Layout 2020: Move all Fragment code to the fragment_tree directory

This is a simple code organization change with no behavior change with
the idea of making Layout 2020 easier to understand by new folks to the
project. The idea is that we will have a cleaner separation between the
different parts of layout ie one directory for the fragment tree and one
(currently multiple) directory for the box tree.

components/layout_2020/fragment_tree/box_fragment.rs

@@ -0,0 +1,280 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use super::{BaseFragment, BaseFragmentInfo, CollapsedBlockMargins, Fragment};
+use crate::cell::ArcRefCell;
+use crate::geom::flow_relative::{Rect, Sides};
+use crate::geom::{PhysicalPoint, PhysicalRect, PhysicalSides, PhysicalSize};
+use gfx_traits::print_tree::PrintTree;
+use servo_arc::Arc as ServoArc;
+use style::computed_values::overflow_x::T as ComputedOverflow;
+use style::computed_values::position::T as ComputedPosition;
+use style::properties::ComputedValues;
+use style::values::computed::{CSSPixelLength, Length, LengthPercentage, LengthPercentageOrAuto};
+use style::Zero;
+
+#[derive(Serialize)]
+pub(crate) struct BoxFragment {
+    pub base: BaseFragment,
+
+    #[serde(skip_serializing)]
+    pub style: ServoArc<ComputedValues>,
+    pub children: Vec<ArcRefCell<Fragment>>,
+
+    /// From the containing block’s start corner…?
+    /// This might be broken when the containing block is in a different writing mode:
+    /// https://drafts.csswg.org/css-writing-modes/#orthogonal-flows
+    pub content_rect: Rect<Length>,
+
+    pub padding: Sides<Length>,
+    pub border: Sides<Length>,
+    pub margin: Sides<Length>,
+
+    pub clearance: Length,
+
+    pub block_margins_collapsed_with_children: CollapsedBlockMargins,
+
+    /// The scrollable overflow of this box fragment.
+    pub scrollable_overflow_from_children: PhysicalRect<Length>,
+
+    /// Whether or not this box was overconstrained in the given dimension.
+    overconstrained: PhysicalSize<bool>,
+}
+
+impl BoxFragment {
+    pub fn new(
+        base_fragment_info: BaseFragmentInfo,
+        style: ServoArc<ComputedValues>,
+        children: Vec<Fragment>,
+        content_rect: Rect<Length>,
+        padding: Sides<Length>,
+        border: Sides<Length>,
+        margin: Sides<Length>,
+        clearance: Length,
+        block_margins_collapsed_with_children: CollapsedBlockMargins,
+    ) -> BoxFragment {
+        let position = style.get_box().position;
+        let insets = style.get_position();
+        let width_overconstrained = position == ComputedPosition::Relative &&
+            !insets.left.is_auto() &&
+            !insets.right.is_auto();
+        let height_overconstrained = position == ComputedPosition::Relative &&
+            !insets.left.is_auto() &&
+            !insets.bottom.is_auto();
+
+        Self::new_with_overconstrained(
+            base_fragment_info,
+            style,
+            children,
+            content_rect,
+            padding,
+            border,
+            margin,
+            clearance,
+            block_margins_collapsed_with_children,
+            PhysicalSize::new(width_overconstrained, height_overconstrained),
+        )
+    }
+
+    pub fn new_with_overconstrained(
+        base_fragment_info: BaseFragmentInfo,
+        style: ServoArc<ComputedValues>,
+        children: Vec<Fragment>,
+        content_rect: Rect<Length>,
+        padding: Sides<Length>,
+        border: Sides<Length>,
+        margin: Sides<Length>,
+        clearance: Length,
+        block_margins_collapsed_with_children: CollapsedBlockMargins,
+        overconstrained: PhysicalSize<bool>,
+    ) -> BoxFragment {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let containing_block = PhysicalRect::zero();
+        let scrollable_overflow_from_children =
+            children.iter().fold(PhysicalRect::zero(), |acc, child| {
+                acc.union(&child.scrollable_overflow(&containing_block))
+            });
+
+        BoxFragment {
+            base: base_fragment_info.into(),
+            style,
+            children: children
+                .into_iter()
+                .map(|fragment| ArcRefCell::new(fragment))
+                .collect(),
+            content_rect,
+            padding,
+            border,
+            margin,
+            clearance,
+            block_margins_collapsed_with_children,
+            scrollable_overflow_from_children,
+            overconstrained,
+        }
+    }
+
+    pub fn scrollable_overflow(
+        &self,
+        containing_block: &PhysicalRect<Length>,
+    ) -> PhysicalRect<Length> {
+        let physical_padding_rect = self
+            .padding_rect()
+            .to_physical(self.style.writing_mode, containing_block);
+
+        let content_origin = self
+            .content_rect
+            .start_corner
+            .to_physical(self.style.writing_mode);
+        physical_padding_rect.union(
+            &self
+                .scrollable_overflow_from_children
+                .translate(content_origin.to_vector()),
+        )
+    }
+
+    pub fn padding_rect(&self) -> Rect<Length> {
+        self.content_rect.inflate(&self.padding)
+    }
+
+    pub fn border_rect(&self) -> Rect<Length> {
+        self.padding_rect().inflate(&self.border)
+    }
+
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.new_level(format!(
+            "Box\
+                \nbase={:?}\
+                \ncontent={:?}\
+                \npadding rect={:?}\
+                \nborder rect={:?}\
+                \nclearance={:?}\
+                \nscrollable_overflow={:?}\
+                \noverflow={:?} / {:?}",
+            self.base,
+            self.content_rect,
+            self.padding_rect(),
+            self.border_rect(),
+            self.clearance,
+            self.scrollable_overflow(&PhysicalRect::zero()),
+            self.style.get_box().overflow_x,
+            self.style.get_box().overflow_y,
+        ));
+
+        for child in &self.children {
+            child.borrow().print(tree);
+        }
+        tree.end_level();
+    }
+
+    pub fn scrollable_overflow_for_parent(
+        &self,
+        containing_block: &PhysicalRect<Length>,
+    ) -> PhysicalRect<Length> {
+        let mut overflow = self
+            .border_rect()
+            .to_physical(self.style.writing_mode, containing_block);
+
+        if self.style.get_box().overflow_y != ComputedOverflow::Visible &&
+            self.style.get_box().overflow_x != ComputedOverflow::Visible
+        {
+            return overflow;
+        }
+
+        // https://www.w3.org/TR/css-overflow-3/#scrollable
+        // Only include the scrollable overflow of a child box if it has overflow: visible.
+        let scrollable_overflow = self.scrollable_overflow(&containing_block);
+        let bottom_right = PhysicalPoint::new(
+            overflow.max_x().max(scrollable_overflow.max_x()),
+            overflow.max_y().max(scrollable_overflow.max_y()),
+        );
+
+        if self.style.get_box().overflow_y == ComputedOverflow::Visible {
+            overflow.origin.y = overflow.origin.y.min(scrollable_overflow.origin.y);
+            overflow.size.height = bottom_right.y - overflow.origin.y;
+        }
+
+        if self.style.get_box().overflow_x == ComputedOverflow::Visible {
+            overflow.origin.x = overflow.origin.x.min(scrollable_overflow.origin.x);
+            overflow.size.width = bottom_right.x - overflow.origin.x;
+        }
+
+        overflow
+    }
+
+    pub(crate) fn calculate_resolved_insets_if_positioned(
+        &self,
+        containing_block: &PhysicalRect<CSSPixelLength>,
+    ) -> PhysicalSides<CSSPixelLength> {
+        let position = self.style.get_box().position;
+        debug_assert_ne!(
+            position,
+            ComputedPosition::Static,
+            "Should not call this method on statically positioned box."
+        );
+
+        let (cb_width, cb_height) = (containing_block.width(), containing_block.height());
+        let content_rect = self
+            .content_rect
+            .to_physical(self.style.writing_mode, &containing_block);
+
+        // "A resolved value special case property like top defined in another
+        // specification If the property applies to a positioned element and the
+        // resolved value of the display property is not none or contents, and
+        // the property is not over-constrained, then the resolved value is the
+        // used value. Otherwise the resolved value is the computed value."
+        // https://drafts.csswg.org/cssom/#resolved-values
+        let insets = self.style.get_position();
+        if position == ComputedPosition::Relative {
+            let get_resolved_axis =
+                |start: &LengthPercentageOrAuto,
+                 end: &LengthPercentageOrAuto,
+                 container_length: CSSPixelLength| {
+                    let start = start.map(|v| v.percentage_relative_to(container_length));
+                    let end = end.map(|v| v.percentage_relative_to(container_length));
+                    match (start.non_auto(), end.non_auto()) {
+                        (None, None) => (Length::zero(), Length::zero()),
+                        (None, Some(end)) => (-end, end),
+                        (Some(start), None) => (start, -start),
+                        // This is the overconstrained case, for which the resolved insets will
+                        // simply be the computed insets.
+                        (Some(start), Some(end)) => (start, end),
+                    }
+                };
+            let (left, right) = get_resolved_axis(&insets.left, &insets.right, cb_width);
+            let (top, bottom) = get_resolved_axis(&insets.top, &insets.bottom, cb_height);
+            return PhysicalSides::new(top, right, bottom, left);
+        }
+
+        debug_assert!(
+            position == ComputedPosition::Fixed || position == ComputedPosition::Absolute,
+            "Got unknown position."
+        );
+
+        let resolve = |value: &LengthPercentageOrAuto, container_length| {
+            value
+                .auto_is(LengthPercentage::zero)
+                .percentage_relative_to(container_length)
+        };
+
+        let (top, bottom) = if self.overconstrained.height {
+            (
+                resolve(&insets.top, cb_height),
+                resolve(&insets.bottom, cb_height),
+            )
+        } else {
+            (content_rect.origin.y, cb_height - content_rect.max_y())
+        };
+        let (left, right) = if self.overconstrained.width {
+            (
+                resolve(&insets.left, cb_width),
+                resolve(&insets.right, cb_width),
+            )
+        } else {
+            (content_rect.origin.x, cb_width - content_rect.max_x())
+        };
+
+        PhysicalSides::new(top, right, bottom, left)
+    }
+}

components/layout_2020/fragment_tree/containing_block.rs

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use crate::fragments::Fragment;
+use crate::fragment_tree::Fragment;
 use style::computed_values::position::T as ComputedPosition;
 
 /// A data structure used to track the containing block when recursing

components/layout_2020/fragment_tree/fragment.rs

@@ -0,0 +1,378 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use super::{BaseFragment, BoxFragment, ContainingBlockManager, HoistedSharedFragment, Tag};
+use crate::cell::ArcRefCell;
+use crate::geom::flow_relative::{Rect, Sides};
+use crate::geom::PhysicalRect;
+use crate::style_ext::ComputedValuesExt;
+use gfx::font::FontMetrics as GfxFontMetrics;
+use gfx::text::glyph::GlyphStore;
+use gfx_traits::print_tree::PrintTree;
+use msg::constellation_msg::{BrowsingContextId, PipelineId};
+use servo_arc::Arc as ServoArc;
+use std::sync::Arc;
+use style::logical_geometry::WritingMode;
+use style::properties::ComputedValues;
+use style::values::computed::Length;
+use style::values::specified::text::TextDecorationLine;
+use style::Zero;
+use webrender_api::{FontInstanceKey, ImageKey};
+
+#[derive(Serialize)]
+pub(crate) enum Fragment {
+    Box(BoxFragment),
+    /// Floating content. A floated fragment is very similar to a normal
+    /// [BoxFragment] but it isn't positioned using normal in block flow
+    /// positioning rules (margin collapse, etc). Instead, they are laid out by
+    /// the [SequentialLayoutState] of their float containing block formatting
+    /// context.
+    Float(BoxFragment),
+    Anonymous(AnonymousFragment),
+    /// Absolute and fixed position fragments are hoisted up so that they
+    /// are children of the BoxFragment that establishes their containing
+    /// blocks, so that they can be laid out properly. When this happens
+    /// an `AbsoluteOrFixedPositioned` fragment is left at the original tree
+    /// position. This allows these hoisted fragments to be painted with
+    /// regard to their original tree order during stacking context tree /
+    /// display list construction.
+    AbsoluteOrFixedPositioned(ArcRefCell<HoistedSharedFragment>),
+    Text(TextFragment),
+    Image(ImageFragment),
+    IFrame(IFrameFragment),
+}
+
+#[derive(Serialize)]
+pub(crate) struct FloatFragment {
+    pub box_fragment: BoxFragment,
+}
+
+#[derive(Serialize)]
+pub(crate) struct CollapsedBlockMargins {
+    pub collapsed_through: bool,
+    pub start: CollapsedMargin,
+    pub end: CollapsedMargin,
+}
+
+#[derive(Clone, Copy, Debug, Serialize)]
+pub(crate) struct CollapsedMargin {
+    max_positive: Length,
+    min_negative: Length,
+}
+
+/// Can contain child fragments with relative coordinates, but does not contribute to painting itself.
+#[derive(Serialize)]
+pub(crate) struct AnonymousFragment {
+    pub base: BaseFragment,
+    pub rect: Rect<Length>,
+    pub children: Vec<ArcRefCell<Fragment>>,
+    pub mode: WritingMode,
+
+    /// The scrollable overflow of this anonymous fragment's children.
+    pub scrollable_overflow: PhysicalRect<Length>,
+}
+
+#[derive(Clone, Copy, Serialize)]
+pub(crate) struct FontMetrics {
+    pub ascent: Length,
+    pub line_gap: Length,
+    pub underline_offset: Length,
+    pub underline_size: Length,
+    pub strikeout_offset: Length,
+    pub strikeout_size: Length,
+}
+
+impl From<&GfxFontMetrics> for FontMetrics {
+    fn from(metrics: &GfxFontMetrics) -> FontMetrics {
+        FontMetrics {
+            ascent: metrics.ascent.into(),
+            line_gap: metrics.line_gap.into(),
+            underline_offset: metrics.underline_offset.into(),
+            underline_size: metrics.underline_size.into(),
+            strikeout_offset: metrics.strikeout_offset.into(),
+            strikeout_size: metrics.strikeout_size.into(),
+        }
+    }
+}
+
+#[derive(Serialize)]
+pub(crate) struct TextFragment {
+    pub base: BaseFragment,
+    #[serde(skip_serializing)]
+    pub parent_style: ServoArc<ComputedValues>,
+    pub rect: Rect<Length>,
+    pub font_metrics: FontMetrics,
+    #[serde(skip_serializing)]
+    pub font_key: FontInstanceKey,
+    pub glyphs: Vec<Arc<GlyphStore>>,
+    /// A flag that represents the _used_ value of the text-decoration property.
+    pub text_decoration_line: TextDecorationLine,
+}
+
+#[derive(Serialize)]
+pub(crate) struct ImageFragment {
+    pub base: BaseFragment,
+    #[serde(skip_serializing)]
+    pub style: ServoArc<ComputedValues>,
+    pub rect: Rect<Length>,
+    #[serde(skip_serializing)]
+    pub image_key: ImageKey,
+}
+
+#[derive(Serialize)]
+pub(crate) struct IFrameFragment {
+    pub base: BaseFragment,
+    pub pipeline_id: PipelineId,
+    pub browsing_context_id: BrowsingContextId,
+    pub rect: Rect<Length>,
+    #[serde(skip_serializing)]
+    pub style: ServoArc<ComputedValues>,
+}
+
+impl Fragment {
+    pub fn offset_inline(&mut self, offset: &Length) {
+        let position = match self {
+            Fragment::Box(f) => &mut f.content_rect.start_corner,
+            Fragment::Float(_) | Fragment::AbsoluteOrFixedPositioned(_) => return,
+            Fragment::Anonymous(f) => &mut f.rect.start_corner,
+            Fragment::Text(f) => &mut f.rect.start_corner,
+            Fragment::Image(f) => &mut f.rect.start_corner,
+            Fragment::IFrame(f) => &mut f.rect.start_corner,
+        };
+
+        position.inline += *offset;
+    }
+
+    pub fn base(&self) -> Option<&BaseFragment> {
+        Some(match self {
+            Fragment::Box(fragment) => &fragment.base,
+            Fragment::Text(fragment) => &fragment.base,
+            Fragment::AbsoluteOrFixedPositioned(_) => return None,
+            Fragment::Anonymous(fragment) => &fragment.base,
+            Fragment::Image(fragment) => &fragment.base,
+            Fragment::IFrame(fragment) => &fragment.base,
+            Fragment::Float(fragment) => &fragment.base,
+        })
+    }
+
+    pub fn tag(&self) -> Option<Tag> {
+        self.base().and_then(|base| base.tag)
+    }
+
+    pub fn print(&self, tree: &mut PrintTree) {
+        match self {
+            Fragment::Box(fragment) => fragment.print(tree),
+            Fragment::Float(fragment) => {
+                tree.new_level(format!("Float"));
+                fragment.print(tree);
+                tree.end_level();
+            },
+            Fragment::AbsoluteOrFixedPositioned(_) => {
+                tree.add_item("AbsoluteOrFixedPositioned".to_string());
+            },
+            Fragment::Anonymous(fragment) => fragment.print(tree),
+            Fragment::Text(fragment) => fragment.print(tree),
+            Fragment::Image(fragment) => fragment.print(tree),
+            Fragment::IFrame(fragment) => fragment.print(tree),
+        }
+    }
+
+    pub fn scrollable_overflow(
+        &self,
+        containing_block: &PhysicalRect<Length>,
+    ) -> PhysicalRect<Length> {
+        match self {
+            Fragment::Box(fragment) | Fragment::Float(fragment) => {
+                fragment.scrollable_overflow_for_parent(&containing_block)
+            },
+            Fragment::AbsoluteOrFixedPositioned(_) => PhysicalRect::zero(),
+            Fragment::Anonymous(fragment) => fragment.scrollable_overflow.clone(),
+            Fragment::Text(fragment) => fragment
+                .rect
+                .to_physical(fragment.parent_style.writing_mode, &containing_block),
+            Fragment::Image(fragment) => fragment
+                .rect
+                .to_physical(fragment.style.writing_mode, &containing_block),
+            Fragment::IFrame(fragment) => fragment
+                .rect
+                .to_physical(fragment.style.writing_mode, &containing_block),
+        }
+    }
+
+    pub(crate) fn find<T>(
+        &self,
+        manager: &ContainingBlockManager<PhysicalRect<Length>>,
+        level: usize,
+        process_func: &mut impl FnMut(&Fragment, usize, &PhysicalRect<Length>) -> Option<T>,
+    ) -> Option<T> {
+        let containing_block = manager.get_containing_block_for_fragment(self);
+        if let Some(result) = process_func(self, level, containing_block) {
+            return Some(result);
+        }
+
+        match self {
+            Fragment::Box(fragment) | Fragment::Float(fragment) => {
+                let content_rect = fragment
+                    .content_rect
+                    .to_physical(fragment.style.writing_mode, containing_block)
+                    .translate(containing_block.origin.to_vector());
+                let padding_rect = fragment
+                    .padding_rect()
+                    .to_physical(fragment.style.writing_mode, containing_block)
+                    .translate(containing_block.origin.to_vector());
+                let new_manager = if fragment
+                    .style
+                    .establishes_containing_block_for_all_descendants()
+                {
+                    manager.new_for_absolute_and_fixed_descendants(&content_rect, &padding_rect)
+                } else if fragment
+                    .style
+                    .establishes_containing_block_for_absolute_descendants()
+                {
+                    manager.new_for_absolute_descendants(&content_rect, &padding_rect)
+                } else {
+                    manager.new_for_non_absolute_descendants(&content_rect)
+                };
+
+                fragment
+                    .children
+                    .iter()
+                    .find_map(|child| child.borrow().find(&new_manager, level + 1, process_func))
+            },
+            Fragment::Anonymous(fragment) => {
+                let content_rect = fragment
+                    .rect
+                    .to_physical(fragment.mode, containing_block)
+                    .translate(containing_block.origin.to_vector());
+                let new_manager = manager.new_for_non_absolute_descendants(&content_rect);
+                fragment
+                    .children
+                    .iter()
+                    .find_map(|child| child.borrow().find(&new_manager, level + 1, process_func))
+            },
+            _ => None,
+        }
+    }
+}
+
+impl AnonymousFragment {
+    pub fn new(rect: Rect<Length>, children: Vec<Fragment>, mode: WritingMode) -> Self {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let containing_block = PhysicalRect::zero();
+        let content_origin = rect.start_corner.to_physical(mode);
+        let scrollable_overflow = children.iter().fold(PhysicalRect::zero(), |acc, child| {
+            acc.union(
+                &child
+                    .scrollable_overflow(&containing_block)
+                    .translate(content_origin.to_vector()),
+            )
+        });
+        AnonymousFragment {
+            base: BaseFragment::anonymous(),
+            rect,
+            children: children
+                .into_iter()
+                .map(|fragment| ArcRefCell::new(fragment))
+                .collect(),
+            mode,
+            scrollable_overflow,
+        }
+    }
+
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.new_level(format!(
+            "Anonymous\
+                \nrect={:?}\
+                \nscrollable_overflow={:?}",
+            self.rect, self.scrollable_overflow
+        ));
+
+        for child in &self.children {
+            child.borrow().print(tree);
+        }
+        tree.end_level();
+    }
+}
+
+impl TextFragment {
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.add_item(format!(
+            "Text num_glyphs={}",
+            self.glyphs
+                .iter()
+                .map(|glyph_store| glyph_store.len().0)
+                .sum::<isize>()
+        ));
+    }
+}
+
+impl ImageFragment {
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.add_item(format!(
+            "Image\
+                \nrect={:?}",
+            self.rect
+        ));
+    }
+}
+
+impl IFrameFragment {
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.add_item(format!(
+            "IFrame\
+                \npipeline={:?} rect={:?}",
+            self.pipeline_id, self.rect
+        ));
+    }
+}
+
+impl CollapsedBlockMargins {
+    pub fn from_margin(margin: &Sides<Length>) -> Self {
+        Self {
+            collapsed_through: false,
+            start: CollapsedMargin::new(margin.block_start),
+            end: CollapsedMargin::new(margin.block_end),
+        }
+    }
+
+    pub fn zero() -> Self {
+        Self {
+            collapsed_through: false,
+            start: CollapsedMargin::zero(),
+            end: CollapsedMargin::zero(),
+        }
+    }
+}
+
+impl CollapsedMargin {
+    pub fn zero() -> Self {
+        Self {
+            max_positive: Length::zero(),
+            min_negative: Length::zero(),
+        }
+    }
+
+    pub fn new(margin: Length) -> Self {
+        Self {
+            max_positive: margin.max(Length::zero()),
+            min_negative: margin.min(Length::zero()),
+        }
+    }
+
+    pub fn adjoin(&self, other: &Self) -> Self {
+        Self {
+            max_positive: self.max_positive.max(other.max_positive),
+            min_negative: self.min_negative.min(other.min_negative),
+        }
+    }
+
+    pub fn adjoin_assign(&mut self, other: &Self) {
+        *self = self.adjoin(other);
+    }
+
+    pub fn solve(&self) -> Length {
+        self.max_positive + self.min_negative
+    }
+}

components/layout_2020/fragment_tree/fragment_tree.rs

@@ -0,0 +1,204 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use super::{ContainingBlockManager, Fragment, Tag};
+use crate::cell::ArcRefCell;
+use crate::display_list::StackingContext;
+use crate::flow::CanvasBackground;
+use crate::geom::PhysicalRect;
+use app_units::Au;
+use euclid::default::{Point2D, Rect, Size2D};
+use fxhash::FxHashSet;
+use gfx_traits::print_tree::PrintTree;
+use style::animation::AnimationSetKey;
+use style::dom::OpaqueNode;
+use style::values::computed::Length;
+
+#[derive(Serialize)]
+pub struct FragmentTree {
+    /// Fragments at the top-level of the tree.
+    ///
+    /// If the root element has `display: none`, there are zero fragments.
+    /// Otherwise, there is at least one:
+    ///
+    /// * The first fragment is generated by the root element.
+    /// * There may be additional fragments generated by positioned boxes
+    ///   that have the initial containing block.
+    pub(crate) root_fragments: Vec<ArcRefCell<Fragment>>,
+
+    /// The scrollable overflow rectangle for the entire tree
+    /// https://drafts.csswg.org/css-overflow/#scrollable
+    pub(crate) scrollable_overflow: PhysicalRect<Length>,
+
+    /// The containing block used in the layout of this fragment tree.
+    pub(crate) initial_containing_block: PhysicalRect<Length>,
+
+    /// https://drafts.csswg.org/css-backgrounds/#special-backgrounds
+    #[serde(skip)]
+    pub(crate) canvas_background: CanvasBackground,
+}
+
+impl FragmentTree {
+    pub(crate) fn build_display_list(
+        &self,
+        builder: &mut crate::display_list::DisplayListBuilder,
+        root_stacking_context: &StackingContext,
+    ) {
+        // Paint the canvas’ background (if any) before/under everything else
+        root_stacking_context.build_canvas_background_display_list(
+            builder,
+            self,
+            &self.initial_containing_block,
+        );
+        root_stacking_context.build_display_list(builder);
+    }
+
+    pub fn print(&self) {
+        let mut print_tree = PrintTree::new("Fragment Tree".to_string());
+        for fragment in &self.root_fragments {
+            fragment.borrow().print(&mut print_tree);
+        }
+    }
+
+    pub fn scrollable_overflow(&self) -> webrender_api::units::LayoutSize {
+        webrender_api::units::LayoutSize::from_untyped(Size2D::new(
+            self.scrollable_overflow.size.width.px(),
+            self.scrollable_overflow.size.height.px(),
+        ))
+    }
+
+    pub(crate) fn find<T>(
+        &self,
+        mut process_func: impl FnMut(&Fragment, usize, &PhysicalRect<Length>) -> Option<T>,
+    ) -> Option<T> {
+        let info = ContainingBlockManager {
+            for_non_absolute_descendants: &self.initial_containing_block,
+            for_absolute_descendants: None,
+            for_absolute_and_fixed_descendants: &self.initial_containing_block,
+        };
+        self.root_fragments
+            .iter()
+            .find_map(|child| child.borrow().find(&info, 0, &mut process_func))
+    }
+
+    pub fn remove_nodes_in_fragment_tree_from_set(&self, set: &mut FxHashSet<AnimationSetKey>) {
+        self.find(|fragment, _, _| {
+            let tag = fragment.tag()?;
+            set.remove(&AnimationSetKey::new(tag.node, tag.pseudo));
+            None::<()>
+        });
+    }
+
+    pub fn get_content_box_for_node(&self, requested_node: OpaqueNode) -> Rect<Au> {
+        let mut bounding_box = PhysicalRect::zero();
+        let tag_to_find = Tag::new(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None::<()>;
+            }
+
+            let fragment_relative_rect = match fragment {
+                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment
+                    .border_rect()
+                    .to_physical(fragment.style.writing_mode, &containing_block),
+                Fragment::Text(fragment) => fragment
+                    .rect
+                    .to_physical(fragment.parent_style.writing_mode, &containing_block),
+                Fragment::AbsoluteOrFixedPositioned(_) |
+                Fragment::Image(_) |
+                Fragment::IFrame(_) |
+                Fragment::Anonymous(_) => return None,
+            };
+
+            bounding_box = fragment_relative_rect
+                .translate(containing_block.origin.to_vector())
+                .union(&bounding_box);
+            None::<()>
+        });
+
+        Rect::new(
+            Point2D::new(
+                Au::from_f32_px(bounding_box.origin.x.px()),
+                Au::from_f32_px(bounding_box.origin.y.px()),
+            ),
+            Size2D::new(
+                Au::from_f32_px(bounding_box.size.width.px()),
+                Au::from_f32_px(bounding_box.size.height.px()),
+            ),
+        )
+    }
+
+    pub fn get_border_dimensions_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
+        let tag_to_find = Tag::new(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None;
+            }
+
+            let (style, padding_rect) = match fragment {
+                Fragment::Box(fragment) => (&fragment.style, fragment.padding_rect()),
+                _ => return None,
+            };
+
+            // https://drafts.csswg.org/cssom-view/#dom-element-clienttop
+            // " If the element has no associated CSS layout box or if the
+            //   CSS layout box is inline, return zero." For this check we
+            // also explicitly ignore the list item portion of the display
+            // style.
+            let display = &style.get_box().display;
+            if display.inside() == style::values::specified::box_::DisplayInside::Flow &&
+                display.outside() == style::values::specified::box_::DisplayOutside::Inline
+            {
+                return Some(Rect::zero());
+            }
+
+            let padding_rect = padding_rect.to_physical(style.writing_mode, &containing_block);
+            let border = style.get_border();
+            Some(Rect::new(
+                Point2D::new(
+                    border.border_left_width.px() as i32,
+                    border.border_top_width.px() as i32,
+                ),
+                Size2D::new(
+                    padding_rect.size.width.px() as i32,
+                    padding_rect.size.height.px() as i32,
+                ),
+            ))
+        })
+        .unwrap_or_else(Rect::zero)
+    }
+
+    pub fn get_scroll_area_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
+        let mut scroll_area: PhysicalRect<Length> = PhysicalRect::zero();
+        let tag_to_find = Tag::new(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None::<()>;
+            }
+
+            scroll_area = match fragment {
+                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment
+                    .scrollable_overflow(&containing_block)
+                    .translate(containing_block.origin.to_vector()),
+                Fragment::Text(_) |
+                Fragment::AbsoluteOrFixedPositioned(_) |
+                Fragment::Image(_) |
+                Fragment::IFrame(_) |
+                Fragment::Anonymous(_) => return None,
+            };
+            None::<()>
+        });
+
+        Rect::new(
+            Point2D::new(
+                scroll_area.origin.x.px() as i32,
+                scroll_area.origin.y.px() as i32,
+            ),
+            Size2D::new(
+                scroll_area.size.width.px() as i32,
+                scroll_area.size.height.px() as i32,
+            ),
+        )
+    }
+}

components/layout_2020/fragment_tree/hoisted_shared_fragment.rs

@@ -0,0 +1,68 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use super::Fragment;
+use crate::{cell::ArcRefCell, geom::flow_relative::Vec2};
+use style::values::computed::{Length, LengthPercentage};
+
+/// A reference to a Fragment which is shared between `HoistedAbsolutelyPositionedBox`
+/// and its placeholder `AbsoluteOrFixedPositionedFragment` in the original tree position.
+/// This will be used later in order to paint this hoisted box in tree order.
+#[derive(Serialize)]
+pub(crate) struct HoistedSharedFragment {
+    pub fragment: Option<ArcRefCell<Fragment>>,
+    pub box_offsets: Vec2<AbsoluteBoxOffsets>,
+}
+
+impl HoistedSharedFragment {
+    pub(crate) fn new(box_offsets: Vec2<AbsoluteBoxOffsets>) -> Self {
+        HoistedSharedFragment {
+            fragment: None,
+            box_offsets,
+        }
+    }
+}
+
+impl HoistedSharedFragment {
+    /// In some cases `inset: auto`-positioned elements do not know their precise
+    /// position until after they're hoisted. This lets us adjust auto values
+    /// after the fact.
+    pub(crate) fn adjust_offsets(&mut self, offsets: Vec2<Length>) {
+        self.box_offsets.inline.adjust_offset(offsets.inline);
+        self.box_offsets.block.adjust_offset(offsets.block);
+    }
+}
+
+#[derive(Clone, Debug, Serialize)]
+pub(crate) enum AbsoluteBoxOffsets {
+    StaticStart {
+        start: Length,
+    },
+    Start {
+        start: LengthPercentage,
+    },
+    End {
+        end: LengthPercentage,
+    },
+    Both {
+        start: LengthPercentage,
+        end: LengthPercentage,
+    },
+}
+
+impl AbsoluteBoxOffsets {
+    pub(crate) fn both_specified(&self) -> bool {
+        match self {
+            AbsoluteBoxOffsets::Both { .. } => return true,
+            _ => return false,
+        }
+    }
+
+    pub(crate) fn adjust_offset(&mut self, new_offset: Length) {
+        match *self {
+            AbsoluteBoxOffsets::StaticStart { ref mut start } => *start = new_offset,
+            _ => (),
+        }
+    }
+}

components/layout_2020/fragment_tree/mod.rs

@@ -2,8 +2,16 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-mod base;
+mod base_fragment;
+mod box_fragment;
 mod containing_block;
+mod fragment;
+mod fragment_tree;
+mod hoisted_shared_fragment;
 
-pub(crate) use base::*;
+pub(crate) use base_fragment::*;
+pub(crate) use box_fragment::*;
 pub(crate) use containing_block::*;
+pub(crate) use fragment::*;
+pub use fragment_tree::*;
+pub(crate) use hoisted_shared_fragment::*;