we / crates / render / src / lib.rs

//! Display list, software rasterizer, Metal GPU compositor.
//!
//! Walks a layout tree, generates paint commands, and rasterizes them
//! into a BGRA pixel buffer suitable for display via CoreGraphics.

use std::collections::HashMap;

use we_css::values::Color;
use we_dom::NodeId;
use we_image::pixel::Image;
use we_layout::{BoxType, LayoutBox, LayoutTree, Rect, TextLine, SCROLLBAR_WIDTH};
use we_style::computed::{BorderStyle, Overflow, Position, TextDecoration, Visibility};
use we_text::font::Font;

/// Scroll state: maps NodeId of scrollable boxes to their (scroll_x, scroll_y) offsets.
pub type ScrollState = HashMap<NodeId, (f32, f32)>;

/// Scroll bar track color (light gray).
const SCROLLBAR_TRACK_COLOR: Color = Color {
    r: 230,
    g: 230,
    b: 230,
    a: 255,
};

/// Scroll bar thumb color (darker gray).
const SCROLLBAR_THUMB_COLOR: Color = Color {
    r: 160,
    g: 160,
    b: 160,
    a: 255,
};

/// A paint command in the display list.
#[derive(Debug)]
pub enum PaintCommand {
    /// Fill a rectangle with a solid color.
    FillRect {
        x: f32,
        y: f32,
        width: f32,
        height: f32,
        color: Color,
    },
    /// Draw a text fragment at a position with styling.
    DrawGlyphs {
        line: TextLine,
        font_size: f32,
        color: Color,
    },
    /// Draw an image at a position with given display dimensions.
    DrawImage {
        x: f32,
        y: f32,
        width: f32,
        height: f32,
        node_id: NodeId,
    },
    /// Push a clip rectangle onto the clip stack. All subsequent paint
    /// commands are clipped to the intersection of all active clip rects.
    PushClip {
        x: f32,
        y: f32,
        width: f32,
        height: f32,
    },
    /// Pop the most recent clip rectangle off the clip stack.
    PopClip,
}

/// A flat list of paint commands in painter's order.
pub type DisplayList = Vec<PaintCommand>;

/// Build a display list from a layout tree.
///
/// Walks the tree in depth-first pre-order (painter's order):
/// backgrounds first, then borders, then text on top.
pub fn build_display_list(tree: &LayoutTree) -> DisplayList {
    build_display_list_with_scroll(tree, &HashMap::new())
}

/// Build a display list from a layout tree with scroll state.
///
/// `page_scroll` is the viewport-level scroll offset (x, y) applied to all content.
/// `scroll_state` maps element NodeIds to their per-element scroll offsets.
pub fn build_display_list_with_scroll(
    tree: &LayoutTree,
    scroll_state: &ScrollState,
) -> DisplayList {
    let mut list = DisplayList::new();
    paint_box(&tree.root, &mut list, (0.0, 0.0), scroll_state);
    list
}

/// Build a display list with page-level scrolling.
///
/// `page_scroll_y` shifts all content vertically (viewport-level scroll).
pub fn build_display_list_with_page_scroll(
    tree: &LayoutTree,
    page_scroll_y: f32,
    scroll_state: &ScrollState,
) -> DisplayList {
    let mut list = DisplayList::new();
    paint_box(&tree.root, &mut list, (0.0, -page_scroll_y), scroll_state);
    list
}

/// Returns `true` if a box is positioned (absolute or fixed).
fn is_positioned(b: &LayoutBox) -> bool {
    b.position == Position::Absolute || b.position == Position::Fixed
}

fn paint_box(
    layout_box: &LayoutBox,
    list: &mut DisplayList,
    translate: (f32, f32),
    scroll_state: &ScrollState,
) {
    let visible = layout_box.visibility == Visibility::Visible;
    let tx = translate.0;
    let ty = translate.1;

    if visible {
        paint_background(layout_box, list, tx, ty);
        paint_borders(layout_box, list, tx, ty);

        // Emit image paint command for replaced elements.
        if let Some((rw, rh)) = layout_box.replaced_size {
            if let Some(node_id) = node_id_from_box_type(&layout_box.box_type) {
                list.push(PaintCommand::DrawImage {
                    x: layout_box.rect.x + tx,
                    y: layout_box.rect.y + ty,
                    width: rw,
                    height: rh,
                    node_id,
                });
            }
        }

        paint_text(layout_box, list, tx, ty);
    }

    // Determine if this box is scrollable.
    let scrollable =
        layout_box.overflow == Overflow::Scroll || layout_box.overflow == Overflow::Auto;

    // If this box has overflow clipping, push a clip rect for the padding box.
    let clips = layout_box.overflow != Overflow::Visible;
    if clips {
        let clip = padding_box(layout_box);
        list.push(PaintCommand::PushClip {
            x: clip.x + tx,
            y: clip.y + ty,
            width: clip.width,
            height: clip.height,
        });
    }

    // Compute child translate: adds scroll offset for scrollable boxes.
    let mut child_translate = translate;
    if scrollable {
        if let Some(node_id) = node_id_from_box_type(&layout_box.box_type) {
            if let Some(&(sx, sy)) = scroll_state.get(&node_id) {
                child_translate.0 -= sx;
                child_translate.1 -= sy;
            }
        }
    }

    // Check if any children are positioned (absolute/fixed).
    let has_positioned = layout_box.children.iter().any(is_positioned);

    if has_positioned {
        // CSS stacking context ordering:
        // 1. Positioned children with negative z-index
        // 2. In-flow (non-positioned) children in tree order
        // 3. Positioned children with z-index >= 0 (or auto) in z-index order

        // Collect positioned children indices, partitioned by z-index sign.
        let mut negative_z: Vec<usize> = Vec::new();
        let mut non_negative_z: Vec<usize> = Vec::new();

        for (i, child) in layout_box.children.iter().enumerate() {
            if is_positioned(child) {
                let z = child.z_index.unwrap_or(0);
                if z < 0 {
                    negative_z.push(i);
                } else {
                    non_negative_z.push(i);
                }
            }
        }

        // Sort by z-index (stable sort preserves tree order for equal z-index).
        negative_z.sort_by_key(|&i| layout_box.children[i].z_index.unwrap_or(0));
        non_negative_z.sort_by_key(|&i| layout_box.children[i].z_index.unwrap_or(0));

        // Paint negative z-index positioned children.
        for &i in &negative_z {
            paint_box(&layout_box.children[i], list, child_translate, scroll_state);
        }

        // Paint in-flow children in tree order.
        for child in &layout_box.children {
            if !is_positioned(child) {
                paint_box(child, list, child_translate, scroll_state);
            }
        }

        // Paint non-negative z-index positioned children.
        for &i in &non_negative_z {
            paint_box(&layout_box.children[i], list, child_translate, scroll_state);
        }
    } else {
        // No positioned children — paint all in tree order.
        for child in &layout_box.children {
            paint_box(child, list, child_translate, scroll_state);
        }
    }

    if clips {
        list.push(PaintCommand::PopClip);
    }

    // Paint scroll bars after PopClip so they're not clipped by the container.
    if scrollable && visible {
        paint_scrollbars(layout_box, list, tx, ty, scroll_state);
    }
}

/// Compute the padding box rectangle for a layout box.
/// The padding box is the content area expanded by padding.
fn padding_box(layout_box: &LayoutBox) -> Rect {
    Rect {
        x: layout_box.rect.x - layout_box.padding.left,
        y: layout_box.rect.y - layout_box.padding.top,
        width: layout_box.rect.width + layout_box.padding.left + layout_box.padding.right,
        height: layout_box.rect.height + layout_box.padding.top + layout_box.padding.bottom,
    }
}

/// Extract the NodeId from a BoxType, if it has one.
fn node_id_from_box_type(box_type: &BoxType) -> Option<NodeId> {
    match box_type {
        BoxType::Block(id) | BoxType::Inline(id) => Some(*id),
        BoxType::TextRun { node, .. } => Some(*node),
        BoxType::Anonymous => None,
    }
}

fn paint_background(layout_box: &LayoutBox, list: &mut DisplayList, tx: f32, ty: f32) {
    let bg = layout_box.background_color;
    // Only paint if the background is not fully transparent and the box has area.
    if bg.a == 0 {
        return;
    }
    if layout_box.rect.width > 0.0 && layout_box.rect.height > 0.0 {
        // Background covers the padding box (content + padding), not including border.
        list.push(PaintCommand::FillRect {
            x: layout_box.rect.x + tx,
            y: layout_box.rect.y + ty,
            width: layout_box.rect.width,
            height: layout_box.rect.height,
            color: bg,
        });
    }
}

fn paint_borders(layout_box: &LayoutBox, list: &mut DisplayList, tx: f32, ty: f32) {
    let b = &layout_box.border;
    let r = &layout_box.rect;
    let styles = &layout_box.border_styles;
    let colors = &layout_box.border_colors;

    // Border box starts at content origin minus padding and border.
    let bx = r.x - layout_box.padding.left - b.left + tx;
    let by = r.y - layout_box.padding.top - b.top + ty;
    let bw = b.left + layout_box.padding.left + r.width + layout_box.padding.right + b.right;
    let bh = b.top + layout_box.padding.top + r.height + layout_box.padding.bottom + b.bottom;

    // Top border
    if b.top > 0.0 && styles[0] != BorderStyle::None && styles[0] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx,
            y: by,
            width: bw,
            height: b.top,
            color: colors[0],
        });
    }
    // Right border
    if b.right > 0.0 && styles[1] != BorderStyle::None && styles[1] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx + bw - b.right,
            y: by,
            width: b.right,
            height: bh,
            color: colors[1],
        });
    }
    // Bottom border
    if b.bottom > 0.0 && styles[2] != BorderStyle::None && styles[2] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx,
            y: by + bh - b.bottom,
            width: bw,
            height: b.bottom,
            color: colors[2],
        });
    }
    // Left border
    if b.left > 0.0 && styles[3] != BorderStyle::None && styles[3] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx,
            y: by,
            width: b.left,
            height: bh,
            color: colors[3],
        });
    }
}

fn paint_text(layout_box: &LayoutBox, list: &mut DisplayList, tx: f32, ty: f32) {
    for line in &layout_box.lines {
        let color = line.color;
        let font_size = line.font_size;

        // Record index before pushing glyphs so we can insert the background
        // before the text in painter's order.
        let glyph_idx = list.len();

        // Create a translated copy of the text line.
        let mut translated_line = line.clone();
        translated_line.x += tx;
        translated_line.y += ty;

        list.push(PaintCommand::DrawGlyphs {
            line: translated_line,
            font_size,
            color,
        });

        // Draw underline as a 1px line below the baseline.
        if line.text_decoration == TextDecoration::Underline && line.width > 0.0 {
            let baseline_y = line.y + ty + font_size;
            let underline_y = baseline_y + 2.0;
            list.push(PaintCommand::FillRect {
                x: line.x + tx,
                y: underline_y,
                width: line.width,
                height: 1.0,
                color,
            });
        }

        // Draw inline background if not transparent.
        if line.background_color.a > 0 && line.width > 0.0 {
            list.insert(
                glyph_idx,
                PaintCommand::FillRect {
                    x: line.x + tx,
                    y: line.y + ty,
                    width: line.width,
                    height: font_size * 1.2,
                    color: line.background_color,
                },
            );
        }
    }
}

/// Paint scroll bars for a scrollable box.
fn paint_scrollbars(
    layout_box: &LayoutBox,
    list: &mut DisplayList,
    tx: f32,
    ty: f32,
    scroll_state: &ScrollState,
) {
    let pad = padding_box(layout_box);
    let viewport_height = pad.height;
    let content_height = layout_box.content_height;

    // Determine whether scroll bars should be shown.
    let show_vertical = match layout_box.overflow {
        Overflow::Scroll => true,
        Overflow::Auto => content_height > viewport_height,
        _ => false,
    };

    if !show_vertical || viewport_height <= 0.0 {
        return;
    }

    // Scroll bar track: right edge of the padding box.
    let track_x = pad.x + pad.width - SCROLLBAR_WIDTH + tx;
    let track_y = pad.y + ty;
    let track_height = viewport_height;

    // Paint the track background.
    list.push(PaintCommand::FillRect {
        x: track_x,
        y: track_y,
        width: SCROLLBAR_WIDTH,
        height: track_height,
        color: SCROLLBAR_TRACK_COLOR,
    });

    // Compute thumb size and position.
    let max_content = content_height.max(viewport_height);
    let thumb_ratio = viewport_height / max_content;
    let thumb_height = (thumb_ratio * track_height).max(20.0).min(track_height);

    // Get current scroll offset.
    let scroll_y = node_id_from_box_type(&layout_box.box_type)
        .and_then(|id| scroll_state.get(&id))
        .map(|&(_, sy)| sy)
        .unwrap_or(0.0);

    let max_scroll = (content_height - viewport_height).max(0.0);
    let scroll_ratio = if max_scroll > 0.0 {
        scroll_y / max_scroll
    } else {
        0.0
    };
    let thumb_y = track_y + scroll_ratio * (track_height - thumb_height);

    // Paint the thumb.
    list.push(PaintCommand::FillRect {
        x: track_x,
        y: thumb_y,
        width: SCROLLBAR_WIDTH,
        height: thumb_height,
        color: SCROLLBAR_THUMB_COLOR,
    });
}

/// An axis-aligned clip rectangle.
#[derive(Debug, Clone, Copy)]
struct ClipRect {
    x0: f32,
    y0: f32,
    x1: f32,
    y1: f32,
}

impl ClipRect {
    /// Intersect two clip rects, returning the overlapping region.
    /// Returns None if they don't overlap.
    fn intersect(self, other: ClipRect) -> Option<ClipRect> {
        let x0 = self.x0.max(other.x0);
        let y0 = self.y0.max(other.y0);
        let x1 = self.x1.min(other.x1);
        let y1 = self.y1.min(other.y1);
        if x0 < x1 && y0 < y1 {
            Some(ClipRect { x0, y0, x1, y1 })
        } else {
            None
        }
    }
}

/// Software renderer that paints a display list into a BGRA pixel buffer.
pub struct Renderer {
    width: u32,
    height: u32,
    /// BGRA pixel data, row-major, top-to-bottom.
    buffer: Vec<u8>,
    /// Stack of clip rectangles. When non-empty, all drawing is clipped to
    /// the intersection of all active clip rects.
    clip_stack: Vec<ClipRect>,
}

impl Renderer {
    /// Create a new renderer with the given dimensions.
    /// The buffer is initialized to white.
    pub fn new(width: u32, height: u32) -> Renderer {
        let size = (width as usize) * (height as usize) * 4;
        let mut buffer = vec![0u8; size];
        // Fill with white (BGRA: B=255, G=255, R=255, A=255).
        for pixel in buffer.chunks_exact_mut(4) {
            pixel[0] = 255; // B
            pixel[1] = 255; // G
            pixel[2] = 255; // R
            pixel[3] = 255; // A
        }
        Renderer {
            width,
            height,
            buffer,
            clip_stack: Vec::new(),
        }
    }

    /// Compute the effective clip rect from the clip stack.
    /// Returns None if the clip stack is empty (no clipping).
    fn active_clip(&self) -> Option<ClipRect> {
        if self.clip_stack.is_empty() {
            return None;
        }
        // Start with the full buffer as the initial rect, then intersect.
        let mut result = ClipRect {
            x0: 0.0,
            y0: 0.0,
            x1: self.width as f32,
            y1: self.height as f32,
        };
        for clip in &self.clip_stack {
            match result.intersect(*clip) {
                Some(r) => result = r,
                None => {
                    return Some(ClipRect {
                        x0: 0.0,
                        y0: 0.0,
                        x1: 0.0,
                        y1: 0.0,
                    })
                }
            }
        }
        Some(result)
    }

    /// Paint a layout tree into the pixel buffer.
    pub fn paint(
        &mut self,
        layout_tree: &LayoutTree,
        font: &Font,
        images: &HashMap<NodeId, &Image>,
    ) {
        self.paint_with_scroll(layout_tree, font, images, 0.0, &HashMap::new());
    }

    /// Paint a layout tree with scroll state into the pixel buffer.
    pub fn paint_with_scroll(
        &mut self,
        layout_tree: &LayoutTree,
        font: &Font,
        images: &HashMap<NodeId, &Image>,
        page_scroll_y: f32,
        scroll_state: &ScrollState,
    ) {
        let display_list =
            build_display_list_with_page_scroll(layout_tree, page_scroll_y, scroll_state);
        for cmd in &display_list {
            match cmd {
                PaintCommand::FillRect {
                    x,
                    y,
                    width,
                    height,
                    color,
                } => {
                    self.fill_rect(*x, *y, *width, *height, *color);
                }
                PaintCommand::DrawGlyphs {
                    line,
                    font_size,
                    color,
                } => {
                    self.draw_text_line(line, *font_size, *color, font);
                }
                PaintCommand::DrawImage {
                    x,
                    y,
                    width,
                    height,
                    node_id,
                } => {
                    if let Some(image) = images.get(node_id) {
                        self.draw_image(*x, *y, *width, *height, image);
                    }
                }
                PaintCommand::PushClip {
                    x,
                    y,
                    width,
                    height,
                } => {
                    self.clip_stack.push(ClipRect {
                        x0: *x,
                        y0: *y,
                        x1: *x + *width,
                        y1: *y + *height,
                    });
                }
                PaintCommand::PopClip => {
                    self.clip_stack.pop();
                }
            }
        }
    }

    /// Get the BGRA pixel data.
    pub fn pixels(&self) -> &[u8] {
        &self.buffer
    }

    /// Width in pixels.
    pub fn width(&self) -> u32 {
        self.width
    }

    /// Height in pixels.
    pub fn height(&self) -> u32 {
        self.height
    }

    /// Fill a rectangle with a solid color.
    pub fn fill_rect(&mut self, x: f32, y: f32, width: f32, height: f32, color: Color) {
        let mut fx0 = x;
        let mut fy0 = y;
        let mut fx1 = x + width;
        let mut fy1 = y + height;

        // Apply clip rect if active.
        if let Some(clip) = self.active_clip() {
            fx0 = fx0.max(clip.x0);
            fy0 = fy0.max(clip.y0);
            fx1 = fx1.min(clip.x1);
            fy1 = fy1.min(clip.y1);
            if fx0 >= fx1 || fy0 >= fy1 {
                return;
            }
        }

        let x0 = (fx0 as i32).max(0) as u32;
        let y0 = (fy0 as i32).max(0) as u32;
        let x1 = (fx1 as i32).max(0).min(self.width as i32) as u32;
        let y1 = (fy1 as i32).max(0).min(self.height as i32) as u32;

        if color.a == 255 {
            // Fully opaque — direct write.
            for py in y0..y1 {
                for px in x0..x1 {
                    self.set_pixel(px, py, color);
                }
            }
        } else if color.a > 0 {
            // Semi-transparent — alpha blend.
            let alpha = color.a as u32;
            let inv_alpha = 255 - alpha;
            for py in y0..y1 {
                for px in x0..x1 {
                    let offset = ((py * self.width + px) * 4) as usize;
                    let dst_b = self.buffer[offset] as u32;
                    let dst_g = self.buffer[offset + 1] as u32;
                    let dst_r = self.buffer[offset + 2] as u32;
                    self.buffer[offset] =
                        ((color.b as u32 * alpha + dst_b * inv_alpha) / 255) as u8;
                    self.buffer[offset + 1] =
                        ((color.g as u32 * alpha + dst_g * inv_alpha) / 255) as u8;
                    self.buffer[offset + 2] =
                        ((color.r as u32 * alpha + dst_r * inv_alpha) / 255) as u8;
                    self.buffer[offset + 3] = 255;
                }
            }
        }
    }

    /// Draw a line of text using the font to render glyphs.
    fn draw_text_line(&mut self, line: &TextLine, font_size: f32, color: Color, font: &Font) {
        let positioned = font.render_text(&line.text, font_size);

        for glyph in &positioned {
            if let Some(ref bitmap) = glyph.bitmap {
                // Glyph origin: line position + glyph horizontal offset.
                let gx = line.x + glyph.x + bitmap.bearing_x as f32;
                // Baseline is at line.y + font_size (approximate: baseline at ~80% of font_size).
                // bearing_y is distance from baseline to top of glyph (positive = above baseline).
                let baseline_y = line.y + font_size;
                let gy = baseline_y - bitmap.bearing_y as f32;

                self.composite_glyph(gx, gy, bitmap, color);
            }
        }
    }

    /// Composite a grayscale glyph bitmap onto the buffer with anti-aliasing.
    fn composite_glyph(
        &mut self,
        x: f32,
        y: f32,
        bitmap: &we_text::font::rasterizer::GlyphBitmap,
        color: Color,
    ) {
        let x0 = x as i32;
        let y0 = y as i32;
        let clip = self.active_clip();

        for by in 0..bitmap.height {
            for bx in 0..bitmap.width {
                let px = x0 + bx as i32;
                let py = y0 + by as i32;

                if px < 0 || py < 0 || px >= self.width as i32 || py >= self.height as i32 {
                    continue;
                }

                // Apply clip rect.
                if let Some(ref c) = clip {
                    if (px as f32) < c.x0
                        || (px as f32) >= c.x1
                        || (py as f32) < c.y0
                        || (py as f32) >= c.y1
                    {
                        continue;
                    }
                }

                let coverage = bitmap.data[(by * bitmap.width + bx) as usize];
                if coverage == 0 {
                    continue;
                }

                let px = px as u32;
                let py = py as u32;

                // Alpha-blend the glyph coverage with the text color onto the background.
                let alpha = (coverage as u32 * color.a as u32) / 255;
                if alpha == 0 {
                    continue;
                }

                let offset = ((py * self.width + px) * 4) as usize;
                let dst_b = self.buffer[offset] as u32;
                let dst_g = self.buffer[offset + 1] as u32;
                let dst_r = self.buffer[offset + 2] as u32;

                // Source-over compositing.
                let inv_alpha = 255 - alpha;
                self.buffer[offset] = ((color.b as u32 * alpha + dst_b * inv_alpha) / 255) as u8;
                self.buffer[offset + 1] =
                    ((color.g as u32 * alpha + dst_g * inv_alpha) / 255) as u8;
                self.buffer[offset + 2] =
                    ((color.r as u32 * alpha + dst_r * inv_alpha) / 255) as u8;
                self.buffer[offset + 3] = 255; // Fully opaque.
            }
        }
    }

    /// Draw an RGBA8 image scaled to the given display dimensions.
    ///
    /// Uses nearest-neighbor sampling for scaling. The source image is in RGBA8
    /// format; the buffer is BGRA. Alpha compositing is performed for semi-transparent pixels.
    fn draw_image(&mut self, x: f32, y: f32, width: f32, height: f32, image: &Image) {
        if image.width == 0 || image.height == 0 || width <= 0.0 || height <= 0.0 {
            return;
        }

        let mut fx0 = x;
        let mut fy0 = y;
        let mut fx1 = x + width;
        let mut fy1 = y + height;

        if let Some(clip) = self.active_clip() {
            fx0 = fx0.max(clip.x0);
            fy0 = fy0.max(clip.y0);
            fx1 = fx1.min(clip.x1);
            fy1 = fy1.min(clip.y1);
            if fx0 >= fx1 || fy0 >= fy1 {
                return;
            }
        }

        let dst_x0 = (fx0 as i32).max(0) as u32;
        let dst_y0 = (fy0 as i32).max(0) as u32;
        let dst_x1 = (fx1 as i32).max(0).min(self.width as i32) as u32;
        let dst_y1 = (fy1 as i32).max(0).min(self.height as i32) as u32;

        let scale_x = image.width as f32 / width;
        let scale_y = image.height as f32 / height;

        for dst_y in dst_y0..dst_y1 {
            let src_y = ((dst_y as f32 - y) * scale_y) as u32;
            let src_y = src_y.min(image.height - 1);

            for dst_x in dst_x0..dst_x1 {
                let src_x = ((dst_x as f32 - x) * scale_x) as u32;
                let src_x = src_x.min(image.width - 1);

                let src_offset = ((src_y * image.width + src_x) * 4) as usize;
                let r = image.data[src_offset] as u32;
                let g = image.data[src_offset + 1] as u32;
                let b = image.data[src_offset + 2] as u32;
                let a = image.data[src_offset + 3] as u32;

                if a == 0 {
                    continue;
                }

                let dst_offset = ((dst_y * self.width + dst_x) * 4) as usize;

                if a == 255 {
                    // Fully opaque — direct write (RGBA → BGRA).
                    self.buffer[dst_offset] = b as u8;
                    self.buffer[dst_offset + 1] = g as u8;
                    self.buffer[dst_offset + 2] = r as u8;
                    self.buffer[dst_offset + 3] = 255;
                } else {
                    // Alpha blend.
                    let inv_a = 255 - a;
                    let dst_b = self.buffer[dst_offset] as u32;
                    let dst_g = self.buffer[dst_offset + 1] as u32;
                    let dst_r = self.buffer[dst_offset + 2] as u32;
                    self.buffer[dst_offset] = ((b * a + dst_b * inv_a) / 255) as u8;
                    self.buffer[dst_offset + 1] = ((g * a + dst_g * inv_a) / 255) as u8;
                    self.buffer[dst_offset + 2] = ((r * a + dst_r * inv_a) / 255) as u8;
                    self.buffer[dst_offset + 3] = 255;
                }
            }
        }
    }

    /// Set a single pixel to the given color (no blending).
    fn set_pixel(&mut self, x: u32, y: u32, color: Color) {
        if x >= self.width || y >= self.height {
            return;
        }
        let offset = ((y * self.width + x) * 4) as usize;
        self.buffer[offset] = color.b; // B
        self.buffer[offset + 1] = color.g; // G
        self.buffer[offset + 2] = color.r; // R
        self.buffer[offset + 3] = color.a; // A
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use we_dom::Document;
    use we_style::computed::{extract_stylesheets, resolve_styles};
    use we_text::font::Font;

    fn test_font() -> Font {
        let paths = [
            "/System/Library/Fonts/Geneva.ttf",
            "/System/Library/Fonts/Monaco.ttf",
        ];
        for path in &paths {
            let p = std::path::Path::new(path);
            if p.exists() {
                return Font::from_file(p).expect("failed to parse font");
            }
        }
        panic!("no test font found");
    }

    fn layout_doc(doc: &Document) -> we_layout::LayoutTree {
        let font = test_font();
        let sheets = extract_stylesheets(doc);
        let styled = resolve_styles(doc, &sheets, (800.0, 600.0)).unwrap();
        we_layout::layout(
            &styled,
            doc,
            800.0,
            600.0,
            &font,
            &std::collections::HashMap::new(),
        )
    }

    #[test]
    fn renderer_new_white_background() {
        let r = Renderer::new(10, 10);
        assert_eq!(r.width(), 10);
        assert_eq!(r.height(), 10);
        assert_eq!(r.pixels().len(), 10 * 10 * 4);
        // All pixels should be white (BGRA: 255, 255, 255, 255).
        for pixel in r.pixels().chunks_exact(4) {
            assert_eq!(pixel, [255, 255, 255, 255]);
        }
    }

    #[test]
    fn fill_rect_basic() {
        let mut r = Renderer::new(20, 20);
        let red = Color::new(255, 0, 0, 255);
        r.fill_rect(5.0, 5.0, 10.0, 10.0, red);

        // Pixel at (7, 7) should be red (BGRA: 0, 0, 255, 255).
        let offset = ((7 * 20 + 7) * 4) as usize;
        assert_eq!(r.pixels()[offset], 0); // B
        assert_eq!(r.pixels()[offset + 1], 0); // G
        assert_eq!(r.pixels()[offset + 2], 255); // R
        assert_eq!(r.pixels()[offset + 3], 255); // A

        // Pixel at (0, 0) should still be white.
        assert_eq!(r.pixels()[0], 255); // B
        assert_eq!(r.pixels()[1], 255); // G
        assert_eq!(r.pixels()[2], 255); // R
        assert_eq!(r.pixels()[3], 255); // A
    }

    #[test]
    fn fill_rect_clipping() {
        let mut r = Renderer::new(10, 10);
        let blue = Color::new(0, 0, 255, 255);
        // Rect extends beyond the buffer — should not panic.
        r.fill_rect(-5.0, -5.0, 20.0, 20.0, blue);

        // All pixels should be blue (BGRA: 255, 0, 0, 255).
        for pixel in r.pixels().chunks_exact(4) {
            assert_eq!(pixel, [255, 0, 0, 255]);
        }
    }

    #[test]
    fn display_list_from_empty_layout() {
        let doc = Document::new();
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets, (800.0, 600.0));
        if let Some(styled) = styled {
            let tree = we_layout::layout(
                &styled,
                &doc,
                800.0,
                600.0,
                &font,
                &std::collections::HashMap::new(),
            );
            let list = build_display_list(&tree);
            assert!(list.len() <= 1);
        }
    }

    #[test]
    fn display_list_has_background_and_text() {
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let p = doc.create_element("p");
        let text = doc.create_text("Hello world");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, p);
        doc.append_child(p, text);

        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let has_text = list
            .iter()
            .any(|c| matches!(c, PaintCommand::DrawGlyphs { .. }));

        assert!(has_text, "should have at least one DrawGlyphs");
    }

    #[test]
    fn paint_simple_page() {
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let p = doc.create_element("p");
        let text = doc.create_text("Hello");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, p);
        doc.append_child(p, text);

        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(800, 600);
        renderer.paint(&tree, &font, &HashMap::new());

        let pixels = renderer.pixels();

        // The buffer should have some non-white pixels (from text rendering).
        let has_non_white = pixels.chunks_exact(4).any(|p| p != [255, 255, 255, 255]);
        assert!(
            has_non_white,
            "rendered page should have non-white pixels from text"
        );
    }

    #[test]
    fn bgra_format_correct() {
        let mut r = Renderer::new(1, 1);
        let color = Color::new(100, 150, 200, 255);
        r.set_pixel(0, 0, color);
        let pixels = r.pixels();
        // BGRA format.
        assert_eq!(pixels[0], 200); // B
        assert_eq!(pixels[1], 150); // G
        assert_eq!(pixels[2], 100); // R
        assert_eq!(pixels[3], 255); // A
    }

    #[test]
    fn paint_heading_produces_larger_glyphs() {
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let h1 = doc.create_element("h1");
        let h1_text = doc.create_text("Big");
        let p = doc.create_element("p");
        let p_text = doc.create_text("Small");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, h1);
        doc.append_child(h1, h1_text);
        doc.append_child(body, p);
        doc.append_child(p, p_text);

        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        // There should be DrawGlyphs commands with different font sizes.
        let font_sizes: Vec<f32> = list
            .iter()
            .filter_map(|c| match c {
                PaintCommand::DrawGlyphs { font_size, .. } => Some(*font_size),
                _ => None,
            })
            .collect();

        assert!(font_sizes.len() >= 2, "should have at least 2 text lines");
        // h1 has font_size 32, p has font_size 16.
        assert!(
            font_sizes.iter().any(|&s| s > 20.0),
            "should have a heading with large font size"
        );
        assert!(
            font_sizes.iter().any(|&s| s < 20.0),
            "should have a paragraph with normal font size"
        );
    }

    #[test]
    fn renderer_zero_size() {
        let r = Renderer::new(0, 0);
        assert_eq!(r.pixels().len(), 0);
    }

    #[test]
    fn glyph_compositing_anti_aliased() {
        // Render text and verify we get anti-aliased (partially transparent) pixels.
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let p = doc.create_element("p");
        let text = doc.create_text("MMMMM");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, p);
        doc.append_child(p, text);

        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(800, 600);
        renderer.paint(&tree, &font, &HashMap::new());

        let pixels = renderer.pixels();
        // Find pixels that are not pure white and not pure black.
        // These represent anti-aliased edges of glyphs.
        let mut has_intermediate = false;
        for pixel in pixels.chunks_exact(4) {
            let b = pixel[0];
            let g = pixel[1];
            let r = pixel[2];
            // Anti-aliased pixel: gray between white and black.
            if r > 0 && r < 255 && r == g && g == b {
                has_intermediate = true;
                break;
            }
        }
        assert!(
            has_intermediate,
            "should have anti-aliased (gray) pixels from glyph compositing"
        );
    }

    #[test]
    fn css_color_renders_correctly() {
        let html_str = r#"<!DOCTYPE html>
<html>
<head><style>p { color: red; }</style></head>
<body><p>Red text</p></body>
</html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets, (800.0, 600.0)).unwrap();
        let tree = we_layout::layout(
            &styled,
            &doc,
            800.0,
            600.0,
            &font,
            &std::collections::HashMap::new(),
        );

        let list = build_display_list(&tree);
        let text_colors: Vec<&Color> = list
            .iter()
            .filter_map(|c| match c {
                PaintCommand::DrawGlyphs { color, .. } => Some(color),
                _ => None,
            })
            .collect();

        assert!(!text_colors.is_empty());
        // Text should be red.
        assert_eq!(*text_colors[0], Color::rgb(255, 0, 0));
    }

    #[test]
    fn css_background_color_renders() {
        let html_str = r#"<!DOCTYPE html>
<html>
<head><style>div { background-color: yellow; }</style></head>
<body><div>Content</div></body>
</html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets, (800.0, 600.0)).unwrap();
        let tree = we_layout::layout(
            &styled,
            &doc,
            800.0,
            600.0,
            &font,
            &std::collections::HashMap::new(),
        );

        let list = build_display_list(&tree);
        let fill_colors: Vec<&Color> = list
            .iter()
            .filter_map(|c| match c {
                PaintCommand::FillRect { color, .. } => Some(color),
                _ => None,
            })
            .collect();

        // Should have a yellow fill rect for the div background.
        assert!(fill_colors.iter().any(|c| **c == Color::rgb(255, 255, 0)));
    }

    #[test]
    fn border_rendering() {
        let html_str = r#"<!DOCTYPE html>
<html>
<head><style>div { border: 2px solid red; }</style></head>
<body><div>Bordered</div></body>
</html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets, (800.0, 600.0)).unwrap();
        let tree = we_layout::layout(
            &styled,
            &doc,
            800.0,
            600.0,
            &font,
            &std::collections::HashMap::new(),
        );

        let list = build_display_list(&tree);
        let red_fills: Vec<_> = list
            .iter()
            .filter(|c| matches!(c, PaintCommand::FillRect { color, .. } if *color == Color::rgb(255, 0, 0)))
            .collect();

        // Should have 4 border fills (top, right, bottom, left).
        assert_eq!(red_fills.len(), 4, "should have 4 border edges");
    }

    // --- Visibility painting tests ---

    #[test]
    fn visibility_hidden_not_painted() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
div { visibility: hidden; background-color: red; width: 100px; height: 100px; }
</style></head>
<body><div>Hidden</div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        // No red background should be in the display list.
        let red_fills = list.iter().filter(|c| {
            matches!(c, PaintCommand::FillRect { color, .. } if *color == Color::rgb(255, 0, 0))
        });
        assert_eq!(red_fills.count(), 0, "hidden element should not be painted");

        // No text should be rendered for the hidden element.
        let hidden_text = list.iter().filter(
            |c| matches!(c, PaintCommand::DrawGlyphs { line, .. } if line.text == "Hidden"),
        );
        assert_eq!(
            hidden_text.count(),
            0,
            "hidden element text should not be painted"
        );
    }

    #[test]
    fn visibility_visible_child_of_hidden_parent_is_painted() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.parent { visibility: hidden; }
.child { visibility: visible; }
</style></head>
<body>
<div class="parent"><p class="child">Visible</p></div>
</body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        // The visible child's text should appear in the display list.
        let visible_text = list.iter().filter(
            |c| matches!(c, PaintCommand::DrawGlyphs { line, .. } if line.text.contains("Visible")),
        );
        assert!(
            visible_text.count() > 0,
            "visible child of hidden parent should be painted"
        );
    }

    #[test]
    fn visibility_collapse_not_painted() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
div { visibility: collapse; background-color: blue; width: 50px; height: 50px; }
</style></head>
<body><div>Collapsed</div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let blue_fills = list.iter().filter(|c| {
            matches!(c, PaintCommand::FillRect { color, .. } if *color == Color::rgb(0, 0, 255))
        });
        assert_eq!(
            blue_fills.count(),
            0,
            "collapse element should not be painted"
        );
    }

    // --- Overflow clipping tests ---

    #[test]
    fn overflow_hidden_generates_clip_commands() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: hidden; width: 100px; height: 50px; }
</style></head>
<body><div class="container"><p>Content</p></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let push_count = list
            .iter()
            .filter(|c| matches!(c, PaintCommand::PushClip { .. }))
            .count();
        let pop_count = list
            .iter()
            .filter(|c| matches!(c, PaintCommand::PopClip))
            .count();

        assert!(push_count >= 1, "overflow:hidden should emit PushClip");
        assert_eq!(push_count, pop_count, "PushClip/PopClip must be balanced");
    }

    #[test]
    fn overflow_visible_no_clip_commands() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: visible; width: 100px; height: 50px; }
</style></head>
<body><div class="container"><p>Content</p></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let push_count = list
            .iter()
            .filter(|c| matches!(c, PaintCommand::PushClip { .. }))
            .count();

        assert_eq!(
            push_count, 0,
            "overflow:visible should not emit clip commands"
        );
    }

    #[test]
    fn overflow_hidden_clips_child_background() {
        // A tall child inside a short overflow:hidden container.
        // The child's red background should be clipped to the container's bounds.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: hidden; width: 100px; height: 50px; }
.child { background-color: red; width: 100px; height: 200px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(200, 200);
        renderer.paint(&tree, &font, &HashMap::new());

        let pixels = renderer.pixels();

        // Pixel at (50, 25) — inside the container — should be red.
        let inside_offset = ((25 * 200 + 50) * 4) as usize;
        assert_eq!(pixels[inside_offset], 0, "B inside should be 0 (red)");
        assert_eq!(
            pixels[inside_offset + 2],
            255,
            "R inside should be 255 (red)"
        );

        // Pixel at (50, 100) — outside the container (below 50px) — should be white.
        let outside_offset = ((100 * 200 + 50) * 4) as usize;
        assert_eq!(
            pixels[outside_offset], 255,
            "B outside should be 255 (white)"
        );
        assert_eq!(
            pixels[outside_offset + 1],
            255,
            "G outside should be 255 (white)"
        );
        assert_eq!(
            pixels[outside_offset + 2],
            255,
            "R outside should be 255 (white)"
        );
    }

    #[test]
    fn overflow_auto_clips_like_hidden() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: auto; width: 100px; height: 50px; }
.child { background-color: blue; width: 100px; height: 200px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(200, 200);
        renderer.paint(&tree, &font, &HashMap::new());

        let pixels = renderer.pixels();

        // Pixel at (50, 100) — below the container — should be white (clipped).
        let outside_offset = ((100 * 200 + 50) * 4) as usize;
        assert_eq!(
            pixels[outside_offset], 255,
            "overflow:auto should clip content below container"
        );
    }

    #[test]
    fn overflow_scroll_clips_like_hidden() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: scroll; width: 100px; height: 50px; }
.child { background-color: green; width: 100px; height: 200px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(200, 200);
        renderer.paint(&tree, &font, &HashMap::new());

        let pixels = renderer.pixels();

        // Pixel at (50, 100) — below the container — should be white (clipped).
        let outside_offset = ((100 * 200 + 50) * 4) as usize;
        assert_eq!(
            pixels[outside_offset], 255,
            "overflow:scroll should clip content below container"
        );
    }

    #[test]
    fn nested_overflow_clips_intersect() {
        // Outer: 200x200, inner: 100x100, both overflow:hidden.
        // A large red child should only appear in the inner 100x100 area.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.outer { overflow: hidden; width: 200px; height: 200px; }
.inner { overflow: hidden; width: 100px; height: 100px; }
.child { background-color: red; width: 500px; height: 500px; }
</style></head>
<body>
<div class="outer"><div class="inner"><div class="child"></div></div></div>
</body></html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(300, 300);
        renderer.paint(&tree, &font, &HashMap::new());

        let pixels = renderer.pixels();

        // Pixel at (50, 50) — inside inner container — should be red.
        let inside_offset = ((50 * 300 + 50) * 4) as usize;
        assert_eq!(pixels[inside_offset + 2], 255, "should be red inside inner");

        // Pixel at (150, 50) — inside outer but outside inner — should be white.
        let between_offset = ((50 * 300 + 150) * 4) as usize;
        assert_eq!(
            pixels[between_offset], 255,
            "should be white outside inner clip"
        );

        // Pixel at (250, 50) — outside both — should be white.
        let outside_offset = ((50 * 300 + 250) * 4) as usize;
        assert_eq!(
            pixels[outside_offset], 255,
            "should be white outside both clips"
        );
    }

    #[test]
    fn clip_rect_intersect_basic() {
        let a = ClipRect {
            x0: 0.0,
            y0: 0.0,
            x1: 100.0,
            y1: 100.0,
        };
        let b = ClipRect {
            x0: 50.0,
            y0: 50.0,
            x1: 150.0,
            y1: 150.0,
        };
        let c = a.intersect(b).unwrap();
        assert_eq!(c.x0, 50.0);
        assert_eq!(c.y0, 50.0);
        assert_eq!(c.x1, 100.0);
        assert_eq!(c.y1, 100.0);
    }

    #[test]
    fn clip_rect_no_overlap() {
        let a = ClipRect {
            x0: 0.0,
            y0: 0.0,
            x1: 50.0,
            y1: 50.0,
        };
        let b = ClipRect {
            x0: 100.0,
            y0: 100.0,
            x1: 200.0,
            y1: 200.0,
        };
        assert!(a.intersect(b).is_none());
    }

    #[test]
    fn display_none_not_in_display_list() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.gone { display: none; background-color: green; }
</style></head>
<body>
<p>Visible</p>
<div class="gone">Gone</div>
</body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let green_fills = list.iter().filter(|c| {
            matches!(c, PaintCommand::FillRect { color, .. } if *color == Color::rgb(0, 128, 0))
        });
        assert_eq!(
            green_fills.count(),
            0,
            "display:none element should not be in display list"
        );
    }

    // --- Overflow scrolling tests ---

    #[test]
    fn overflow_scroll_renders_scrollbar_always() {
        // overflow:scroll should always render scroll bars, even when content fits.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: scroll; width: 200px; height: 200px; }
.child { height: 50px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        // Should have scroll bar track and thumb FillRect commands.
        let scrollbar_fills: Vec<_> = list
            .iter()
            .filter(|c| {
                matches!(c, PaintCommand::FillRect { color, .. }
                    if *color == SCROLLBAR_TRACK_COLOR || *color == SCROLLBAR_THUMB_COLOR)
            })
            .collect();

        assert!(
            scrollbar_fills.len() >= 2,
            "overflow:scroll should render track and thumb (got {} fills)",
            scrollbar_fills.len()
        );
    }

    #[test]
    fn overflow_auto_scrollbar_only_when_overflows() {
        // overflow:auto with content that fits should NOT show scroll bars.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: auto; width: 200px; height: 200px; }
.child { height: 50px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let scrollbar_fills = list.iter().filter(|c| {
            matches!(c, PaintCommand::FillRect { color, .. }
                if *color == SCROLLBAR_TRACK_COLOR || *color == SCROLLBAR_THUMB_COLOR)
        });

        assert_eq!(
            scrollbar_fills.count(),
            0,
            "overflow:auto should not render scroll bars when content fits"
        );
    }

    #[test]
    fn overflow_auto_scrollbar_when_content_overflows() {
        // overflow:auto with content taller than container should show scroll bars.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: auto; width: 200px; height: 100px; }
.child { background-color: red; height: 500px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let scrollbar_fills: Vec<_> = list
            .iter()
            .filter(|c| {
                matches!(c, PaintCommand::FillRect { color, .. }
                    if *color == SCROLLBAR_TRACK_COLOR || *color == SCROLLBAR_THUMB_COLOR)
            })
            .collect();

        assert!(
            scrollbar_fills.len() >= 2,
            "overflow:auto should render scroll bars when content overflows (got {} fills)",
            scrollbar_fills.len()
        );
    }

    #[test]
    fn scroll_offset_shifts_content() {
        // Scrolling should shift content within a scroll container.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: scroll; width: 200px; height: 100px; }
.child { background-color: red; width: 200px; height: 500px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let tree = layout_doc(&doc);

        // Find the container's NodeId to set scroll offset.
        let container_id = tree
            .root
            .iter()
            .find_map(|b| {
                if b.overflow == Overflow::Scroll {
                    node_id_from_box_type(&b.box_type)
                } else {
                    None
                }
            })
            .expect("should find scroll container");

        // Without scroll: pixel at (50, 50) should be red (inside child).
        let mut renderer = Renderer::new(300, 300);
        renderer.paint(&tree, &font, &HashMap::new());
        let pixels = renderer.pixels();
        let offset_50_50 = ((50 * 300 + 50) * 4) as usize;
        assert_eq!(
            pixels[offset_50_50 + 2],
            255,
            "before scroll: (50,50) should be red"
        );

        // With scroll offset 60px: content shifts up, so pixel at (50, 50) should
        // still be red (content extends to 500px), but pixel at (50, 0) should now
        // show content that was at y=60.
        let mut scroll_state = HashMap::new();
        scroll_state.insert(container_id, (0.0, 60.0));
        let mut renderer2 = Renderer::new(300, 300);
        renderer2.paint_with_scroll(&tree, &font, &HashMap::new(), 0.0, &scroll_state);
        let pixels2 = renderer2.pixels();

        // After scrolling 60px, content starts at y=-60 in the viewport.
        // The container clips to its bounds, so pixel at (50, 50) is still visible
        // red content (it was at y=110 in original content, now at y=50).
        assert_eq!(
            pixels2[offset_50_50 + 2],
            255,
            "after scroll: (50,50) should still be red (content is tall)"
        );
    }

    #[test]
    fn scroll_bar_thumb_proportional() {
        // Thumb size should be proportional to viewport/content ratio.
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.container { overflow: scroll; width: 200px; height: 100px; }
.child { height: 400px; }
</style></head>
<body><div class="container"><div class="child"></div></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        // Find the thumb FillRect (SCROLLBAR_THUMB_COLOR).
        let thumb = list.iter().find(|c| {
            matches!(c, PaintCommand::FillRect { color, .. } if *color == SCROLLBAR_THUMB_COLOR)
        });

        assert!(thumb.is_some(), "should have a scroll bar thumb");
        if let Some(PaintCommand::FillRect { height, .. }) = thumb {
            // Container height is 100, content is 400.
            // Thumb ratio = 100/400 = 0.25, track height = 100.
            // Thumb height = max(0.25 * 100, 20) = 25.
            assert!(
                *height >= 20.0 && *height <= 100.0,
                "thumb height {} should be proportional and within bounds",
                height
            );
        }
    }

    #[test]
    fn page_scroll_shifts_all_content() {
        let html_str = r#"<!DOCTYPE html>
<html><head><style>
body { margin: 0; }
.block { background-color: red; width: 100px; height: 100px; }
</style></head>
<body><div class="block"></div></body></html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let tree = layout_doc(&doc);

        // Without page scroll: red at (50, 50).
        let mut r1 = Renderer::new(200, 200);
        r1.paint(&tree, &font, &HashMap::new());
        let offset = ((50 * 200 + 50) * 4) as usize;
        assert_eq!(r1.pixels()[offset + 2], 255, "should be red without scroll");

        // With page scroll 80px: the red block shifts up by 80px.
        // Pixel at (50, 50) was at y=130 in content, which is beyond the 100px block.
        let mut r2 = Renderer::new(200, 200);
        r2.paint_with_scroll(&tree, &font, &HashMap::new(), 80.0, &HashMap::new());
        let pixels2 = r2.pixels();
        // At (50, 50) with 80px scroll: this shows content at y=130, which is white.
        assert_eq!(
            pixels2[offset], 255,
            "should be white at (50,50) with 80px page scroll"
        );

        // Pixel at (50, 10) with 80px scroll shows content at y=90, which is still red.
        let offset_10 = ((10 * 200 + 50) * 4) as usize;
        assert_eq!(
            pixels2[offset_10 + 2],
            255,
            "should be red at (50,10) with 80px page scroll"
        );
    }
}