obscura/crates/obscura-render/src/lib.rs

//! obscura-render: the optional scoped render layer for Obscura.
//!
//! Obscura's default build has no layout or paint engine, which is the source
//! of its speed and low memory. This crate adds a render layer behind a feature
//! flag: real CSS box geometry (so getBoundingClientRect, elementFromPoint, and
//! IntersectionObserver return true values) and, with the paint feature,
//! rasterized PNG screenshots.
//!
//! Phase 1 (this file): the layout core. A `LayoutNode` tree plus a viewport is
//! laid out with taffy and the resulting border-box geometry is returned per
//! node. Later phases build the `LayoutNode` tree from the live DOM + computed
//! styles, feed geometry back to JS, and add text + paint.

use taffy::prelude::*;

pub mod css;
pub use css::Stylesheet;

pub mod style;
pub use style::compute_style;

pub mod dom;
pub use dom::{layout_dom, DomLayout};

#[cfg(feature = "paint")]
mod paint;
#[cfg(feature = "paint")]
pub use paint::{paint_dom, screenshot_png};

/// An axis-aligned rectangle in CSS pixels, relative to the containing block.
#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub struct Rect {
    pub x: f32,
    pub y: f32,
    pub width: f32,
    pub height: f32,
}

impl Rect {
    /// The overlap of two rects, or `None` if they do not intersect (or the
    /// overlap is degenerate). Used to accumulate an ancestor clip chain for
    /// `overflow: hidden`.
    pub fn intersect(&self, other: &Rect) -> Option<Rect> {
        let x0 = self.x.max(other.x);
        let y0 = self.y.max(other.y);
        let x1 = (self.x + self.width).min(other.x + other.width);
        let y1 = (self.y + self.height).min(other.y + other.height);
        if x1 > x0 && y1 > y0 {
            Some(Rect { x: x0, y: y0, width: x1 - x0, height: y1 - y0 })
        } else {
            None
        }
    }
}

/// Per-edge box values (margin / padding / border) in CSS pixels.
#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub struct Edges {
    pub top: f32,
    pub right: f32,
    pub bottom: f32,
    pub left: f32,
}

/// The display modes obscura-render cares about for phase 1. Inline text layout
/// arrives with the text/paint phase and is folded in then.
#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub enum Display {
    #[default]
    Block,
    Flex,
    Grid,
    Inline,
    #[allow(dead_code)]
    None,
}

#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub enum Dimension {
    #[default]
    Auto,
    Px(f32),
    Percent(f32),
}

/// The subset of CSS that influences box layout. Expanded in later phases.
#[derive(Debug, Clone, Default)]
pub struct LayoutStyle {
    pub display: Display,
    pub width: Dimension,
    pub height: Dimension,
    pub min_width: Dimension,
    pub min_height: Dimension,
    pub max_width: Dimension,
    pub max_height: Dimension,
    pub margin: Edges,
    pub padding: Edges,
    pub border: Edges,
    /// RGBA for the paint step. Parsed always (cheap), used only with `paint`.
    pub background_color: Option<[u8; 4]>,
    /// The first `url(...)` reference from `background`/`background-image`
    /// (gradients and repeat keywords in the same shorthand are ignored: we
    /// paint the referenced image, not the gradient layer).
    pub background_image: Option<String>,
    /// `background-size`, in px, when given as explicit length(s) (a bare
    /// `10px` applies to both axes, matching how small square icons are
    /// almost always sized). `None` means "fill the whole box" (our fallback
    /// when the value is a keyword we don't evaluate, `cover`/`contain`, or
    /// absent with an already icon-sized box like the HN vote arrow).
    pub background_size: Option<(f32, f32)>,
    /// `background-position` as a 0.0-1.0 fraction per axis (0,0 = default
    /// top-left; 1,0.5 = "right center"). Only meaningful alongside
    /// `background_size`: without a known image size there is no leftover
    /// box space to position within.
    pub background_position: (f32, f32),
    /// Foreground (text) color for the paint step.
    pub color: Option<[u8; 4]>,
    pub border_color: Option<[u8; 4]>,
    pub font_size: Option<f32>,
    pub font_weight: Option<String>,
    pub align_items: Option<taffy::AlignItems>,
    pub flex_direction: Option<taffy::FlexDirection>,
    pub flex_wrap: Option<taffy::FlexWrap>,
    pub justify_content: Option<taffy::JustifyContent>,
    pub flex_grow: Option<f32>,
    pub flex_shrink: Option<f32>,

    // CSS Grid. Tracks are stored as taffy sizing functions; `grid_areas` is the
    // parsed `grid-template-areas` matrix (one Vec per row, `.` for a null cell),
    // resolved to line placements on children in a later pass.
    pub grid_template_columns: Vec<taffy::TrackSizingFunction>,
    pub grid_template_rows: Vec<taffy::TrackSizingFunction>,
    pub grid_areas: Option<Vec<Vec<String>>>,
    pub grid_area_name: Option<String>,
    pub grid_column: Option<taffy::Line<taffy::GridPlacement>>,
    pub grid_row: Option<taffy::Line<taffy::GridPlacement>>,
    pub column_gap: Option<f32>,
    pub row_gap: Option<f32>,

    // Positioning. `position: absolute|fixed` takes the box out of normal flow.
    pub position: Option<taffy::Position>,
    pub inset: [Option<f32>; 4], // top, right, bottom, left

    /// `overflow`/-x/-y other than `visible`: clips this element's descendants
    /// to its border box during paint. This is what makes the ubiquitous
    /// "visually-hidden but accessible" pattern (a 1x1 absolutely-positioned,
    /// clipped box used for skip-links and screen-reader-only labels) actually
    /// invisible instead of painting its text wherever it lands.
    pub overflow_hidden: bool,
}

/// A node in the input layout tree. `text` is carried for the paint phase; it
/// does not affect layout in phase 1 (inline/text layout comes later).
#[derive(Debug, Clone)]
pub struct LayoutNode {
    pub style: LayoutStyle,
    #[allow(dead_code)]
    pub text: Option<String>,
    pub children: Vec<LayoutNode>,
}

impl LayoutNode {
    pub fn leaf(style: LayoutStyle) -> Self {
        LayoutNode { style, text: None, children: Vec::new() }
    }
}

/// Computed geometry for one node and its subtree.
#[derive(Debug, Clone, Default)]
pub struct NodeRect {
    /// Border box, in viewport coordinates.
    pub border_box: Rect,
    pub children: Vec<NodeRect>,
}

/// Lay out `root` within a `viewport` (width, height) in CSS pixels and return
/// the border-box geometry per node, mirroring the input tree.
pub fn layout(root: &LayoutNode, viewport: (f32, f32)) -> NodeRect {
    let mut tree: TaffyTree = TaffyTree::new();
    let root_id = build_node(&mut tree, root);
    let _ = tree.compute_layout(
        root_id,
        taffy::Size {
            width: taffy::AvailableSpace::Definite(viewport.0),
            height: taffy::AvailableSpace::Definite(viewport.1),
        },
    );
    read_node(&tree, root_id)
}

fn build_node(tree: &mut TaffyTree, node: &LayoutNode) -> NodeId {
    let style = to_taffy_style(&node.style);
    if node.children.is_empty() {
        tree.new_leaf(style).expect("taffy new_leaf")
    } else {
        let child_ids: Vec<NodeId> =
            node.children.iter().map(|c| build_node(tree, c)).collect();
        tree.new_with_children(style, &child_ids).expect("taffy new_with_children")
    }
}

fn read_node(tree: &TaffyTree, id: NodeId) -> NodeRect {
    let layout = tree.layout(id).expect("taffy layout");
    NodeRect {
        border_box: Rect {
            x: layout.location.x,
            y: layout.location.y,
            width: layout.size.width,
            height: layout.size.height,
        },
        children: tree
            .children(id)
            .unwrap_or_default()
            .iter()
            .map(|&cid| read_node(tree, cid))
            .collect(),
    }
}

pub(crate) fn to_taffy_style(style: &LayoutStyle) -> Style {
    let mut s = Style::DEFAULT;

    // A block box that wants non-default cross-axis alignment (from
    // text-align: center/right, the only way our engine currently sets
    // align_items on a block-level element) needs a flex column to have any
    // effect at all: taffy's native block algorithm (used for plain
    // Display::Block) has no align-items concept whatsoever, it only ever
    // places block children at the start edge. Promote such boxes to a flex
    // column, matching the same column-flex approximation already used for
    // elements like <td> and <center>.
    let promote_for_alignment = style.display == Display::Block
        && matches!(style.align_items, Some(taffy::AlignItems::Center) | Some(taffy::AlignItems::FlexEnd));

    s.display = match style.display {
        Display::Block if promote_for_alignment => taffy::style::Display::Flex,
        Display::Block => taffy::style::Display::Block,
        Display::Flex => taffy::style::Display::Flex,
        Display::Grid => taffy::style::Display::Grid,
        Display::Inline => taffy::style::Display::Flex,
        Display::None => taffy::style::Display::None,
    };
    if promote_for_alignment {
        s.flex_direction = taffy::FlexDirection::Column;
    }
    if let Some(fd) = style.flex_direction {
        s.flex_direction = fd;
    }
    if let Some(fw) = style.flex_wrap {
        s.flex_wrap = fw;
    } else if style.display == Display::Inline {
        s.flex_direction = taffy::FlexDirection::Row;
        s.flex_wrap = taffy::FlexWrap::Wrap;
    } else {
        s.flex_wrap = taffy::FlexWrap::NoWrap;
    }
    s.size = taffy::Size {
        width: dimension(style.width),
        height: dimension(style.height),
    };
    s.min_size = taffy::Size {
        width: dimension(style.min_width),
        height: dimension(style.min_height),
    };
    s.max_size = taffy::Size {
        width: dimension(style.max_width),
        height: dimension(style.max_height),
    };
    if let Some(ai) = style.align_items {
        s.align_items = Some(ai);
    }
    if let Some(jc) = style.justify_content {
        s.justify_content = Some(jc);
    }
    if let Some(fg) = style.flex_grow {
        s.flex_grow = fg;
    }
    if let Some(fs) = style.flex_shrink {
        s.flex_shrink = fs;
    }

    // Grid container tracks and gaps.
    if style.display == Display::Grid {
        if !style.grid_template_columns.is_empty() {
            s.grid_template_columns = style.grid_template_columns.clone();
        }
        if !style.grid_template_rows.is_empty() {
            s.grid_template_rows = style.grid_template_rows.clone();
        }
    }
    let cg = style.column_gap.unwrap_or(0.0);
    let rg = style.row_gap.unwrap_or(0.0);
    s.gap = taffy::Size {
        width: taffy::style::LengthPercentage::Length(cg),
        height: taffy::style::LengthPercentage::Length(rg),
    };

    // Grid item placement (resolved from grid-area names or explicit lines).
    if let Some(gc) = style.grid_column {
        s.grid_column = gc;
    }
    if let Some(gr) = style.grid_row {
        s.grid_row = gr;
    }

    // Positioning. Absolute/fixed take the box out of flow.
    if let Some(pos) = style.position {
        s.position = pos;
        s.inset = taffy::Rect {
            top: inset_lpa(style.inset[0]),
            right: inset_lpa(style.inset[1]),
            bottom: inset_lpa(style.inset[2]),
            left: inset_lpa(style.inset[3]),
        };
    }

    s.margin = rect_auto(style.margin);
    s.padding = rect_lp(style.padding);
    s.border = rect_lp(style.border);
    s
}

fn inset_lpa(v: Option<f32>) -> taffy::style::LengthPercentageAuto {
    match v {
        Some(px) => taffy::style::LengthPercentageAuto::Length(px),
        None => taffy::style::LengthPercentageAuto::Auto,
    }
}

fn dimension(v: Dimension) -> taffy::style::Dimension {
    match v {
        Dimension::Px(px) => taffy::style::Dimension::Length(px),
        Dimension::Percent(p) => taffy::style::Dimension::Percent(p),
        Dimension::Auto => taffy::style::Dimension::Auto,
    }
}

fn rect_lp(e: Edges) -> taffy::Rect<taffy::style::LengthPercentage> {
    taffy::Rect {
        top: taffy::style::LengthPercentage::Length(e.top),
        right: taffy::style::LengthPercentage::Length(e.right),
        bottom: taffy::style::LengthPercentage::Length(e.bottom),
        left: taffy::style::LengthPercentage::Length(e.left),
    }
}

fn rect_auto(e: Edges) -> taffy::Rect<taffy::style::LengthPercentageAuto> {
    taffy::Rect {
        top: taffy::style::LengthPercentageAuto::Length(e.top),
        right: taffy::style::LengthPercentageAuto::Length(e.right),
        bottom: taffy::style::LengthPercentageAuto::Length(e.bottom),
        left: taffy::style::LengthPercentageAuto::Length(e.left),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn make_box(display: Display, w: f32, h: f32) -> LayoutStyle {
        LayoutStyle { display, width: Dimension::Px(w), height: Dimension::Px(h), ..Default::default() }
    }

    #[test]
    fn block_children_stack_vertically() {
        // A 1000px-wide viewport, two fixed-size block children: they should
        // stack top-to-bottom at the expected y offsets.
        let root = LayoutNode {
            style: make_box(Display::Block, 1000.0, 800.0),
            text: None,
            children: vec![
                LayoutNode::leaf(make_box(Display::Block, 1000.0, 50.0)),
                LayoutNode::leaf(make_box(Display::Block, 1000.0, 30.0)),
            ],
        };
        let out = layout(&root, (1000.0, 800.0));
        assert_eq!(out.border_box.width, 1000.0);
        assert_eq!(out.children.len(), 2);
        assert_eq!(out.children[0].border_box.height, 50.0);
        assert_eq!(out.children[1].border_box.height, 30.0);
        // Second block begins where the first ended.
        assert!(
            (out.children[1].border_box.y - out.children[0].border_box.y).abs()
                >= out.children[0].border_box.height - 0.01,
            "blocks should stack: c0.y={} c1.y={}",
            out.children[0].border_box.y,
            out.children[1].border_box.y
        );
    }

    #[test]
    fn flex_row_lays_out_horizontally() {
        let root = LayoutNode {
            style: LayoutStyle { display: Display::Flex, width: Dimension::Px(600.0), height: Dimension::Px(100.0), ..Default::default() },
            text: None,
            children: vec![
                LayoutNode::leaf(make_box(Display::Block, 200.0, 100.0)),
                LayoutNode::leaf(make_box(Display::Block, 200.0, 100.0)),
            ],
        };
        let out = layout(&root, (600.0, 400.0));
        assert_eq!(out.border_box.width, 600.0);
        assert_eq!(out.children.len(), 2);
        // In a row the second child is to the right of the first.
        assert!(
            out.children[1].border_box.x > out.children[0].border_box.x,
            "flex row should place children horizontally: c0.x={} c1.x={}",
            out.children[0].border_box.x,
            out.children[1].border_box.x
        );
    }

    #[test]
    fn padding_insets_the_box() {
        let root = LayoutNode {
            style: LayoutStyle {
                display: Display::Block,
                width: Dimension::Px(100.0),
                height: Dimension::Px(100.0),
                padding: Edges { top: 10.0, right: 10.0, bottom: 10.0, left: 10.0 },
                ..Default::default()
            },
            text: None,
            children: vec![],
        };
        let out = layout(&root, (1000.0, 800.0));
        // Border box still matches the requested size; padding affects content,
        // not the outer border box here.
        assert_eq!(out.border_box.width, 100.0);
    }
}