range.end.font_generation != font_generation
        {
            return;
        }
        let current_frame = &mut *self.current_frame.write();
        let previous_frame = &mut *self.previous_frame.lock();

        for key in &previous_frame.used_lines[range.start.lines_index..range.end.lines_index] {
            if let Some((key, line)) = previous_frame.lines.remove_entry(key) {
                current_frame.lines.insert(key, line);
            }
            current_frame.used_lines.push(key.clone());
        }

        for key in &previous_frame.used_wrapped_lines
            [range.start.wrapped_lines_index..range.end.wrapped_lines_index]
        {
            if let Some((key, line)) = previous_frame.wrapped_lines.remove_entry(key) {
                current_frame.wrapped_lines.insert(key, line);
            }
            current_frame.used_wrapped_lines.push(key.clone());
        }

        for key in &previous_frame.used_lines_by_hash
            [range.start.lines_by_hash_index..range.end.lines_by_hash_index]
        {
            if let Some((key, line)) = previous_frame.lines_by_hash.remove_entry(key) {
                current_frame.lines_by_hash.insert(key, line);
            }
            current_frame.used_lines_by_hash.push(key.clone());
        }

        for key in &previous_frame.used_wrapped_lines_by_hash
            [range.start.wrapped_lines_by_hash_index..range.end.wrapped_lines_by_hash_index]
        {
            if let Some((key, line)) = previous_frame.wrapped_lines_by_hash.remove_entry(key) {
                current_frame.wrapped_lines_by_hash.insert(key, line);
            }
            current_frame.used_wrapped_lines_by_hash.push(key.clone());
        }
    }

    /// Drops layouts created after `index`, keeping the cache consistent.
    pub fn truncate_layouts(&self, index: LineLayoutIndex) {
        let font_generation = self.clear_if_font_generation_changed();
        if index.font_generation != font_generation {
            return;
        }
        let current_frame = &mut *self.current_frame.write();
        current_frame.used_lines.truncate(index.lines_index);
        current_frame
            .used_wrapped_lines
            .truncate(index.wrapped_lines_index);
        current_frame
            .used_lines_by_hash
            .truncate(index.lines_by_hash_index);
        current_frame
            .used_wrapped_lines_by_hash
            .truncate(index.wrapped_lines_by_hash_index);
    }

    /// Ages the current frame into the previous frame and clears the current one.
    pub fn finish_frame(&self) {
        let _font_generation = self.clear_if_font_generation_changed();
        let mut curr_frame = self.current_frame.write();
        let mut prev_frame = self.previous_frame.lock();
        std::mem::swap(&mut *prev_frame, &mut *curr_frame);
        curr_frame.lines.clear();
        curr_frame.wrapped_lines.clear();
        curr_frame.used_lines.clear();
        curr_frame.used_wrapped_lines.clear();

        curr_frame.lines_by_hash.clear();
        curr_frame.wrapped_lines_by_hash.clear();
        curr_frame.used_lines_by_hash.clear();
        curr_frame.used_wrapped_lines_by_hash.clear();
    }

    /// Shapes and wraps `text`, reusing a cached layout when the inputs match.
    pub fn layout_wrapped_line<Text>(
        &self,
        text: Text,
        font_size: Pixels,
        runs: &[FontRun],
        wrap_width: Option<Pixels>,
        max_lines: Option<usize>,
    ) -> Arc<WrappedLineLayout>
    where
        Text: AsRef<str>,
        SharedString: From<Text>,
    {
        let _font_generation = self.clear_if_font_generation_changed();
        let key = &CacheKeyRef {
            text: text.as_ref(),
            font_size,
            runs,
            wrap_width,
            force_width: None,
        } as &dyn AsCacheKeyRef;

        let current_frame = self.current_frame.upgradable_read();
        if let Some(layout) = current_frame.wrapped_lines.get(key) {
            return layout.clone();
        }

        let previous_frame_entry = self.previous_frame.lock().wrapped_lines.remove_entry(key);
        if let Some((key, layout)) = previous_frame_entry {
            let mut current_frame = RwLockUpgradableReadGuard::upgrade(current_frame);
            current_frame
                .wrapped_lines
                .insert(key.clone(), layout.clone());
            current_frame.used_wrapped_lines.push(key);
            layout
        } else {
            drop(current_frame);
            let text = SharedString::from(text);
            let unwrapped_layout = self.layout_line::<&SharedString>(&text, font_size, runs, None);
            let wrap_boundaries = if let Some(wrap_width) = wrap_width {
                compute_wrap_boundaries(&unwrapped_layout, text.as_ref(), wrap_width, max_lines)
            } else {
                SmallVec::new()
            };
            let layout = Arc::new(WrappedLineLayout {
                unwrapped_layout,
                wrap_boundaries,
                wrap_width,
            });
            let key = Arc::new(CacheKey {
                text,
                font_size,
                runs: SmallVec::from(runs),
                wrap_width,
                force_width: None,
            });

            let mut current_frame = self.current_frame.write();
            current_frame
                .wrapped_lines
                .insert(key.clone(), layout.clone());
            current_frame.used_wrapped_lines.push(key);

            layout
        }
    }

    /// Shapes a single line of `text`, reusing a cached layout when the inputs match.
    pub fn layout_line<Text>(
        &self,
        text: Text,
        font_size: Pixels,
        runs: &[FontRun],
        force_width: Option<Pixels>,
    ) -> Arc<LineLayout>
    where
        Text: AsRef<str>,
        SharedString: From<Text>,
    {
        let _font_generation = self.clear_if_font_generation_changed();
        let key = &CacheKeyRef {
            text: text.as_ref(),
            font_size,
            runs,
            wrap_width: None,
            force_width,
        } as &dyn AsCacheKeyRef;

        let current_frame = self.current_frame.upgradable_read();
        if let Some(layout) = current_frame.lines.get(key) {
            return layout.clone();
        }

        let mut current_frame = RwLockUpgradableReadGuard::upgrade(current_frame);
        if let Some((key, layout)) = self.previous_frame.lock().lines.remove_entry(key) {
            current_frame.lines.insert(key.clone(), layout.clone());
            current_frame.used_lines.push(key);
            layout
        } else {
            let text = SharedString::from(text);
            let mut layout = self
                .platform_text_system
                .layout_line(&text, font_size, runs);

            if let Some(force_width) = force_width {
                apply_force_width_to_layout(&mut layout, force_width);
            }

            let key = Arc::new(CacheKey {
                text,
                font_size,
                runs: SmallVec::from(runs),
                wrap_width: None,
                force_width,
            });
            let layout = Arc::new(layout);
            current_frame.lines.insert(key.clone(), layout.clone());
            current_frame.used_lines.push(key);
            layout
        }
    }

    /// Try to retrieve a previously-shaped line layout using a caller-provided content hash.
    ///
    /// This is a *non-allocating* cache probe: it does not materialize any text. If the layout
    /// is not already cached in either the current frame or previous frame, returns `None`.
    ///
    /// Contract (caller enforced):
    /// - Same `text_hash` implies identical text content (collision risk accepted by caller).
    /// - `text_len` should be the UTF-8 byte length of the text (helps reduce accidental collisions).
    pub fn try_layout_line_by_hash(
        &self,
        text_hash: u64,
        text_len: usize,
        font_size: Pixels,
        runs: &[FontRun],
        force_width: Option<Pixels>,
    ) -> Option<Arc<LineLayout>> {
        let _font_generation = self.clear_if_font_generation_changed();
        let key_ref = HashedCacheKeyRef {
            text_hash,
            text_len,
            font_size,
            runs,
            wrap_width: None,
            force_width,
        };

        let current_frame = self.current_frame.read();
        if let Some((_, layout)) = current_frame.lines_by_hash.iter().find(|(key, _)| {
            HashedCacheKeyRef {
                text_hash: key.text_hash,
                text_len: key.text_len,
                font_size: key.font_size,
                runs: key.runs.as_slice(),
                wrap_width: key.wrap_width,
                force_width: key.force_width,
            } == key_ref
        }) {
            return Some(layout.clone());
        }

        let previous_frame = self.previous_frame.lock();
        if let Some((_, layout)) = previous_frame.lines_by_hash.iter().find(|(key, _)| {
            HashedCacheKeyRef {
                text_hash: key.text_hash,
                text_len: key.text_len,
                font_size: key.font_size,
                runs: key.runs.as_slice(),
                wrap_width: key.wrap_width,
                force_width: key.force_width,
            } == key_ref
        }) {
            return Some(layout.clone());
        }

        None
    }

    /// Layout a line of text using a caller-provided content hash as the cache key.
    ///
    /// This enables cache hits without materializing a contiguous `SharedString` for `text`.
    /// If the cache misses, `materialize_text` is invoked to produce the `SharedString` for shaping.
    ///
    /// Contract (caller enforced):
    /// - Same `text_hash` implies identical text content (collision risk accepted by caller).
    /// - `text_len` should be the UTF-8 byte length of the text (helps reduce accidental collisions).
    pub fn layout_line_by_hash(
        &self,
        text_hash: u64,
        text_len: usize,
        font_size: Pixels,
        runs: &[FontRun],
        force_width: Option<Pixels>,
        materialize_text: impl FnOnce() -> SharedString,
    ) -> Arc<LineLayout> {
        let _font_generation = self.clear_if_font_generation_changed();
        let key_ref = HashedCacheKeyRef {
            text_hash,
            text_len,
            font_size,
            runs,
            wrap_width: None,
            force_width,
        };

        // Fast path: already cached (no allocation).
        let current_frame = self.current_frame.upgradable_read();
        if let Some((_, layout)) = current_frame.lines_by_hash.iter().find(|(key, _)| {
            HashedCacheKeyRef {
                text_hash: key.text_hash,
                text_len: key.text_len,
                font_size: key.font_size,
                runs: key.runs.as_slice(),
                wrap_width: key.wrap_width,
                force_width: key.force_width,
            } == key_ref
        }) {
            return layout.clone();
        }

        let mut current_frame = RwLockUpgradableReadGuard::upgrade(current_frame);

        // Try to reuse from previous frame without allocating; do a linear scan to find a matching key.
        // (We avoid `drain()` here because it would eagerly move all entries.)
        let mut previous_frame = self.previous_frame.lock();
        if let Some(existing_key) = previous_frame
            .used_lines_by_hash
            .iter()
            .find(|key| {
                HashedCacheKeyRef {
                    text_hash: key.text_hash,
                    text_len: key.text_len,
                    font_size: key.font_size,
                    runs: key.runs.as_slice(),
                    wrap_width: key.wrap_width,
                    force_width: key.force_width,
                } == key_ref
            })
            .cloned()
        {
            if let Some((key, layout)) = previous_frame.lines_by_hash.remove_entry(&existing_key) {
                current_frame
                    .lines_by_hash
                    .insert(key.clone(), layout.clone());
                current_frame.used_lines_by_hash.push(key);
                return layout;
            }
        }

        let text = materialize_text();
        let mut layout = self
            .platform_text_system
            .layout_line(&text, font_size, runs);

        if let Some(force_width) = force_width {
            apply_force_width_to_layout(&mut layout, force_width);
        }

        let key = Arc::new(HashedCacheKey {
            text_hash,
            text_len,
            font_size,
            runs: SmallVec::from(runs),
            wrap_width: None,
            force_width,
        });
        let layout = Arc::new(layout);
        current_frame
            .lines_by_hash
            .insert(key.clone(), layout.clone());
        current_frame.used_lines_by_hash.push(key);
        layout
    }

    fn clear_if_font_generation_changed(&self) -> usize {
        let font_generation = self.font_generation.load(Ordering::Acquire);
        if self.cached_font_generation.load(Ordering::Acquire) == font_generation {
            return font_generation;
        }

        let mut current_frame = self.current_frame.write();
        if self.cached_font_generation.load(Ordering::Acquire) == font_generation {
            return font_generation;
        }

        *current_frame = FrameCache::default();
        *self.previous_frame.lock() = FrameCache::default();
        self.cached_font_generation
            .store(font_generation, Ordering::Release);
        font_generation
    }
}

// Combining marks (e.g. Thai vowel signs, Arabic diacritics) are shaped by
// HarfBuzz at the same x position as their base character. The force-width
// loop must not advance the cell counter for these zero-advance glyphs,
// otherwise they get displaced into the next cell. We detect them by checking
// whether shaped x has advanced by at least half a cell beyond the last base.
fn apply_force_width_to_layout(layout: &mut LineLayout, force_width: Pixels) {
    let mut glyph_pos: usize = 0;
    // NEG_INFINITY ensures the first glyph is always classified as a base.
    let mut last_base_shaped_x = px(f32::NEG_INFINITY);
    let mut last_base_actual_x = px(0.);

    for run in layout.runs.iter_mut() {
        for glyph in run.glyphs.iter_mut() {
            let shaped_x = glyph.position.x;

            if shaped_x > last_base_shaped_x + force_width * 0.5 {
                let forced_x = glyph_pos * force_width;
                if (shaped_x - forced_x).abs() > px(1.) {
                    glyph.position.x = forced_x;
                }
                last_base_shaped_x = shaped_x;
                last_base_actual_x = glyph.position.x;
                glyph_pos += 1;
            } else {
                glyph.position.x = last_base_actual_x + (shaped_x - last_base_shaped_x);
            }
        }
    }
}

/// A laid out and styled line of text
trait AsCacheKeyRef {
    fn as_cache_key_ref(&self) -> CacheKeyRef<'_>;
}

#[derive(Clone, Debug, Eq)]
struct CacheKey {
    text: SharedString,
    font_size: Pixels,
    runs: SmallVec<[FontRun; 1]>,
    wrap_width: Option<Pixels>,
    force_width: Option<Pixels>,
}

#[derive(Copy, Clone, PartialEq, Eq, Hash)]
struct CacheKeyRef<'a> {
    text: &'a str,
    font_size: Pixels,
    runs: &'a [FontRun],
    wrap_width: Option<Pixels>,
    force_width: Option<Pixels>,
}

#[derive(Clone, Debug)]
struct HashedCacheKey {
    text_hash: u64,
    text_len: usize,
    font_size: Pixels,
    runs: SmallVec<[FontRun; 1]>,
    wrap_width: Option<Pixels>,
    force_width: Option<Pixels>,
}

#[derive(Copy, Clone)]
struct HashedCacheKeyRef<'a> {
    text_hash: u64,
    text_len: usize,
    font_size: Pixels,
    runs: &'a [FontRun],
    wrap_width: Option<Pixels>,
    force_width: Option<Pixels>,
}

impl PartialEq for dyn AsCacheKeyRef + '_ {
    fn eq(&self, other: &dyn AsCacheKeyRef) -> bool {
        self.as_cache_key_ref() == other.as_cache_key_ref()
    }
}

impl PartialEq for HashedCacheKey {
    fn eq(&self, other: &Self) -> bool {
        self.text_hash == other.text_hash
            && self.text_len == other.text_len
            && self.font_size == other.font_size
            && self.runs.as_slice() == other.runs.as_slice()
            && self.wrap_width == other.wrap_width
            && self.force_width == other.force_width
    }
}

impl Eq for HashedCacheKey {}

impl Hash for HashedCacheKey {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.text_hash.hash(state);
        self.text_len.hash(state);
        self.font_size.hash(state);
        self.runs.as_slice().hash(state);
        self.wrap_width.hash(state);
        self.force_width.hash(state);
    }
}

impl PartialEq for HashedCacheKeyRef<'_> {
    fn eq(&self, other: &Self) -> bool {
        self.text_hash == other.text_hash
            && self.text_len == other.text_len
            && self.font_size == other.font_size
            && self.runs == other.runs
            && self.wrap_width == other.wrap_width
            && self.force_width == other.force_width
    }
}

impl Eq for HashedCacheKeyRef<'_> {}

impl Hash for HashedCacheKeyRef<'_> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.text_hash.hash(state);
        self.text_len.hash(state);
        self.font_size.hash(state);
        self.runs.hash(state);
        self.wrap_width.hash(state);
        self.force_width.hash(state);
    }
}

impl Eq for dyn AsCacheKeyRef + '_ {}

impl Hash for dyn AsCacheKeyRef + '_ {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.as_cache_key_ref().hash(state)
    }
}

impl AsCacheKeyRef for CacheKey {
    fn as_cache_key_ref(&self) -> CacheKeyRef<'_> {
        CacheKeyRef {
            text: &self.text,
            font_size: self.font_size,
            runs: self.runs.as_slice(),
            wrap_width: self.wrap_width,
            force_width: self.force_width,
        }
    }
}

impl PartialEq for CacheKey {
    fn eq(&self, other: &Self) -> bool {
        self.as_cache_key_ref().eq(&other.as_cache_key_ref())
    }
}

impl Hash for CacheKey {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.as_cache_key_ref().hash(state);
    }
}

impl<'a> Borrow<dyn AsCacheKeyRef + 'a> for Arc<CacheKey> {
    fn borrow(&self) -> &(dyn AsCacheKeyRef + 'a) {
        self.as_ref() as &dyn AsCacheKeyRef
    }
}

impl AsCacheKeyRef for CacheKeyRef<'_> {
    fn as_cache_key_ref(&self) -> CacheKeyRef<'_> {
        *self
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use gpui_engine::{FontId, GlyphId, ShapedGlyph, ShapedRun};
    use gpui_types::point;

    fn glyph_at(x: f32, index: usize) -> ShapedGlyph {
        ShapedGlyph {
            id: GlyphId(0),
            position: point(px(x), px(0.)),
            index,
            is_emoji: false,
        }
    }

    fn make_layout(glyphs: Vec<ShapedGlyph>) -> LineLayout {
        LineLayout {
            font_size: px(16.),
            width: px(100.),
            ascent: px(12.),
            descent: px(4.),
            runs: vec![ShapedRun {
                font_id: FontId(0),
                glyphs,
            }],
            len: 0,
        }
    }

    fn glyph_x_positions(layout: &LineLayout) -> Vec<f32> {
        layout.runs[0]
            .glyphs
            .iter()
            .map(|g| f32::from(g.position.x))
            .collect()
    }

    #[test]
    fn test_force_width_latin_unchanged() {
        let cell_width = px(8.);
        let mut layout = make_layout(vec![glyph_at(0., 0), glyph_at(8., 1), glyph_at(16., 2)]);

        apply_force_width_to_layout(&mut layout, cell_width);

        let positions = glyph_x_positions(&layout);
        assert_eq!(positions, vec![0., 8., 16.]);
    }

    #[test]
    fn test_force_width_combining_marks_not_advanced() {
        let cell_width = px(8.);
        // Simulates Thai "กี" — base consonant at x=0, combining vowel also at x=0
        let mut layout = make_layout(vec![
            glyph_at(0., 0), // ก (base)
            glyph_at(0., 3), // ี (combining mark, same x)
        ]);

        apply_force_width_to_layout(&mut layout, cell_width);

        let positions = glyph_x_positions(&layout);
        assert_eq!(positions, vec![0., 0.]);
    }

    #[test]
    fn test_force_width_base_after_combining_mark() {
        let cell_width = px(8.);
        let mut layout = make_layout(vec![glyph_at(0., 0), glyph_at(0., 3), glyph_at(8., 6)]);

        apply_force_width_to_layout(&mut layout, cell_width);

        let positions = glyph_x_positions(&layout);
        assert_eq!(positions, vec![0., 0., 8.]);
    }

    #[test]
    fn test_force_width_multiple_combining_marks() {
        let cell_width = px(8.);
        // Simulates "ก้" — base + vowel + tone mark (two combining marks stacked)
        let mut layout = make_layout(vec![
            glyph_at(0., 0), // ก (base)
            glyph_at(0., 3), // vowel (combining)
            glyph_at(0., 6), // tone mark (combining)
            glyph_at(8., 9), // next base
        ]);

        apply_force_width_to_layout(&mut layout, cell_width);

        let positions = glyph_x_positions(&layout);
        assert_eq!(positions, vec![0., 0., 0., 8.]);
    }

    #[test]
    fn test_force_width_corrects_drifted_base_positions() {
        let cell_width = px(8.);
        // Font metrics don't perfectly match cell grid — glyphs drift >1px from cell boundary
        let mut layout = make_layout(vec![
            glyph_at(0.5, 0),  // within 1px tolerance, kept as-is
            glyph_at(10.2, 1), // >1px off from 8.0, corrected
            glyph_at(19.8, 2), // >1px off from 16.0, corrected
        ]);

        apply_force_width_to_layout(&mut layout, cell_width);

        let positions = glyph_x_positions(&layout);
        assert_eq!(positions, vec![0.5, 8., 16.]);
    }

    #[test]
    fn test_force_width_combining_mark_after_within_tolerance_base() {
        let cell_width = px(8.);
        // Base glyph is within 1px of grid so it keeps its shaped position.
        // The combining mark must align to the base's actual position, not the grid slot.
        let mut layout = make_layout(vec![glyph_at(0.5, 0), glyph_at(0.5, 3)]);

        apply_force_width_to_layout(&mut layout, cell_width);

        let positions = glyph_x_positions(&layout);
        assert_eq!(positions, vec![0.5, 0.5]);
    }
}