servo/components/layout_2020/flow/mod.rs

/* 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/. */

//! Flow layout, also known as block-and-inline layout.

use std::ops::DerefMut;

use crate::cell::ArcRefCell;
use crate::context::LayoutContext;
use crate::flow::float::{ClearSide, ContainingBlockPositionInfo, FloatBox, SequentialLayoutState};
use crate::flow::inline::InlineFormattingContext;
use crate::formatting_contexts::{
    IndependentFormattingContext, IndependentLayout, NonReplacedFormattingContext,
};
use crate::fragment_tree::{
    BaseFragmentInfo, BoxFragment, CollapsedBlockMargins, CollapsedMargin, Fragment,
};
use crate::geom::flow_relative::{Rect, Sides, Vec2};
use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
use crate::replaced::ReplacedContent;
use crate::sizing::{self, ContentSizes};
use crate::style_ext::{ComputedValuesExt, PaddingBorderMargin};
use crate::ContainingBlock;
use rayon::iter::{IndexedParallelIterator, IntoParallelRefIterator, ParallelIterator};
use rayon_croissant::ParallelIteratorExt;
use servo_arc::Arc;
use style::logical_geometry::WritingMode;
use style::properties::ComputedValues;
use style::values::computed::{Length, LengthOrAuto};
use style::Zero;

mod construct;
pub mod float;
pub mod inline;
mod root;

pub use root::{BoxTree, CanvasBackground};

#[derive(Debug, Serialize)]
pub(crate) struct BlockFormattingContext {
    pub contents: BlockContainer,
    pub contains_floats: bool,
}

#[derive(Debug, Serialize)]
pub(crate) enum BlockContainer {
    BlockLevelBoxes(Vec<ArcRefCell<BlockLevelBox>>),
    InlineFormattingContext(InlineFormattingContext),
}

#[derive(Debug, Serialize)]
pub(crate) enum BlockLevelBox {
    SameFormattingContextBlock {
        base_fragment_info: BaseFragmentInfo,
        #[serde(skip_serializing)]
        style: Arc<ComputedValues>,
        contents: BlockContainer,
    },
    OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
    OutOfFlowFloatBox(FloatBox),
    Independent(IndependentFormattingContext),
}

struct FlowLayout {
    pub fragments: Vec<Fragment>,
    pub content_block_size: Length,
    pub collapsible_margins_in_children: CollapsedBlockMargins,
}

#[derive(Clone, Copy)]
struct CollapsibleWithParentStartMargin(bool);

impl BlockFormattingContext {
    pub(super) fn layout(
        &self,
        layout_context: &LayoutContext,
        positioning_context: &mut PositioningContext,
        containing_block: &ContainingBlock,
        tree_rank: usize,
    ) -> IndependentLayout {
        let mut sequential_layout_state = if self.contains_floats || !layout_context.use_rayon {
            Some(SequentialLayoutState::new())
        } else {
            None
        };

        let flow_layout = self.contents.layout(
            layout_context,
            positioning_context,
            containing_block,
            tree_rank,
            sequential_layout_state.as_mut(),
            CollapsibleWithParentStartMargin(false),
        );
        debug_assert!(
            !flow_layout
                .collapsible_margins_in_children
                .collapsed_through
        );

        IndependentLayout {
            fragments: flow_layout.fragments,
            content_block_size: flow_layout.content_block_size +
                flow_layout.collapsible_margins_in_children.end.solve(),
        }
    }
}

impl BlockContainer {
    fn layout(
        &self,
        layout_context: &LayoutContext,
        positioning_context: &mut PositioningContext,
        containing_block: &ContainingBlock,
        tree_rank: usize,
        sequential_layout_state: Option<&mut SequentialLayoutState>,
        collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
    ) -> FlowLayout {
        match self {
            BlockContainer::BlockLevelBoxes(child_boxes) => layout_block_level_children(
                layout_context,
                positioning_context,
                child_boxes,
                containing_block,
                tree_rank,
                sequential_layout_state,
                collapsible_with_parent_start_margin,
            ),
            BlockContainer::InlineFormattingContext(ifc) => ifc.layout(
                layout_context,
                positioning_context,
                containing_block,
                tree_rank,
                sequential_layout_state,
            ),
        }
    }

    pub(super) fn inline_content_sizes(
        &self,
        layout_context: &LayoutContext,
        writing_mode: WritingMode,
    ) -> ContentSizes {
        match &self {
            Self::BlockLevelBoxes(boxes) if layout_context.use_rayon => boxes
                .par_iter()
                .map(|box_| {
                    box_.borrow_mut()
                        .inline_content_sizes(layout_context, writing_mode)
                })
                .reduce(ContentSizes::zero, ContentSizes::max),
            Self::BlockLevelBoxes(boxes) => boxes
                .iter()
                .map(|box_| {
                    box_.borrow_mut()
                        .inline_content_sizes(layout_context, writing_mode)
                })
                .reduce(ContentSizes::max)
                .unwrap_or_else(ContentSizes::zero),
            Self::InlineFormattingContext(context) => {
                context.inline_content_sizes(layout_context, writing_mode)
            },
        }
    }
}

fn layout_block_level_children(
    layout_context: &LayoutContext,
    positioning_context: &mut PositioningContext,
    child_boxes: &[ArcRefCell<BlockLevelBox>],
    containing_block: &ContainingBlock,
    tree_rank: usize,
    mut sequential_layout_state: Option<&mut SequentialLayoutState>,
    collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
) -> FlowLayout {
    match sequential_layout_state {
        Some(ref mut sequential_layout_state) => layout_block_level_children_sequentially(
            layout_context,
            positioning_context,
            child_boxes,
            containing_block,
            tree_rank,
            sequential_layout_state,
            collapsible_with_parent_start_margin,
        ),
        None => layout_block_level_children_in_parallel(
            layout_context,
            positioning_context,
            child_boxes,
            containing_block,
            tree_rank,
            collapsible_with_parent_start_margin,
        ),
    }
}

fn layout_block_level_children_in_parallel(
    layout_context: &LayoutContext,
    positioning_context: &mut PositioningContext,
    child_boxes: &[ArcRefCell<BlockLevelBox>],
    containing_block: &ContainingBlock,
    tree_rank: usize,
    collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
) -> FlowLayout {
    let mut placement_state = PlacementState::new(collapsible_with_parent_start_margin);

    let fragments = positioning_context.adjust_static_positions(tree_rank, |positioning_context| {
        let collects_for_nearest_positioned_ancestor =
            positioning_context.collects_for_nearest_positioned_ancestor();
        let mut fragments: Vec<Fragment> = child_boxes
            .par_iter()
            .enumerate()
            .mapfold_reduce_into(
                positioning_context,
                |positioning_context, (tree_rank, box_)| {
                    box_.borrow_mut().layout(
                        layout_context,
                        positioning_context,
                        containing_block,
                        tree_rank,
                        /* sequential_layout_state = */ None,
                    )
                },
                || PositioningContext::new_for_rayon(collects_for_nearest_positioned_ancestor),
                PositioningContext::append,
            )
            .collect();
        for fragment in fragments.iter_mut() {
            placement_state.place_fragment(fragment);
        }
        fragments
    });

    FlowLayout {
        fragments,
        content_block_size: placement_state.current_block_direction_position,
        collapsible_margins_in_children: placement_state.collapsible_margins_in_children(),
    }
}

fn layout_block_level_children_sequentially(
    layout_context: &LayoutContext,
    positioning_context: &mut PositioningContext,
    child_boxes: &[ArcRefCell<BlockLevelBox>],
    containing_block: &ContainingBlock,
    tree_rank: usize,
    sequential_layout_state: &mut SequentialLayoutState,
    collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
) -> FlowLayout {
    let mut placement_state = PlacementState::new(collapsible_with_parent_start_margin);

    let fragments = positioning_context.adjust_static_positions(tree_rank, |positioning_context| {
        // Because floats are involved, we do layout for this block formatting context in tree
        // order without parallelism. This enables mutable access to a `SequentialLayoutState` that
        // tracks every float encountered so far (again in tree order).
        child_boxes
            .iter()
            .enumerate()
            .map(|(tree_rank, child_box)| {
                let mut fragment = child_box.borrow_mut().layout(
                    layout_context,
                    positioning_context,
                    containing_block,
                    tree_rank,
                    Some(&mut *sequential_layout_state),
                );

                placement_state.place_fragment(&mut fragment);
                placement_state
                    .adjust_positions_of_float_children(&mut fragment, sequential_layout_state);
                fragment
            })
            .collect()
    });

    FlowLayout {
        fragments,
        content_block_size: placement_state.current_block_direction_position,
        collapsible_margins_in_children: placement_state.collapsible_margins_in_children(),
    }
}

impl BlockLevelBox {
    fn layout(
        &mut self,
        layout_context: &LayoutContext,
        positioning_context: &mut PositioningContext,
        containing_block: &ContainingBlock,
        tree_rank: usize,
        sequential_layout_state: Option<&mut SequentialLayoutState>,
    ) -> Fragment {
        match self {
            BlockLevelBox::SameFormattingContextBlock {
                base_fragment_info: tag,
                style,
                contents,
            } => Fragment::Box(positioning_context.layout_maybe_position_relative_fragment(
                layout_context,
                containing_block,
                style,
                |positioning_context| {
                    layout_in_flow_non_replaced_block_level(
                        layout_context,
                        positioning_context,
                        containing_block,
                        *tag,
                        style,
                        NonReplacedContents::SameFormattingContextBlock(contents),
                        tree_rank,
                        sequential_layout_state,
                    )
                },
            )),
            BlockLevelBox::Independent(independent) => match independent {
                IndependentFormattingContext::Replaced(replaced) => {
                    Fragment::Box(positioning_context.layout_maybe_position_relative_fragment(
                        layout_context,
                        containing_block,
                        &replaced.style,
                        |_positioning_context| {
                            layout_in_flow_replaced_block_level(
                                containing_block,
                                replaced.base_fragment_info,
                                &replaced.style,
                                &replaced.contents,
                                sequential_layout_state,
                            )
                        },
                    ))
                },
                IndependentFormattingContext::NonReplaced(non_replaced) => {
                    Fragment::Box(positioning_context.layout_maybe_position_relative_fragment(
                        layout_context,
                        containing_block,
                        &non_replaced.style,
                        |positioning_context| {
                            layout_in_flow_non_replaced_block_level(
                                layout_context,
                                positioning_context,
                                containing_block,
                                non_replaced.base_fragment_info,
                                &non_replaced.style,
                                NonReplacedContents::EstablishesAnIndependentFormattingContext(
                                    non_replaced,
                                ),
                                tree_rank,
                                sequential_layout_state,
                            )
                        },
                    ))
                },
            },
            BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(box_) => {
                let hoisted_box = AbsolutelyPositionedBox::to_hoisted(
                    box_.clone(),
                    // This is incorrect, however we do not know the
                    // correct positioning until later, in place_block_level_fragment,
                    // and this value will be adjusted there
                    Vec2::zero(),
                    tree_rank,
                    containing_block,
                );
                let hoisted_fragment = hoisted_box.fragment.clone();
                positioning_context.push(hoisted_box);
                Fragment::AbsoluteOrFixedPositioned(hoisted_fragment)
            },
            BlockLevelBox::OutOfFlowFloatBox(box_) => box_.layout(
                layout_context,
                positioning_context,
                containing_block,
                sequential_layout_state,
            ),
        }
    }

    fn inline_content_sizes(
        &mut self,
        layout_context: &LayoutContext,
        containing_block_writing_mode: WritingMode,
    ) -> ContentSizes {
        match self {
            Self::SameFormattingContextBlock {
                style, contents, ..
            } => sizing::outer_inline(style, containing_block_writing_mode, || {
                contents.inline_content_sizes(layout_context, style.writing_mode)
            }),
            Self::Independent(independent) => independent
                .outer_inline_content_sizes(layout_context, containing_block_writing_mode),
            BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(_) => ContentSizes::zero(),
            BlockLevelBox::OutOfFlowFloatBox(_box_) => {
                // TODO: Actually implement that.
                ContentSizes::zero()
            },
        }
    }
}

enum NonReplacedContents<'a> {
    SameFormattingContextBlock(&'a BlockContainer),
    EstablishesAnIndependentFormattingContext(&'a NonReplacedFormattingContext),
}

/// https://drafts.csswg.org/css2/visudet.html#blockwidth
/// https://drafts.csswg.org/css2/visudet.html#normal-block
fn layout_in_flow_non_replaced_block_level(
    layout_context: &LayoutContext,
    positioning_context: &mut PositioningContext,
    containing_block: &ContainingBlock,
    base_fragment_info: BaseFragmentInfo,
    style: &Arc<ComputedValues>,
    block_level_kind: NonReplacedContents,
    tree_rank: usize,
    mut sequential_layout_state: Option<&mut SequentialLayoutState>,
) -> BoxFragment {
    let pbm = style.padding_border_margin(containing_block);
    let box_size = style.content_box_size(containing_block, &pbm);
    let max_box_size = style.content_max_box_size(containing_block, &pbm);
    let min_box_size = style
        .content_min_box_size(containing_block, &pbm)
        .auto_is(Length::zero);

    // https://drafts.csswg.org/css2/visudet.html#min-max-widths
    let solve_inline_margins = |inline_size| {
        solve_inline_margins_for_in_flow_block_level(containing_block, &pbm, inline_size)
    };
    let (mut inline_size, mut inline_margins) =
        if let Some(inline_size) = box_size.inline.non_auto() {
            (inline_size, solve_inline_margins(inline_size))
        } else {
            let margin_inline_start = pbm.margin.inline_start.auto_is(Length::zero);
            let margin_inline_end = pbm.margin.inline_end.auto_is(Length::zero);
            let inline_size = containing_block.inline_size -
                pbm.padding_border_sums.inline -
                margin_inline_start -
                margin_inline_end;
            (inline_size, (margin_inline_start, margin_inline_end))
        };
    if let Some(max_inline_size) = max_box_size.inline {
        if inline_size > max_inline_size {
            inline_size = max_inline_size;
            inline_margins = solve_inline_margins(inline_size);
        }
    }
    if inline_size < min_box_size.inline {
        inline_size = min_box_size.inline;
        inline_margins = solve_inline_margins(inline_size);
    }

    let margin = Sides {
        inline_start: inline_margins.0,
        inline_end: inline_margins.1,
        block_start: pbm.margin.block_start.auto_is(Length::zero),
        block_end: pbm.margin.block_end.auto_is(Length::zero),
    };

    // https://drafts.csswg.org/css2/visudet.html#min-max-heights
    let mut block_size = box_size.block;
    if let LengthOrAuto::LengthPercentage(ref mut block_size) = block_size {
        *block_size = block_size.clamp_between_extremums(min_box_size.block, max_box_size.block);
    }

    let containing_block_for_children = ContainingBlock {
        inline_size,
        block_size,
        style,
    };

    // https://drafts.csswg.org/css-writing-modes/#orthogonal-flows
    assert_eq!(
        containing_block.style.writing_mode, containing_block_for_children.style.writing_mode,
        "Mixed writing modes are not supported yet"
    );

    let block_is_same_formatting_context = match block_level_kind {
        NonReplacedContents::SameFormattingContextBlock(_) => true,
        NonReplacedContents::EstablishesAnIndependentFormattingContext(_) => false,
    };

    let start_margin_can_collapse_with_children = block_is_same_formatting_context &&
        pbm.padding.block_start == Length::zero() &&
        pbm.border.block_start == Length::zero();
    let end_margin_can_collapse_with_children = block_is_same_formatting_context &&
        pbm.padding.block_end == Length::zero() &&
        pbm.border.block_end == Length::zero() &&
        block_size == LengthOrAuto::Auto;

    let mut clearance = Length::zero();
    let parent_containing_block_position_info;
    match sequential_layout_state {
        None => parent_containing_block_position_info = None,
        Some(ref mut sequential_layout_state) => {
            sequential_layout_state.adjoin_assign(&CollapsedMargin::new(margin.block_start));
            if !start_margin_can_collapse_with_children {
                sequential_layout_state.collapse_margins();
            }

            // Introduce clearance if necessary.
            let clear_side = ClearSide::from_style(style);
            clearance = sequential_layout_state.calculate_clearance(clear_side);

            // NB: This will be a no-op if we're collapsing margins with our children since that
            // can only happen if we have no block-start padding and border.
            sequential_layout_state.advance_block_position(
                pbm.padding.block_start + pbm.border.block_start + clearance,
            );

            // We are about to lay out children. Update the offset between the block formatting
            // context and the containing block that we create for them. This offset is used to
            // ajust BFC relative coordinates to coordinates that are relative to our content box.
            // Our content box establishes the containing block for non-abspos children, including
            // floats.
            let inline_start = sequential_layout_state
                .floats
                .containing_block_info
                .inline_start +
                pbm.padding.inline_start +
                pbm.border.inline_start +
                margin.inline_start;
            let new_cb_offsets = ContainingBlockPositionInfo {
                block_start: sequential_layout_state.bfc_relative_block_position,
                block_start_margins_not_collapsed: sequential_layout_state.current_margin,
                inline_start,
                inline_end: inline_start + inline_size,
            };
            parent_containing_block_position_info =
                Some(sequential_layout_state.update_all_containing_block_offsets(new_cb_offsets));
        },
    };

    let mut block_margins_collapsed_with_children = CollapsedBlockMargins::from_margin(&margin);

    let fragments;
    let mut content_block_size;
    match block_level_kind {
        NonReplacedContents::SameFormattingContextBlock(contents) => {
            let flow_layout = contents.layout(
                layout_context,
                positioning_context,
                &containing_block_for_children,
                tree_rank,
                sequential_layout_state.as_mut().map(|x| &mut **x),
                CollapsibleWithParentStartMargin(start_margin_can_collapse_with_children),
            );

            fragments = flow_layout.fragments;
            content_block_size = flow_layout.content_block_size;

            // Update margins.
            let mut collapsible_margins_in_children = flow_layout.collapsible_margins_in_children;
            if start_margin_can_collapse_with_children {
                block_margins_collapsed_with_children
                    .start
                    .adjoin_assign(&collapsible_margins_in_children.start);
                if collapsible_margins_in_children.collapsed_through {
                    block_margins_collapsed_with_children
                        .start
                        .adjoin_assign(&std::mem::replace(
                            &mut collapsible_margins_in_children.end,
                            CollapsedMargin::zero(),
                        ));
                }
            }
            if end_margin_can_collapse_with_children {
                block_margins_collapsed_with_children
                    .end
                    .adjoin_assign(&collapsible_margins_in_children.end);
            } else {
                content_block_size += collapsible_margins_in_children.end.solve();
            }
            block_margins_collapsed_with_children.collapsed_through =
                collapsible_margins_in_children.collapsed_through &&
                    block_is_same_formatting_context &&
                    pbm.padding_border_sums.block == Length::zero() &&
                    block_size.auto_is(|| Length::zero()) == Length::zero() &&
                    min_box_size.block == Length::zero();
        },
        NonReplacedContents::EstablishesAnIndependentFormattingContext(non_replaced) => {
            let independent_layout = non_replaced.layout(
                layout_context,
                positioning_context,
                &containing_block_for_children,
                tree_rank,
            );
            fragments = independent_layout.fragments;
            content_block_size = independent_layout.content_block_size;
        },
    };

    let block_size = block_size.auto_is(|| {
        content_block_size.clamp_between_extremums(min_box_size.block, max_box_size.block)
    });

    if let Some(ref mut sequential_layout_state) = sequential_layout_state {
        // Now that we're done laying out our children, we can restore the
        // parent's containing block position information.
        sequential_layout_state
            .update_all_containing_block_offsets(parent_containing_block_position_info.unwrap());

        // Account for padding and border. We also might have to readjust the
        // `bfc_relative_block_position` if it was different from the content size (i.e. was
        // non-`auto` and/or was affected by min/max block size).
        sequential_layout_state.advance_block_position(
            (block_size - content_block_size) + pbm.padding.block_end + pbm.border.block_end,
        );

        if !end_margin_can_collapse_with_children {
            sequential_layout_state.collapse_margins();
        }
        sequential_layout_state.adjoin_assign(&CollapsedMargin::new(margin.block_end));
    }

    let content_rect = Rect {
        start_corner: Vec2 {
            block: pbm.padding.block_start + pbm.border.block_start + clearance,
            inline: pbm.padding.inline_start + pbm.border.inline_start + margin.inline_start,
        },
        size: Vec2 {
            block: block_size,
            inline: inline_size,
        },
    };

    BoxFragment::new(
        base_fragment_info,
        style.clone(),
        fragments,
        content_rect,
        pbm.padding,
        pbm.border,
        margin,
        clearance,
        block_margins_collapsed_with_children,
    )
}

/// https://drafts.csswg.org/css2/visudet.html#block-replaced-width
/// https://drafts.csswg.org/css2/visudet.html#inline-replaced-width
/// https://drafts.csswg.org/css2/visudet.html#inline-replaced-height
fn layout_in_flow_replaced_block_level<'a>(
    containing_block: &ContainingBlock,
    base_fragment_info: BaseFragmentInfo,
    style: &Arc<ComputedValues>,
    replaced: &ReplacedContent,
    mut sequential_layout_state: Option<&mut SequentialLayoutState>,
) -> BoxFragment {
    let pbm = style.padding_border_margin(containing_block);
    let size = replaced.used_size_as_if_inline_element(containing_block, style, None, &pbm);

    let (margin_inline_start, margin_inline_end) =
        solve_inline_margins_for_in_flow_block_level(containing_block, &pbm, size.inline);
    let margin = Sides {
        inline_start: margin_inline_start,
        inline_end: margin_inline_end,
        block_start: pbm.margin.block_start.auto_is(Length::zero),
        block_end: pbm.margin.block_end.auto_is(Length::zero),
    };
    let fragments = replaced.make_fragments(style, size.clone());

    let mut clearance = Length::zero();
    if let Some(ref mut sequential_layout_state) = sequential_layout_state {
        sequential_layout_state.collapse_margins();
        clearance = sequential_layout_state.calculate_clearance(ClearSide::from_style(style));
        sequential_layout_state
            .advance_block_position(pbm.border.block_sum() + pbm.padding.block_sum() + size.block);
    };

    let content_rect = Rect {
        start_corner: Vec2 {
            block: pbm.padding.block_start + pbm.border.block_start + clearance,
            inline: pbm.padding.inline_start + pbm.border.inline_start + margin.inline_start,
        },
        size,
    };
    let block_margins_collapsed_with_children = CollapsedBlockMargins::from_margin(&margin);

    BoxFragment::new(
        base_fragment_info,
        style.clone(),
        fragments,
        content_rect,
        pbm.padding,
        pbm.border,
        margin,
        Length::zero(),
        block_margins_collapsed_with_children,
    )
}

fn solve_inline_margins_for_in_flow_block_level(
    containing_block: &ContainingBlock,
    pbm: &PaddingBorderMargin,
    inline_size: Length,
) -> (Length, Length) {
    let available = containing_block.inline_size - pbm.padding_border_sums.inline - inline_size;
    match (pbm.margin.inline_start, pbm.margin.inline_end) {
        (LengthOrAuto::Auto, LengthOrAuto::Auto) => (available / 2., available / 2.),
        (LengthOrAuto::Auto, LengthOrAuto::LengthPercentage(end)) => (available - end, end),
        (LengthOrAuto::LengthPercentage(start), _) => (start, available - start),
    }
}

// State that we maintain when placing blocks.
//
// In parallel mode, this placement is done after all child blocks are laid out. In sequential
// mode, this is done right after each block is laid out.
pub(crate) struct PlacementState {
    next_in_flow_margin_collapses_with_parent_start_margin: bool,
    start_margin: CollapsedMargin,
    current_margin: CollapsedMargin,
    current_block_direction_position: Length,
}

impl PlacementState {
    fn new(
        collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
    ) -> PlacementState {
        PlacementState {
            next_in_flow_margin_collapses_with_parent_start_margin:
                collapsible_with_parent_start_margin.0,
            start_margin: CollapsedMargin::zero(),
            current_margin: CollapsedMargin::zero(),
            current_block_direction_position: Length::zero(),
        }
    }

    fn place_fragment(&mut self, fragment: &mut Fragment) {
        match fragment {
            Fragment::Box(fragment) => {
                let fragment_block_margins = &fragment.block_margins_collapsed_with_children;
                let fragment_block_size = fragment.clearance +
                    fragment.padding.block_sum() +
                    fragment.border.block_sum() +
                    fragment.content_rect.size.block;

                if self.next_in_flow_margin_collapses_with_parent_start_margin {
                    debug_assert_eq!(self.current_margin.solve(), Length::zero());
                    self.start_margin
                        .adjoin_assign(&fragment_block_margins.start);
                    if fragment_block_margins.collapsed_through {
                        self.start_margin.adjoin_assign(&fragment_block_margins.end);
                        return;
                    }
                    self.next_in_flow_margin_collapses_with_parent_start_margin = false;
                } else {
                    self.current_margin
                        .adjoin_assign(&fragment_block_margins.start);
                }
                fragment.content_rect.start_corner.block +=
                    self.current_margin.solve() + self.current_block_direction_position;
                if fragment_block_margins.collapsed_through {
                    self.current_margin
                        .adjoin_assign(&fragment_block_margins.end);
                    return;
                }
                self.current_block_direction_position +=
                    self.current_margin.solve() + fragment_block_size;
                self.current_margin = fragment_block_margins.end;
            },
            Fragment::AbsoluteOrFixedPositioned(fragment) => {
                let offset = Vec2 {
                    block: self.current_margin.solve() + self.current_block_direction_position,
                    inline: Length::new(0.),
                };
                fragment.borrow_mut().adjust_offsets(offset);
            },
            Fragment::Anonymous(_) | Fragment::Float(_) => {},
            _ => unreachable!(),
        }
    }

    fn collapsible_margins_in_children(&self) -> CollapsedBlockMargins {
        CollapsedBlockMargins {
            collapsed_through: self.next_in_flow_margin_collapses_with_parent_start_margin,
            start: self.start_margin,
            end: self.current_margin,
        }
    }

    /// When Float fragments are created in block flows, they are positioned
    /// relative to the float containing independent block formatting context.
    /// Once we place a float's containing block, this function can be used to
    /// fix up the float position to be relative to the containing block.
    fn adjust_positions_of_float_children(
        &self,
        fragment: &mut Fragment,
        sequential_layout_state: &mut SequentialLayoutState,
    ) {
        let fragment = match fragment {
            Fragment::Box(ref mut fragment) => fragment,
            _ => return,
        };

        // TODO(mrobinson): Will these margins be accurate if this fragment
        // collapses through. Can a fragment collapse through when it has a
        // non-zero sized float inside? The float won't be positioned correctly
        // anyway (see the comment in `floats.rs` about margin collapse), but
        // this might make the result even worse.
        let collapsed_margins = self.collapsible_margins_in_children().start.adjoin(
            &sequential_layout_state
                .floats
                .containing_block_info
                .block_start_margins_not_collapsed,
        );

        let parent_fragment_offset_in_cb = &fragment.content_rect.start_corner;
        let parent_fragment_offset_in_formatting_context = Vec2 {
            inline: sequential_layout_state
                .floats
                .containing_block_info
                .inline_start +
                parent_fragment_offset_in_cb.inline,
            block: sequential_layout_state
                .floats
                .containing_block_info
                .block_start +
                collapsed_margins.solve() +
                parent_fragment_offset_in_cb.block,
        };

        for child_fragment in fragment.children.iter_mut() {
            if let Fragment::Float(box_fragment) = child_fragment.borrow_mut().deref_mut() {
                box_fragment.content_rect.start_corner = &box_fragment.content_rect.start_corner -
                    &parent_fragment_offset_in_formatting_context;
            }
        }
    }
}