servo/components/layout_2020/flow/inline.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/. */

use std::cell::OnceCell;
use std::mem;
use std::vec::IntoIter;

use app_units::Au;
use atomic_refcell::AtomicRef;
use gfx::font::FontMetrics;
use gfx::text::glyph::GlyphStore;
use gfx::text::text_run::GlyphRun;
use log::warn;
use serde::Serialize;
use servo_arc::Arc;
use style::computed_values::white_space::T as WhiteSpace;
use style::context::QuirksMode;
use style::logical_geometry::WritingMode;
use style::properties::ComputedValues;
use style::values::computed::{Length, LengthPercentage};
use style::values::generics::box_::{GenericVerticalAlign, VerticalAlignKeyword};
use style::values::generics::text::LineHeight;
use style::values::specified::box_::DisplayOutside as StyloDisplayOutside;
use style::values::specified::text::{TextAlignKeyword, TextDecorationLine};
use style::values::specified::{TextAlignLast, TextJustify};
use style::Zero;
use webrender_api::FontInstanceKey;
use xi_unicode::{linebreak_property, LineBreakLeafIter};

use super::float::PlacementAmongFloats;
use super::CollapsibleWithParentStartMargin;
use crate::cell::ArcRefCell;
use crate::context::LayoutContext;
use crate::flow::float::{FloatBox, SequentialLayoutState};
use crate::flow::FlowLayout;
use crate::formatting_contexts::IndependentFormattingContext;
use crate::fragment_tree::{
    AnonymousFragment, BaseFragmentInfo, BoxFragment, CollapsedBlockMargins, CollapsedMargin,
    Fragment, HoistedSharedFragment, TextFragment,
};
use crate::geom::{LogicalRect, LogicalVec2};
use crate::positioned::{
    relative_adjustement, AbsolutelyPositionedBox, PositioningContext, PositioningContextLength,
};
use crate::sizing::ContentSizes;
use crate::style_ext::{
    ComputedValuesExt, Display, DisplayGeneratingBox, DisplayOutside, PaddingBorderMargin,
};
use crate::ContainingBlock;

// These constants are the xi-unicode line breaking classes that are defined in
// `table.rs`. Unfortunately, they are only identified by number.
const XI_LINE_BREAKING_CLASS_GL: u8 = 12;
const XI_LINE_BREAKING_CLASS_WJ: u8 = 30;
const XI_LINE_BREAKING_CLASS_ZWJ: u8 = 40;

// From gfxFontConstants.h in Firefox.
static FONT_SUBSCRIPT_OFFSET_RATIO: f32 = 0.20;
static FONT_SUPERSCRIPT_OFFSET_RATIO: f32 = 0.34;

#[derive(Debug, Serialize)]
pub(crate) struct InlineFormattingContext {
    pub(super) inline_level_boxes: Vec<ArcRefCell<InlineLevelBox>>,
    pub(super) text_decoration_line: TextDecorationLine,
    // Whether this IFC contains the 1st formatted line of an element
    // https://www.w3.org/TR/css-pseudo-4/#first-formatted-line
    pub(super) has_first_formatted_line: bool,
    pub(super) contains_floats: bool,
    /// Whether this IFC being constructed currently ends with whitespace. This is used to
    /// implement rule 4 of <https://www.w3.org/TR/css-text-3/#collapse>:
    ///
    /// > Any collapsible space immediately following another collapsible space—even one
    /// > outside the boundary of the inline containing that space, provided both spaces are
    /// > within the same inline formatting context—is collapsed to have zero advance width.
    /// > (It is invisible, but retains its soft wrap opportunity, if any.)
    pub(super) ends_with_whitespace: bool,
}

#[derive(Debug, Serialize)]
pub(crate) enum InlineLevelBox {
    InlineBox(InlineBox),
    TextRun(TextRun),
    OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
    OutOfFlowFloatBox(FloatBox),
    Atomic(IndependentFormattingContext),
}

#[derive(Debug, Serialize)]
pub(crate) struct InlineBox {
    pub base_fragment_info: BaseFragmentInfo,
    #[serde(skip_serializing)]
    pub style: Arc<ComputedValues>,
    pub is_first_fragment: bool,
    pub is_last_fragment: bool,
    pub children: Vec<ArcRefCell<InlineLevelBox>>,
}

/// https://www.w3.org/TR/css-display-3/#css-text-run
#[derive(Debug, Serialize)]
pub(crate) struct TextRun {
    pub base_fragment_info: BaseFragmentInfo,
    #[serde(skip_serializing)]
    pub parent_style: Arc<ComputedValues>,
    pub text: String,
    pub has_uncollapsible_content: bool,
}

/// Information about the current line under construction for a particular
/// [`InlineFormattingContextState`]. This tracks position and size information while
/// [`LineItem`]s are collected and is used as input when those [`LineItem`]s are
/// converted into [`Fragment`]s during the final phase of line layout. Note that this
/// does not store the [`LineItem`]s themselves, as they are stored as part of the
/// nesting state in the [`InlineFormattingContextState`].
struct LineUnderConstruction {
    /// The position where this line will start once it is laid out. This includes any
    /// offset from `text-indent`.
    start_position: LogicalVec2<Length>,

    /// The current inline position in the line being laid out into [`LineItems`] in this
    /// [`InlineFormattingContext`] independent of the depth in the nesting level.
    inline_position: Length,

    /// The maximum block size of all boxes that ended and are in progress in this line.
    /// This uses [`LineBlockSizes`] instead of a simple value, because the final block size
    /// depends on vertical alignment.
    max_block_size: LineBlockSizes,

    /// Whether any active linebox has added a glyph or atomic element to this line, which
    /// indicates that the next run that exceeds the line length can cause a line break.
    has_content: bool,

    /// Whether or not there are floats that did not fit on the current line. Before
    /// the [`LineItems`] of this line are laid out, these floats will need to be
    /// placed directly below this line, but still as children of this line's Fragments.
    has_floats_waiting_to_be_placed: bool,

    /// A rectangular area (relative to the containing block / inline formatting
    /// context boundaries) where we can fit the line box without overlapping floats.
    /// Note that when this is not empty, its start corner takes precedence over
    /// [`LineUnderConstruction::start_position`].
    placement_among_floats: OnceCell<LogicalRect<Length>>,

    /// The LineItems for the current line under construction that have already
    /// been committed to this line.
    line_items: Vec<LineItem>,

    /// The number of justification opportunities in this line.
    justification_opportunities: usize,
}

impl LineUnderConstruction {
    fn new(start_position: LogicalVec2<Length>) -> Self {
        Self {
            inline_position: start_position.inline.clone(),
            start_position: start_position,
            max_block_size: LineBlockSizes::zero(),
            has_content: false,
            has_floats_waiting_to_be_placed: false,
            placement_among_floats: OnceCell::new(),
            line_items: Vec::new(),
            justification_opportunities: 0,
        }
    }

    fn line_block_start_considering_placement_among_floats(&self) -> Length {
        match self.placement_among_floats.get() {
            Some(placement_among_floats) => placement_among_floats.start_corner.block,
            None => self.start_position.block,
        }
    }

    fn replace_placement_among_floats(&mut self, new_placement: LogicalRect<Length>) {
        self.placement_among_floats.take();
        let _ = self.placement_among_floats.set(new_placement);
    }

    /// Trim the trailing whitespace in this line and return the width of the whitespace trimmed.
    fn trim_trailing_whitespace(&mut self) -> Length {
        // From <https://www.w3.org/TR/css-text-3/#white-space-phase-2>:
        // > 3. A sequence of collapsible spaces at the end of a line is removed,
        // >    as well as any trailing U+1680   OGHAM SPACE MARK whose white-space
        // >    property is normal, nowrap, or pre-line.
        let mut whitespace_trimmed = Length::zero();
        let mut spaces_trimmed = 0;
        for item in self.line_items.iter_mut().rev() {
            if !item.trim_whitespace_at_end(&mut whitespace_trimmed, &mut spaces_trimmed) {
                break;
            }
        }
        self.justification_opportunities -= spaces_trimmed;
        whitespace_trimmed
    }
}

/// A block size relative to a line's final baseline. This is to track the size
/// contribution of a particular element of a line above and below the baseline.
/// These sizes can be combined with other baseline relative sizes before the
/// final baseline position is known. The values here are relative to the
/// overall line's baseline and *not* the nested baseline of an inline box.
#[derive(Clone, Debug)]
struct BaselineRelativeSize {
    /// The ascent above the baseline, where a positive value means a larger
    /// ascent. Thus, the top of this size contribution is `baseline_offset -
    /// ascent`.
    ascent: Au,

    /// The descent below the baseline, where a positive value means a larger
    /// descent. Thus, the bottom of this size contribution is `baseline_offset +
    /// descent`.
    descent: Au,
}

impl BaselineRelativeSize {
    fn zero() -> Self {
        Self {
            ascent: Au::zero(),
            descent: Au::zero(),
        }
    }

    fn max(&self, other: &Self) -> Self {
        BaselineRelativeSize {
            ascent: self.ascent.max(other.ascent),
            descent: self.descent.max(other.descent),
        }
    }

    /// Given an offset from the line's root baseline, adjust this [`BaselineRelativeSize`]
    /// by that offset. This is used to adjust a [`BaselineRelativeSize`] for different kinds
    /// of baseline-relative `vertical-align`. This will "move" measured size of a particular
    /// inline box's block size. For example, in the following HTML:
    ///
    /// ```html
    ///     <div>
    ///         <span style="vertical-align: 5px">child content</span>
    ///     </div>
    /// ````
    ///
    /// If this [`BaselineRelativeSize`] is for the `<span>` then the adjustment
    /// passed here would be equivalent to -5px.
    fn adjust_for_nested_baseline_offset(&mut self, baseline_offset: Au) {
        self.ascent -= baseline_offset;
        self.descent += baseline_offset;
    }
}

#[derive(Clone, Debug)]
struct LineBlockSizes {
    line_height: Length,
    baseline_relative_size_for_line_height: Option<BaselineRelativeSize>,
    size_for_baseline_positioning: BaselineRelativeSize,
}

impl LineBlockSizes {
    fn zero() -> Self {
        LineBlockSizes {
            line_height: Length::zero(),
            baseline_relative_size_for_line_height: None,
            size_for_baseline_positioning: BaselineRelativeSize::zero(),
        }
    }

    fn resolve(&self) -> Length {
        let height_from_ascent_and_descent = self
            .baseline_relative_size_for_line_height
            .as_ref()
            .map(|size| Length::from((size.ascent + size.descent).abs()))
            .unwrap_or_else(Length::zero);
        self.line_height.max(height_from_ascent_and_descent)
    }

    fn max(&self, other: &LineBlockSizes) -> LineBlockSizes {
        let baseline_relative_size = match (
            self.baseline_relative_size_for_line_height.as_ref(),
            other.baseline_relative_size_for_line_height.as_ref(),
        ) {
            (Some(our_size), Some(other_size)) => Some(our_size.max(&other_size)),
            (our_size, other_size) => our_size.or(other_size).cloned(),
        };
        Self {
            line_height: self.line_height.max(other.line_height),
            baseline_relative_size_for_line_height: baseline_relative_size,
            size_for_baseline_positioning: self
                .size_for_baseline_positioning
                .max(&other.size_for_baseline_positioning),
        }
    }

    fn max_assign(&mut self, other: &LineBlockSizes) {
        *self = self.max(other);
    }

    fn adjust_for_baseline_offset(&mut self, baseline_offset: Au) {
        self.baseline_relative_size_for_line_height
            .as_mut()
            .map(|size| size.adjust_for_nested_baseline_offset(baseline_offset));
        self.size_for_baseline_positioning
            .adjust_for_nested_baseline_offset(baseline_offset);
    }

    /// From https://drafts.csswg.org/css2/visudet.html#line-height:
    ///  > The inline-level boxes are aligned vertically according to their 'vertical-align'
    ///  > property. In case they are aligned 'top' or 'bottom', they must be aligned so as
    ///  > to minimize the line box height. If such boxes are tall enough, there are multiple
    ///  > solutions and CSS 2 does not define the position of the line box's baseline (i.e.,
    ///  > the position of the strut, see below).
    fn find_baseline_offset(&self) -> Length {
        match self.baseline_relative_size_for_line_height.as_ref() {
            Some(size) => size.ascent.into(),
            None => {
                // This is the case mentinoned above where there are multiple solutions.
                // This code is putting the baseline roughly in the middle of the line.
                let leading = self.resolve() -
                    (self.size_for_baseline_positioning.ascent +
                        self.size_for_baseline_positioning.descent)
                        .into();
                leading.scale_by(0.5) + self.size_for_baseline_positioning.ascent.into()
            },
        }
    }
}

/// The current unbreakable segment under construction for an inline formatting context.
/// Items accumulate here until we reach a soft line break opportunity during processing
/// of inline content or we reach the end of the formatting context.
struct UnbreakableSegmentUnderConstruction {
    /// The size of this unbreakable segment in both dimension.
    inline_size: Length,

    /// The maximum block size that this segment has. This uses [`LineBlockSizes`] instead of a
    /// simple value, because the final block size depends on vertical alignment.
    max_block_size: LineBlockSizes,

    /// The LineItems for the segment under construction
    line_items: Vec<LineItem>,

    /// The depth in the inline box hierarchy at the start of this segment. This is used
    /// to prefix this segment when it is pushed to a new line.
    inline_box_hierarchy_depth: Option<usize>,

    /// Whether any active linebox has added a glyph or atomic element to this line
    /// segment, which indicates that the next run that exceeds the line length can cause
    /// a line break.
    has_content: bool,

    /// The inline size of any trailing whitespace in this segment.
    trailing_whitespace_size: Length,

    /// The number of justification opportunities in this unbreakable segment.
    justification_opportunities: usize,
}

impl UnbreakableSegmentUnderConstruction {
    fn new() -> Self {
        Self {
            inline_size: Length::zero(),
            max_block_size: LineBlockSizes {
                line_height: Length::zero(),
                baseline_relative_size_for_line_height: None,
                size_for_baseline_positioning: BaselineRelativeSize::zero(),
            },
            line_items: Vec::new(),
            inline_box_hierarchy_depth: None,
            has_content: false,
            trailing_whitespace_size: Length::zero(),
            justification_opportunities: 0,
        }
    }

    /// Reset this segment after its contents have been committed to a line.
    fn reset(&mut self) {
        assert!(self.line_items.is_empty()); // Preserve allocated memory.
        self.inline_size = Length::zero();
        self.max_block_size = LineBlockSizes::zero();
        self.inline_box_hierarchy_depth = None;
        self.has_content = false;
        self.trailing_whitespace_size = Length::zero();
        self.justification_opportunities = 0;
    }

    /// Push a single line item to this segment. In addition, record the inline box
    /// hierarchy depth if this is the first segment. The hierarchy depth is used to
    /// duplicate the necessary `StartInlineBox` tokens if this segment is ultimately
    /// placed on a new empty line.
    fn push_line_item(&mut self, line_item: LineItem, inline_box_hierarchy_depth: usize) {
        if self.line_items.is_empty() {
            self.inline_box_hierarchy_depth = Some(inline_box_hierarchy_depth);
        }
        self.line_items.push(line_item);
    }

    /// Trim whitespace from the beginning of this UnbreakbleSegmentUnderConstruction.
    ///
    /// From <https://www.w3.org/TR/css-text-3/#white-space-phase-2>:
    ///
    /// > Then, the entire block is rendered. Inlines are laid out, taking bidi
    /// > reordering into account, and wrapping as specified by the text-wrap
    /// > property. As each line is laid out,
    /// >  1. A sequence of collapsible spaces at the beginning of a line is removed.
    ///
    /// This prevents whitespace from being added to the beginning of a line.
    fn trim_leading_whitespace(&mut self) {
        let mut whitespace_trimmed = Length::zero();
        let mut spaces_trimmed = 0;
        for item in self.line_items.iter_mut() {
            if !item.trim_whitespace_at_start(&mut whitespace_trimmed, &mut spaces_trimmed) {
                break;
            }
        }
        self.inline_size -= whitespace_trimmed;
        self.justification_opportunities -= spaces_trimmed;
    }

    /// Prepare this segment for placement on a new and empty line. This happens when the
    /// segment is too large to fit on the current line and needs to be placed on a new
    /// one.
    fn prepare_for_placement_on_empty_line(
        &mut self,
        line: &LineUnderConstruction,
        current_hierarchy_depth: usize,
    ) {
        self.trim_leading_whitespace();

        // The segment may start in the middle of an already processed inline box. In that
        // case we need to duplicate the `StartInlineBox` tokens as a prefix of the new
        // lines. For instance if the following segment is going to be placed on a new line:
        //
        // line = [StartInlineBox "every"]
        // segment = ["good" EndInlineBox "boy"]
        //
        // Then the segment must be prefixed with `StartInlineBox` before it is committed
        // to the empty line.
        let mut hierarchy_depth = self
            .inline_box_hierarchy_depth
            .unwrap_or(current_hierarchy_depth);
        if hierarchy_depth == 0 {
            return;
        }
        let mut hierarchy = Vec::new();
        let mut skip_depth = 0;
        for item in line.line_items.iter().rev() {
            match item {
                // We need to skip over any inline boxes that are not in our hierarchy. If
                // any inline box ends, we skip until it starts.
                LineItem::StartInlineBox(_) if skip_depth > 0 => skip_depth -= 1,
                LineItem::EndInlineBox => skip_depth += 1,

                // Otherwise copy the inline box to the hierarchy we are collecting.
                LineItem::StartInlineBox(inline_box) => {
                    let mut cloned_inline_box = inline_box.clone();
                    cloned_inline_box.is_first_fragment = false;
                    hierarchy.push(LineItem::StartInlineBox(cloned_inline_box));
                    hierarchy_depth -= 1;
                    if hierarchy_depth == 0 {
                        break;
                    }
                },
                _ => {},
            }
        }

        let segment_items = mem::take(&mut self.line_items);
        self.line_items = hierarchy
            .into_iter()
            .rev()
            .chain(segment_items.into_iter())
            .collect();
    }
}

struct InlineContainerState {
    /// The style of this inline container.
    style: Arc<ComputedValues>,

    /// Whether or not we have processed any content (an atomic element or text) for
    /// this inline box on the current line OR any previous line.
    has_content: bool,

    /// Indicates whether this nesting level have text decorations in effect.
    /// From https://drafts.csswg.org/css-text-decor/#line-decoration
    // "When specified on or propagated to a block container that establishes
    //  an IFC..."
    text_decoration_line: TextDecorationLine,

    /// The block size contribution of this container's default font ie the size of the
    /// "strut." Whether this is integrated into the [`Self::nested_strut_block_sizes`]
    /// depends on the line-height quirk described in
    /// https://quirks.spec.whatwg.org/#the-line-height-calculation-quirk.
    strut_block_sizes: LineBlockSizes,

    /// The strut block size of this inline container maxed with the strut block
    /// sizes of all inline container ancestors. In quirks mode, this will be
    /// zero, until we know that an element has inline content.
    nested_strut_block_sizes: LineBlockSizes,

    /// The baseline offset of this container from the baseline of the line. The is the
    /// cumulative offset of this container and all of its parents. In contrast to the
    /// `vertical-align` property a positive value indicates an offset "below" the
    /// baseline while a negative value indicates one "above" it (when the block direction
    /// is vertical).
    baseline_offset: Au,

    /// The font metrics of the non-fallback font for this container.
    font_metrics: FontMetrics,
}

struct InlineBoxContainerState {
    /// The container state common to both [`InlineBox`] and the root of the
    /// [`InlineFormattingContext`].
    base: InlineContainerState,

    /// The [`BaseFragmentInfo`] of the [`InlineBox`] that this state tracks.
    base_fragment_info: BaseFragmentInfo,

    /// The [`PaddingBorderMargin`] of the [`InlineBox`] that this state tracks.
    pbm: PaddingBorderMargin,

    /// Whether this is the last fragment of this InlineBox. This may not be the case if
    /// the InlineBox is split due to an block-in-inline-split and this is not the last of
    /// that split.
    is_last_fragment: bool,
}

struct InlineFormattingContextState<'a, 'b> {
    positioning_context: &'a mut PositioningContext,
    containing_block: &'b ContainingBlock<'b>,
    sequential_layout_state: Option<&'a mut SequentialLayoutState>,
    layout_context: &'b LayoutContext<'b>,

    /// The [`InlineContainerState`] for the container formed by the root of the
    /// [`InlineFormattingContext`]. This is effectively the "root inline box" described
    /// by https://drafts.csswg.org/css-inline/#model:
    ///
    /// > The block container also generates a root inline box, which is an anonymous
    /// > inline box that holds all of its inline-level contents. (Thus, all text in an
    /// > inline formatting context is directly contained by an inline box, whether the root
    /// > inline box or one of its descendants.) The root inline box inherits from its
    /// > parent block container, but is otherwise unstyleable.
    root_nesting_level: InlineContainerState,

    /// A stack of [`InlineBoxContainerState`] that is used to produce [`LineItem`]s either when we
    /// reach the end of an inline box or when we reach the end of a line. Only at the end
    /// of the inline box is the state popped from the stack.
    inline_box_state_stack: Vec<InlineBoxContainerState>,

    /// A vector of fragment that are laid out. This includes one [`Fragment::Anonymous`]
    /// per line that is currently laid out plus fragments for all floats, which
    /// are currently laid out at the top-level of each [`InlineFormattingContext`].
    fragments: Vec<Fragment>,

    /// Information about the line currently being laid out into [`LineItems`]s.
    current_line: LineUnderConstruction,

    /// Information about the unbreakable line segment currently being laid out into [`LineItems`]s.
    current_line_segment: UnbreakableSegmentUnderConstruction,

    /// The line breaking state for this inline formatting context.
    linebreaker: Option<LineBreakLeafIter>,

    /// After a forced line break (for instance from a `<br>` element) we wait to actually
    /// break the line until seeing more content. This allows ongoing inline boxes to finish,
    /// since in the case where they have no more content they should not be on the next
    /// line.
    ///
    /// For instance:
    ///
    /// ``` html
    ///    <span style="border-right: 30px solid blue;">
    ///         first line<br>
    ///    </span>
    ///    second line
    /// ```
    ///
    /// In this case, the `<span>` should not extend to the second line. If we linebreak
    /// as soon as we encounter the `<br>` the `<span>`'s ending inline borders would be
    /// placed on the second line, because we add those borders in
    /// [`InlineFormattingContextState::finish_inline_box()`].
    linebreak_before_new_content: bool,

    /// Whether or not a soft wrap opportunity is queued. Soft wrap opportunities are
    /// queued after replaced content and they are processed when the next text content
    /// is encountered.
    have_deferred_soft_wrap_opportunity: bool,

    /// Whether or not a soft wrap opportunity should be prevented before the next atomic
    /// element encountered in the inline formatting context. See
    /// `char_prevents_soft_wrap_opportunity_when_before_or_after_atomic` for more
    /// details.
    prevent_soft_wrap_opportunity_before_next_atomic: bool,

    /// Whether or not this InlineFormattingContext has processed any in flow content at all.
    had_inflow_content: bool,

    /// The currently white-space setting of this line. This is stored on the
    /// [`InlineFormattingContextState`] because when a soft wrap opportunity is defined
    /// by the boundary between two characters, the white-space property of their nearest
    /// common ancestor is used.
    white_space: WhiteSpace,

    /// The offset of the last baseline in the inline formatting context that we
    /// are laying out. This is used to propagate baselines to the ancestors of
    /// `display: inline-block` elements.
    last_baseline_offset: Option<Length>,
}

impl<'a, 'b> InlineFormattingContextState<'a, 'b> {
    fn current_inline_container_state(&self) -> &InlineContainerState {
        match self.inline_box_state_stack.last() {
            Some(inline_box_state) => &inline_box_state.base,
            None => &self.root_nesting_level,
        }
    }

    fn current_inline_container_state_mut(&mut self) -> &mut InlineContainerState {
        match self.inline_box_state_stack.last_mut() {
            Some(inline_box_state) => &mut inline_box_state.base,
            None => &mut self.root_nesting_level,
        }
    }

    fn current_line_max_block_size_including_nested_containers(&self) -> LineBlockSizes {
        self.current_inline_container_state()
            .nested_strut_block_sizes
            .max(&self.current_line.max_block_size)
    }

    fn propagate_current_nesting_level_white_space_style(&mut self) {
        let style = match self.inline_box_state_stack.last() {
            Some(inline_box_state) => &inline_box_state.base.style,
            None => self.containing_block.style,
        };
        self.white_space = style.get_inherited_text().white_space;
    }

    /// Start laying out a particular [`InlineBox`] into line items. This will push
    /// a new [`InlineBoxContainerState`] onto [`Self::inline_box_state_stack`].
    fn start_inline_box(&mut self, inline_box: &InlineBox) {
        let mut inline_box_state = InlineBoxContainerState::new(
            inline_box,
            &self.containing_block,
            self.layout_context,
            self.current_inline_container_state(),
            inline_box.is_last_fragment,
        );

        if inline_box.is_first_fragment {
            self.current_line.inline_position += inline_box_state.pbm.padding.inline_start +
                inline_box_state.pbm.border.inline_start +
                inline_box_state
                    .pbm
                    .margin
                    .inline_start
                    .auto_is(Length::zero);
        }

        let line_item = inline_box_state
            .layout_into_line_item(inline_box.is_first_fragment, inline_box.is_last_fragment);
        self.push_line_item_to_unbreakable_segment(LineItem::StartInlineBox(line_item));
        self.inline_box_state_stack.push(inline_box_state);
    }

    /// Finish laying out a particular [`InlineBox`] into line items. This will add the
    /// final [`InlineBoxLineItem`] to the state and pop its state off of
    /// [`Self::inline_box_state_stack`].
    fn finish_inline_box(&mut self) {
        let inline_box_state = match self.inline_box_state_stack.pop() {
            Some(inline_box_state) => inline_box_state,
            None => return, // We are at the root.
        };

        self.push_line_item_to_unbreakable_segment(LineItem::EndInlineBox);
        self.current_line_segment
            .max_block_size
            .max_assign(&inline_box_state.base.nested_strut_block_sizes);

        // If the inline box that we just finished had any content at all, we want to propagate
        // the `white-space` property of its parent to future inline children. This is because
        // when a soft wrap opportunity is defined by the boundary between two elements, the
        // `white-space` used is that of their nearest common ancestor.
        if inline_box_state.base.has_content {
            self.propagate_current_nesting_level_white_space_style();
        }

        if inline_box_state.is_last_fragment {
            let pbm_end = inline_box_state.pbm.padding.inline_end +
                inline_box_state.pbm.border.inline_end +
                inline_box_state.pbm.margin.inline_end.auto_is(Length::zero);
            self.current_line_segment.inline_size += pbm_end;
        }
    }

    fn finish_last_line(&mut self) {
        // We are at the end of the IFC, and we need to do a few things to make sure that
        // the current segment is committed and that the final line is finished.
        //
        // A soft wrap opportunity makes it so the current segment is placed on a new line
        // if it doesn't fit on the current line under construction.
        self.process_soft_wrap_opportunity();

        // `process_soft_line_wrap_opportunity` does not commit the segment to a line if
        // there is no line wrapping, so this forces the segment into the current line.
        self.commit_current_segment_to_line();

        // Finally we finish the line itself and convert all of the LineItems into
        // fragments.
        self.finish_current_line_and_reset(true /* last_line_or_forced_line_break */);
    }

    /// Finish layout of all inline boxes for the current line. This will gather all
    /// [`LineItem`]s and turn them into [`Fragment`]s, then reset the
    /// [`InlineFormattingContextState`] preparing it for laying out a new line.
    fn finish_current_line_and_reset(&mut self, last_line_or_forced_line_break: bool) {
        let whitespace_trimmed = self.current_line.trim_trailing_whitespace();
        let (inline_start_position, justification_adjustment) = self
            .calculate_current_line_inline_start_and_justification_adjustment(
                whitespace_trimmed,
                last_line_or_forced_line_break,
            );

        let block_start_position = self
            .current_line
            .line_block_start_considering_placement_among_floats();
        let had_inline_advance =
            self.current_line.inline_position != self.current_line.start_position.inline;

        let effective_block_advance = if self.current_line.has_content ||
            had_inline_advance ||
            self.linebreak_before_new_content
        {
            self.current_line_max_block_size_including_nested_containers()
        } else {
            LineBlockSizes::zero()
        };

        let block_end_position = block_start_position + effective_block_advance.resolve();
        if let Some(sequential_layout_state) = self.sequential_layout_state.as_mut() {
            // This amount includes both the block size of the line and any extra space
            // added to move the line down in order to avoid overlapping floats.
            let increment = block_end_position - self.current_line.start_position.block;
            sequential_layout_state.advance_block_position(increment);
        }

        let mut line_items = std::mem::take(&mut self.current_line.line_items);
        if self.current_line.has_floats_waiting_to_be_placed {
            place_pending_floats(self, &mut line_items);
        }

        // Set up the new line now that we no longer need the old one.
        self.current_line = LineUnderConstruction::new(LogicalVec2 {
            inline: Length::zero(),
            block: block_end_position,
        });

        let baseline_offset = effective_block_advance.find_baseline_offset();

        let mut state = LineItemLayoutState {
            inline_position: inline_start_position,
            parent_offset: LogicalVec2::zero(),
            baseline_offset,
            ifc_containing_block: self.containing_block,
            positioning_context: &mut self.positioning_context,
            justification_adjustment,
            line_metrics: &LineMetrics {
                block_offset: block_start_position,
                block_size: effective_block_advance.resolve(),
                baseline_block_offset: baseline_offset,
            },
        };

        let positioning_context_length = state.positioning_context.len();
        let mut saw_end = false;
        let fragments = layout_line_items(
            &mut line_items.into_iter(),
            self.layout_context,
            &mut state,
            &mut saw_end,
        );

        let line_had_content =
            !fragments.is_empty() || state.positioning_context.len() != positioning_context_length;

        // If the line doesn't have any fragments, we don't need to add a containing fragment for it.
        if !line_had_content {
            return;
        }

        self.last_baseline_offset = Some(baseline_offset + block_start_position);
        let line_rect = LogicalRect {
            // The inline part of this start offset was taken into account when determining
            // the inline start of the line in `calculate_inline_start_for_current_line` so
            // we do not need to include it in the `start_corner` of the line's main Fragment.
            start_corner: LogicalVec2 {
                inline: Length::zero(),
                block: block_start_position,
            },
            size: LogicalVec2 {
                inline: self.containing_block.inline_size,
                block: effective_block_advance.resolve(),
            },
        };

        state
            .positioning_context
            .adjust_static_position_of_hoisted_fragments_with_offset(
                &line_rect.start_corner,
                positioning_context_length,
            );

        self.fragments
            .push(Fragment::Anonymous(AnonymousFragment::new(
                line_rect,
                fragments,
                self.containing_block.style.writing_mode,
            )));
    }

    /// Given the amount of whitespace trimmed from the line and taking into consideration
    /// the `text-align` property, calculate where the line under construction starts in
    /// the inline axis as well as the adjustment needed for every justification opportunity
    /// to account for `text-align: justify`.
    fn calculate_current_line_inline_start_and_justification_adjustment(
        &self,
        whitespace_trimmed: Length,
        last_line_or_forced_line_break: bool,
    ) -> (Length, Length) {
        enum TextAlign {
            Start,
            Center,
            End,
        }
        let style = self.containing_block.style;
        let line_left_is_inline_start = style.writing_mode.line_left_is_inline_start();
        let mut text_align_keyword = style.clone_text_align();

        if last_line_or_forced_line_break {
            text_align_keyword = match style.clone_text_align_last() {
                TextAlignLast::Auto if text_align_keyword == TextAlignKeyword::Justify => {
                    TextAlignKeyword::Start
                },
                TextAlignLast::Auto => text_align_keyword,
                TextAlignLast::Start => TextAlignKeyword::Start,
                TextAlignLast::End => TextAlignKeyword::End,
                TextAlignLast::Left => TextAlignKeyword::Left,
                TextAlignLast::Right => TextAlignKeyword::Right,
                TextAlignLast::Center => TextAlignKeyword::Center,
                TextAlignLast::Justify => TextAlignKeyword::Justify,
            };
        }

        let text_align = match text_align_keyword {
            TextAlignKeyword::Start => TextAlign::Start,
            TextAlignKeyword::Center => TextAlign::Center,
            TextAlignKeyword::End => TextAlign::End,
            TextAlignKeyword::Left => {
                if line_left_is_inline_start {
                    TextAlign::Start
                } else {
                    TextAlign::End
                }
            },
            TextAlignKeyword::Right => {
                if line_left_is_inline_start {
                    TextAlign::End
                } else {
                    TextAlign::Start
                }
            },
            TextAlignKeyword::Justify => TextAlign::Start,
            TextAlignKeyword::ServoCenter |
            TextAlignKeyword::ServoLeft |
            TextAlignKeyword::ServoRight => {
                // TODO: Implement these modes which seem to be used by quirks mode.
                TextAlign::Start
            },
        };

        let (line_start, available_space) = match self.current_line.placement_among_floats.get() {
            Some(placement_among_floats) => (
                placement_among_floats.start_corner.inline,
                placement_among_floats.size.inline,
            ),
            None => (Length::zero(), self.containing_block.inline_size),
        };

        // Properly handling text-indent requires that we do not align the text
        // into the text-indent.
        // See <https://drafts.csswg.org/css-text/#text-indent-property>
        // "This property specifies the indentation applied to lines of inline content in
        // a block. The indent is treated as a margin applied to the start edge of the
        // line box."
        let text_indent = self.current_line.start_position.inline;
        let line_length = self.current_line.inline_position - whitespace_trimmed - text_indent;
        let adjusted_line_start = line_start +
            match text_align {
                TextAlign::Start => text_indent,
                TextAlign::End => (available_space - line_length).max(text_indent),
                TextAlign::Center => (available_space - line_length + text_indent) / 2.,
            };

        // Calculate the justification adjustment. This is simply the remaining space on the line,
        // dividided by the number of justficiation opportunities that we recorded when building
        // the line.
        let num_justification_opportunities = self.current_line.justification_opportunities as f32;
        let text_justify = self.containing_block.style.clone_text_justify();
        let justification_adjustment = match (text_align_keyword, text_justify) {
            // `text-justify: none` should disable text justification.
            // TODO: Handle more `text-justify` values.
            (TextAlignKeyword::Justify, TextJustify::None) => Length::zero(),
            (TextAlignKeyword::Justify, _) if num_justification_opportunities > 0. => {
                (available_space - line_length) / num_justification_opportunities
            },
            _ => Length::zero(),
        };

        (adjusted_line_start, justification_adjustment)
    }

    fn place_float_fragment(&mut self, fragment: &mut BoxFragment) {
        let state = self
            .sequential_layout_state
            .as_mut()
            .expect("Tried to lay out a float with no sequential placement state!");

        let block_offset_from_containining_block_top = state
            .current_block_position_including_margins() -
            state.current_containing_block_offset();
        state.place_float_fragment(
            fragment,
            CollapsedMargin::zero(),
            block_offset_from_containining_block_top,
        );
    }

    /// Place a FloatLineItem. This is done when an unbreakable segment is committed to
    /// the current line. Placement of FloatLineItems might need to be deferred until the
    /// line is complete in the case that floats stop fitting on the current line.
    ///
    /// When placing floats we do not want to take into account any trailing whitespace on
    /// the line, because that whitespace will be trimmed in the case that the line is
    /// broken. Thus this function takes as an argument the new size (without whitespace) of
    /// the line that these floats are joining.
    fn place_float_line_item_for_commit_to_line(
        &mut self,
        float_item: &mut FloatLineItem,
        line_inline_size_without_trailing_whitespace: Length,
    ) {
        let margin_box = float_item
            .fragment
            .border_rect()
            .inflate(&float_item.fragment.margin);
        let inline_size = margin_box.size.inline.max(Length::zero());

        let available_inline_size = match self.current_line.placement_among_floats.get() {
            Some(placement_among_floats) => placement_among_floats.size.inline,
            None => self.containing_block.inline_size,
        } - line_inline_size_without_trailing_whitespace;

        // If this float doesn't fit on the current line or a previous float didn't fit on
        // the current line, we need to place it starting at the next line BUT still as
        // children of this line's hierarchy of inline boxes (for the purposes of properly
        // parenting in their stacking contexts). Once all the line content is gathered we
        // will place them later.
        let has_content = self.current_line.has_content || self.current_line_segment.has_content;
        let fits_on_line = !has_content || inline_size <= available_inline_size;
        let needs_placement_later =
            self.current_line.has_floats_waiting_to_be_placed || !fits_on_line;

        if needs_placement_later {
            self.current_line.has_floats_waiting_to_be_placed = true;
        } else {
            self.place_float_fragment(&mut float_item.fragment);
            float_item.needs_placement = false;
        }

        // We've added a new float to the IFC, but this may have actually changed the
        // position of the current line. In order to determine that we regenerate the
        // placement among floats for the current line, which may adjust its inline
        // start position.
        let new_placement = self.place_line_among_floats(&LogicalVec2 {
            inline: line_inline_size_without_trailing_whitespace,
            block: self.current_line.max_block_size.resolve(),
        });
        self.current_line
            .replace_placement_among_floats(new_placement);
    }

    /// Given a new potential line size for the current line, create a "placement" for that line.
    /// This tells us whether or not the new potential line will fit in the current block position
    /// or need to be moved. In addition, the placement rect determines the inline start and end
    /// of the line if it's used as the final placement among floats.
    fn place_line_among_floats(
        &self,
        potential_line_size: &LogicalVec2<Length>,
    ) -> LogicalRect<Length> {
        let sequential_layout_state = self
            .sequential_layout_state
            .as_ref()
            .expect("Should not have called this function without having floats.");

        let ifc_offset_in_float_container = LogicalVec2 {
            inline: sequential_layout_state
                .floats
                .containing_block_info
                .inline_start,
            block: sequential_layout_state.current_containing_block_offset(),
        };

        let ceiling = self
            .current_line
            .line_block_start_considering_placement_among_floats();
        let mut placement = PlacementAmongFloats::new(
            &sequential_layout_state.floats,
            ceiling + ifc_offset_in_float_container.block,
            potential_line_size.clone(),
            &PaddingBorderMargin::zero(),
        );

        let mut placement_rect = placement.place();
        placement_rect.start_corner = &placement_rect.start_corner - &ifc_offset_in_float_container;
        placement_rect
    }

    /// Returns true if a new potential line size for the current line would require a line
    /// break. This takes into account floats and will also update the "placement among
    /// floats" for this line if the potential line size would not cause a line break.
    /// Thus, calling this method has side effects and should only be done while in the
    /// process of laying out line content that is always going to be committed to this
    /// line or the next.
    fn new_potential_line_size_causes_line_break(
        &mut self,
        potential_line_size: &LogicalVec2<Length>,
    ) -> bool {
        let available_line_space = if self.sequential_layout_state.is_some() {
            self.current_line
                .placement_among_floats
                .get_or_init(|| self.place_line_among_floats(potential_line_size))
                .size
                .clone()
        } else {
            LogicalVec2 {
                inline: self.containing_block.inline_size,
                block: Length::new(f32::INFINITY),
            }
        };

        let inline_would_overflow = potential_line_size.inline > available_line_space.inline;
        let block_would_overflow = potential_line_size.block > available_line_space.block;

        // The first content that is added to a line cannot trigger a line break and
        // the `white-space` propertly can also prevent all line breaking.
        let can_break = self.current_line.has_content;

        // If this is the first content on the line and we already have a float placement,
        // that means that the placement was initialized by a leading float in the IFC.
        // This placement needs to be updated, because the first line content might push
        // the block start of the line downward. If there is no float placement, we want
        // to make one to properly set the block position of the line.
        if !can_break {
            // Even if we cannot break, adding content to this line might change its position.
            // In that case we need to redo our placement among floats.
            if self.sequential_layout_state.is_some() &&
                (inline_would_overflow || block_would_overflow)
            {
                let new_placement = self.place_line_among_floats(potential_line_size);
                self.current_line
                    .replace_placement_among_floats(new_placement);
            }

            return false;
        }

        // If the potential line is larger than the containing block we do not even need to consider
        // floats. We definitely have to do a linebreak.
        if potential_line_size.inline > self.containing_block.inline_size {
            return true;
        }

        // Not fitting in the block space means that our block size has changed and we had a
        // placement among floats that is no longer valid. This same placement might just
        // need to be expanded or perhaps we need to line break.
        if block_would_overflow {
            // If we have a limited block size then we are wedging this line between floats.
            assert!(self.sequential_layout_state.is_some());
            let new_placement = self.place_line_among_floats(potential_line_size);
            if new_placement.start_corner.block !=
                self.current_line
                    .line_block_start_considering_placement_among_floats()
            {
                return true;
            } else {
                self.current_line
                    .replace_placement_among_floats(new_placement);
                return false;
            }
        }

        // Otherwise the new potential line size will require a newline if it fits in the
        // inline space available for this line. This space may be smaller than the
        // containing block if floats shrink the available inline space.
        inline_would_overflow
    }

    fn defer_forced_line_break(&mut self) {
        // If this hard line break happens in the middle of an unbreakable segment, there are two
        // scenarios:
        //    1. The current portion of the unbreakable segment fits on the current line in which
        //       case we commit it.
        //    2. The current portion of the unbreakable segment does not fit in which case we
        //       need to put it on a new line *before* actually triggering the hard line break.
        //
        // `process_soft_wrap_opportunity` handles both of these cases.
        self.process_soft_wrap_opportunity();

        // Defer the actual line break until we've cleared all ending inline boxes.
        self.linebreak_before_new_content = true;

        // We need to ensure that the appropriate space for a linebox is created even if there
        // was no other content on this line. We mark the line as having content (needing a
        // advance) and having at least the height associated with this nesting of inline boxes.
        self.current_line
            .max_block_size
            .max_assign(&self.current_line_max_block_size_including_nested_containers());

        self.had_inflow_content = true;
    }

    fn possibly_flush_deferred_forced_line_break(&mut self) {
        if !self.linebreak_before_new_content {
            return;
        }

        self.commit_current_segment_to_line();
        self.process_line_break(true /* forced_line_break */);
        self.linebreak_before_new_content = false;
    }

    fn push_line_item_to_unbreakable_segment(&mut self, line_item: LineItem) {
        self.current_line_segment
            .push_line_item(line_item, self.inline_box_state_stack.len());
    }

    fn push_glyph_store_to_unbreakable_segment(
        &mut self,
        glyph_store: std::sync::Arc<GlyphStore>,
        base_fragment_info: BaseFragmentInfo,
        parent_style: &Arc<ComputedValues>,
        font_metrics: &FontMetrics,
        font_key: FontInstanceKey,
    ) {
        self.current_line_segment.justification_opportunities +=
            glyph_store.total_word_separators() as usize;

        let inline_advance = Length::from(glyph_store.total_advance());
        let preserve_spaces = parent_style
            .get_inherited_text()
            .white_space
            .preserve_spaces();
        let is_collapsible_whitespace = glyph_store.is_whitespace() && !preserve_spaces;

        // Normally, the strut is incorporated into the nested block size. In quirks mode though
        // if we find any text that isn't collapsed whitespace, we need to incorporate the strut.
        // TODO(mrobinson): This isn't quite right for situations where collapsible white space
        // ultimately does not collapse because it is between two other pieces of content.
        // TODO(mrobinson): When we have font fallback, this should be calculating the
        // block sizes of the fallback font.
        let quirks_mode = self.layout_context.style_context.quirks_mode() != QuirksMode::NoQuirks;
        let strut_size = if quirks_mode && !is_collapsible_whitespace {
            self.current_inline_container_state()
                .strut_block_sizes
                .clone()
        } else {
            LineBlockSizes::zero()
        };
        self.update_unbreakable_segment_for_new_content(
            &strut_size,
            inline_advance,
            is_collapsible_whitespace,
        );

        match self.current_line_segment.line_items.last_mut() {
            Some(LineItem::TextRun(text_run)) => {
                debug_assert!(font_key == text_run.font_key);
                text_run.text.push(glyph_store);
                return;
            },
            _ => {},
        }

        self.push_line_item_to_unbreakable_segment(LineItem::TextRun(TextRunLineItem {
            text: vec![glyph_store],
            base_fragment_info: base_fragment_info.into(),
            parent_style: parent_style.clone(),
            font_metrics: font_metrics.clone(),
            font_key,
            text_decoration_line: self.current_inline_container_state().text_decoration_line,
        }));
    }

    fn update_unbreakable_segment_for_new_content(
        &mut self,
        block_sizes_of_content: &LineBlockSizes,
        inline_size: Length,
        is_collapsible_whitespace: bool,
    ) {
        if !is_collapsible_whitespace {
            self.current_line_segment.trailing_whitespace_size = Length::zero();
            self.current_line_segment.has_content = true;
            self.had_inflow_content = true;
        } else {
            self.current_line_segment.trailing_whitespace_size = inline_size;
        }

        // This may or may not include the size of the strut depending on the quirks mode setting.
        let container_max_block_size = &self
            .current_inline_container_state()
            .nested_strut_block_sizes
            .clone();
        self.current_line_segment
            .max_block_size
            .max_assign(container_max_block_size);
        self.current_line_segment
            .max_block_size
            .max_assign(block_sizes_of_content);

        self.current_line_segment.inline_size += inline_size;

        // Propagate the whitespace setting to the current nesting level.
        let current_nesting_level = self.current_inline_container_state_mut();
        current_nesting_level.has_content = true;
        self.propagate_current_nesting_level_white_space_style();
    }

    fn process_line_break(&mut self, forced_line_break: bool) {
        self.current_line_segment
            .prepare_for_placement_on_empty_line(
                &self.current_line,
                self.inline_box_state_stack.len(),
            );
        self.finish_current_line_and_reset(forced_line_break);
    }

    /// Process a soft wrap opportunity. This will either commit the current unbreakble
    /// segment to the current line, if it fits within the containing block and float
    /// placement boundaries, or do a line break and then commit the segment.
    fn process_soft_wrap_opportunity(&mut self) {
        if self.current_line_segment.line_items.is_empty() {
            return;
        }
        if !self.white_space.allow_wrap() {
            return;
        }

        let potential_line_size = LogicalVec2 {
            inline: self.current_line.inline_position + self.current_line_segment.inline_size -
                self.current_line_segment.trailing_whitespace_size,
            block: self
                .current_line_max_block_size_including_nested_containers()
                .max(&self.current_line_segment.max_block_size)
                .resolve(),
        };

        if self.new_potential_line_size_causes_line_break(&potential_line_size) {
            self.process_line_break(false /* forced_line_break */);
        }
        self.commit_current_segment_to_line();
    }

    /// Commit the current unbrekable segment to the current line. In addition, this will
    /// place all floats in the unbreakable segment and expand the line dimensions.
    fn commit_current_segment_to_line(&mut self) {
        if self.current_line_segment.line_items.is_empty() {
            return;
        }

        if !self.current_line.has_content {
            self.current_line_segment.trim_leading_whitespace();
        }

        self.current_line.inline_position += self.current_line_segment.inline_size;
        self.current_line.max_block_size = self
            .current_line_max_block_size_including_nested_containers()
            .max(&self.current_line_segment.max_block_size);
        self.current_line.justification_opportunities +=
            self.current_line_segment.justification_opportunities;
        let line_inline_size_without_trailing_whitespace =
            self.current_line.inline_position - self.current_line_segment.trailing_whitespace_size;

        // Place all floats in this unbreakable segment.
        let mut segment_items = mem::take(&mut self.current_line_segment.line_items);
        for item in segment_items.iter_mut() {
            match item {
                LineItem::Float(float_item) => {
                    self.place_float_line_item_for_commit_to_line(
                        float_item,
                        line_inline_size_without_trailing_whitespace,
                    );
                },
                _ => {},
            }
        }

        // If the current line was never placed among floats, we need to do that now based on the
        // new size. Calling `new_potential_line_size_causes_line_break()` here triggers the
        // new line to be positioned among floats. This should never ask for a line
        // break because it is the first content on the line.
        if self.current_line.line_items.is_empty() {
            let will_break = self.new_potential_line_size_causes_line_break(&LogicalVec2 {
                inline: line_inline_size_without_trailing_whitespace,
                block: self.current_line_segment.max_block_size.resolve(),
            });
            assert!(!will_break);
        }

        // Try to merge all TextRuns in the line.
        let to_skip = match (
            self.current_line.line_items.last_mut(),
            segment_items.first_mut(),
        ) {
            (
                Some(LineItem::TextRun(last_line_item)),
                Some(LineItem::TextRun(first_segment_item)),
            ) => {
                last_line_item.text.append(&mut first_segment_item.text);
                1
            },
            _ => 0,
        };

        self.current_line
            .line_items
            .extend(segment_items.into_iter().skip(to_skip));
        self.current_line.has_content |= self.current_line_segment.has_content;

        self.current_line_segment.reset();
    }
}

impl InlineFormattingContext {
    pub(super) fn new(
        text_decoration_line: TextDecorationLine,
        has_first_formatted_line: bool,
        ends_with_whitespace: bool,
    ) -> InlineFormattingContext {
        InlineFormattingContext {
            inline_level_boxes: Default::default(),
            text_decoration_line,
            has_first_formatted_line,
            contains_floats: false,
            ends_with_whitespace,
        }
    }

    // This works on an already-constructed `InlineFormattingContext`,
    // Which would have to change if/when
    // `BlockContainer::construct` parallelize their construction.
    pub(super) fn inline_content_sizes(
        &self,
        layout_context: &LayoutContext,
        containing_block_writing_mode: WritingMode,
    ) -> ContentSizes {
        struct Computation<'a> {
            layout_context: &'a LayoutContext<'a>,
            containing_block_writing_mode: WritingMode,
            paragraph: ContentSizes,
            current_line: ContentSizes,
            /// Size for whitepsace pending to be added to this line.
            pending_whitespace: Length,
            /// Whether or not this IFC has seen any non-whitespace content.
            had_non_whitespace_content_yet: bool,
            /// The global linebreaking state.
            linebreaker: Option<LineBreakLeafIter>,
        }
        impl Computation<'_> {
            fn traverse(&mut self, inline_level_boxes: &[ArcRefCell<InlineLevelBox>]) {
                for inline_level_box in inline_level_boxes {
                    match &mut *inline_level_box.borrow_mut() {
                        InlineLevelBox::InlineBox(inline_box) => {
                            let padding =
                                inline_box.style.padding(self.containing_block_writing_mode);
                            let border = inline_box
                                .style
                                .border_width(self.containing_block_writing_mode);
                            let margin =
                                inline_box.style.margin(self.containing_block_writing_mode);
                            macro_rules! add {
                                ($condition: ident, $side: ident) => {
                                    if inline_box.$condition {
                                        // For margins and paddings, a cyclic percentage is resolved against zero
                                        // for determining intrinsic size contributions.
                                        // https://drafts.csswg.org/css-sizing-3/#min-percentage-contribution
                                        let zero = Length::zero();
                                        let mut length = padding.$side.percentage_relative_to(zero) + border.$side;
                                        if let Some(lp) = margin.$side.non_auto() {
                                            length += lp.percentage_relative_to(zero)
                                        }
                                        self.add_length(length);
                                    }
                                };
                            }

                            add!(is_first_fragment, inline_start);
                            self.traverse(&inline_box.children);
                            add!(is_last_fragment, inline_end);
                        },
                        InlineLevelBox::TextRun(text_run) => {
                            let result = text_run
                                .break_and_shape(self.layout_context, &mut self.linebreaker);
                            let BreakAndShapeResult {
                                runs,
                                break_at_start,
                                ..
                            } = match result {
                                Ok(result) => result,
                                Err(_) => return,
                            };

                            if break_at_start {
                                self.line_break_opportunity()
                            }
                            for run in &runs {
                                let advance = Length::from(run.glyph_store.total_advance());

                                if !run.glyph_store.is_whitespace() {
                                    self.had_non_whitespace_content_yet = true;
                                    self.current_line.min_content += advance;
                                    self.current_line.max_content +=
                                        self.pending_whitespace + advance;
                                    self.pending_whitespace = Length::zero();
                                } else {
                                    // If this run is a forced line break, we *must* break the line
                                    // and start measuring from the inline origin once more.
                                    if text_run
                                        .glyph_run_is_whitespace_ending_with_preserved_newline(run)
                                    {
                                        self.had_non_whitespace_content_yet = true;
                                        self.forced_line_break();
                                        self.current_line = ContentSizes::zero();
                                        continue;
                                    }

                                    // Discard any leading whitespace in the IFC. This will always be trimmed.
                                    if !self.had_non_whitespace_content_yet {
                                        continue;
                                    }

                                    // Wait to take into account other whitespace until we see more content.
                                    // Whitespace at the end of the IFC will always be trimmed.
                                    self.line_break_opportunity();
                                    self.pending_whitespace += advance;
                                }
                            }
                        },
                        InlineLevelBox::Atomic(atomic) => {
                            let outer = atomic.outer_inline_content_sizes(
                                self.layout_context,
                                self.containing_block_writing_mode,
                            );

                            self.current_line.min_content +=
                                self.pending_whitespace + outer.min_content;
                            self.current_line.max_content += outer.max_content;
                            self.pending_whitespace = Length::zero();
                            self.had_non_whitespace_content_yet = true;
                        },
                        InlineLevelBox::OutOfFlowFloatBox(_) |
                        InlineLevelBox::OutOfFlowAbsolutelyPositionedBox(_) => {},
                    }
                }
            }

            fn add_length(&mut self, l: Length) {
                self.current_line.min_content += l;
                self.current_line.max_content += l;
            }

            fn line_break_opportunity(&mut self) {
                self.paragraph
                    .min_content
                    .max_assign(take(&mut self.current_line.min_content));
            }

            fn forced_line_break(&mut self) {
                self.line_break_opportunity();
                self.paragraph
                    .max_content
                    .max_assign(take(&mut self.current_line.max_content));
            }
        }
        fn take<T: Zero>(x: &mut T) -> T {
            std::mem::replace(x, T::zero())
        }
        let mut computation = Computation {
            layout_context,
            containing_block_writing_mode,
            paragraph: ContentSizes::zero(),
            current_line: ContentSizes::zero(),
            pending_whitespace: Length::zero(),
            had_non_whitespace_content_yet: false,
            linebreaker: None,
        };
        computation.traverse(&self.inline_level_boxes);
        computation.forced_line_break();
        computation.paragraph
    }

    pub(super) fn layout(
        &self,
        layout_context: &LayoutContext,
        positioning_context: &mut PositioningContext,
        containing_block: &ContainingBlock,
        sequential_layout_state: Option<&mut SequentialLayoutState>,
        collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
    ) -> FlowLayout {
        let first_line_inline_start = if self.has_first_formatted_line {
            containing_block
                .style
                .get_inherited_text()
                .text_indent
                .to_used_value(containing_block.inline_size.into())
                .into()
        } else {
            Length::zero()
        };

        let style = containing_block.style;
        let mut ifc = InlineFormattingContextState {
            positioning_context,
            containing_block,
            sequential_layout_state,
            layout_context,
            fragments: Vec::new(),
            current_line: LineUnderConstruction::new(LogicalVec2 {
                inline: first_line_inline_start,
                block: Length::zero(),
            }),
            root_nesting_level: InlineContainerState::new(
                style.to_arc(),
                layout_context,
                None, /* parent_container */
                self.text_decoration_line,
                inline_container_needs_strut(style, layout_context, None),
            ),
            linebreaker: None,
            inline_box_state_stack: Vec::new(),
            current_line_segment: UnbreakableSegmentUnderConstruction::new(),
            linebreak_before_new_content: false,
            have_deferred_soft_wrap_opportunity: false,
            prevent_soft_wrap_opportunity_before_next_atomic: false,
            had_inflow_content: false,
            white_space: containing_block.style.get_inherited_text().white_space,
            last_baseline_offset: None,
        };

        // FIXME(pcwalton): This assumes that margins never collapse through inline formatting
        // contexts (i.e. that inline formatting contexts are never empty). Is that right?
        // FIXME(mrobinson): This should not happen if the IFC collapses through.
        if let Some(ref mut sequential_layout_state) = ifc.sequential_layout_state {
            sequential_layout_state.collapse_margins();
            // FIXME(mrobinson): Collapse margins in the containing block offsets as well??
        }

        let mut iterator = InlineBoxChildIter::from_formatting_context(self);
        let mut parent_iterators = Vec::new();
        loop {
            let next = iterator.next();

            // Any new box should flush a pending hard line break.
            if next.is_some() {
                ifc.possibly_flush_deferred_forced_line_break();
            }

            match next {
                Some(child) => match &mut *child.borrow_mut() {
                    InlineLevelBox::InlineBox(inline_box) => {
                        ifc.start_inline_box(inline_box);
                        parent_iterators.push(iterator);
                        iterator = InlineBoxChildIter::from_inline_level_box(child.clone());
                    },
                    InlineLevelBox::TextRun(run) => {
                        run.layout_into_line_items(layout_context, &mut ifc)
                    },
                    InlineLevelBox::Atomic(atomic_formatting_context) => {
                        atomic_formatting_context.layout_into_line_items(layout_context, &mut ifc);
                    },
                    InlineLevelBox::OutOfFlowAbsolutelyPositionedBox(box_) => ifc
                        .push_line_item_to_unbreakable_segment(LineItem::AbsolutelyPositioned(
                            AbsolutelyPositionedLineItem {
                                absolutely_positioned_box: box_.clone(),
                            },
                        )),
                    InlineLevelBox::OutOfFlowFloatBox(float_box) => {
                        float_box.layout_into_line_items(layout_context, &mut ifc);
                    },
                },
                None => {
                    match parent_iterators.pop() {
                        // If we have a parent iterator, then we are working on an
                        // InlineBox and we just finished it.
                        Some(parent_iterator) => {
                            ifc.finish_inline_box();
                            iterator = parent_iterator;
                            continue;
                        },
                        // If we have no more parents, we are at the end of the root
                        // iterator ie at the end of this InlineFormattingContext.
                        None => break,
                    };
                },
            }
        }

        ifc.finish_last_line();

        let mut collapsible_margins_in_children = CollapsedBlockMargins::zero();
        let content_block_size = ifc.current_line.start_position.block;
        collapsible_margins_in_children.collapsed_through = !ifc.had_inflow_content &&
            content_block_size == Length::zero() &&
            collapsible_with_parent_start_margin.0;

        return FlowLayout {
            fragments: ifc.fragments,
            content_block_size,
            collapsible_margins_in_children,
            last_inflow_baseline_offset: ifc.last_baseline_offset,
        };
    }

    /// Return true if this [InlineFormattingContext] is empty for the purposes of ignoring
    /// during box tree construction. An IFC is empty if it only contains TextRuns with
    /// completely collapsible whitespace. When that happens it can be ignored completely.
    pub fn is_empty(&self) -> bool {
        fn inline_level_boxes_are_empty(boxes: &[ArcRefCell<InlineLevelBox>]) -> bool {
            boxes
                .iter()
                .all(|inline_level_box| inline_level_box_is_empty(&*inline_level_box.borrow()))
        }

        fn inline_level_box_is_empty(inline_level_box: &InlineLevelBox) -> bool {
            match inline_level_box {
                InlineLevelBox::InlineBox(_) => false,
                InlineLevelBox::TextRun(text_run) => !text_run.has_uncollapsible_content,
                InlineLevelBox::OutOfFlowAbsolutelyPositionedBox(_) => false,
                InlineLevelBox::OutOfFlowFloatBox(_) => false,
                InlineLevelBox::Atomic(_) => false,
            }
        }

        inline_level_boxes_are_empty(&self.inline_level_boxes)
    }
}

impl InlineContainerState {
    fn new(
        style: Arc<ComputedValues>,
        layout_context: &LayoutContext,
        parent_container: Option<&InlineContainerState>,
        parent_text_decoration_line: TextDecorationLine,
        create_strut: bool,
    ) -> Self {
        let text_decoration_line = parent_text_decoration_line | style.clone_text_decoration_line();
        let font_metrics = font_metrics_from_style(layout_context, &style);
        let line_height = line_height(&style, &font_metrics);

        let mut baseline_offset = Au::zero();
        let mut strut_block_sizes =
            Self::get_block_sizes_with_style(&style, &font_metrics, line_height);
        if let Some(parent_container) = parent_container {
            // The baseline offset from `vertical-align` might adjust where our block size contribution is
            // within the line.
            baseline_offset = parent_container.get_cumulative_baseline_offset_for_child(
                effective_vertical_for_inline_container(&style),
                &strut_block_sizes,
            );
            strut_block_sizes.adjust_for_baseline_offset(baseline_offset);
        }

        let mut nested_block_sizes = parent_container
            .map(|container| container.nested_strut_block_sizes.clone())
            .unwrap_or_else(LineBlockSizes::zero);
        if create_strut {
            nested_block_sizes.max_assign(&strut_block_sizes);
        }

        Self {
            style,
            has_content: false,
            text_decoration_line,
            nested_strut_block_sizes: nested_block_sizes,
            strut_block_sizes,
            baseline_offset,
            font_metrics,
        }
    }

    fn get_block_sizes_with_style(
        style: &ComputedValues,
        font_metrics: &FontMetrics,
        line_height: Length,
    ) -> LineBlockSizes {
        let vertical_align = effective_vertical_for_inline_container(style);
        if !is_baseline_relative(vertical_align) {
            return LineBlockSizes {
                line_height,
                baseline_relative_size_for_line_height: None,
                size_for_baseline_positioning: BaselineRelativeSize::zero(),
            };
        }

        // From https://drafts.csswg.org/css-inline/#inline-height
        // > If line-height computes to `normal` and either `text-box-edge` is `leading` or this
        // > is the root inline box, the font’s line gap metric may also be incorporated
        // > into A and D by adding half to each side as half-leading.
        //
        // `text-box-edge` isn't implemented (and this is a draft specification), so it's
        // always effectively `leading`, which means we always take into account the line gap
        // when `line-height` is normal.
        let mut ascent = font_metrics.ascent;
        let mut descent = font_metrics.descent;
        if style.get_inherited_text().line_height == LineHeight::Normal {
            let half_leading_from_line_gap =
                (font_metrics.line_gap - descent - ascent).scale_by(0.5);
            ascent += half_leading_from_line_gap;
            descent += half_leading_from_line_gap;
        }

        // The ascent and descent we use for computing the line's final line height isn't
        // the same the ascent and descent we use for finding the baseline. For finding
        // the baseline we want the content rect.
        let size_for_baseline_positioning = BaselineRelativeSize { ascent, descent };

        // From https://drafts.csswg.org/css-inline/#inline-height
        // > When its computed line-height is not normal, its layout bounds are derived solely
        // > from metrics of its first available font (ignoring glyphs from other fonts), and
        // > leading is used to adjust the effective A and D to add up to the used line-height.
        // > Calculate the leading L as L = line-height - (A + D). Half the leading (its
        // > half-leading) is added above A of the first available font, and the other half
        // > below D of the first available font, giving an effective ascent above the baseline
        // > of A′ = A + L/2, and an effective descent of D′ = D + L/2.
        //
        // Note that leading might be negative here and the line-height might be zero. In
        // the case where the height is zero, ascent and descent will move to the same
        // point in the block axis.  Even though the contribution to the line height is
        // zero in this case, the line may get some height when taking them into
        // considering with other zero line height boxes that converge on other block axis
        // locations when using the above formula.
        if style.get_inherited_text().line_height != LineHeight::Normal {
            let half_leading =
                (Au::from_f32_px(line_height.px()) - (ascent + descent)).scale_by(0.5);
            ascent = ascent + half_leading;
            descent = descent + half_leading;
        }

        LineBlockSizes {
            line_height,
            baseline_relative_size_for_line_height: Some(BaselineRelativeSize { ascent, descent }),
            size_for_baseline_positioning,
        }
    }

    fn get_block_size_contribution(&self, font_metrics: &FontMetrics) -> LineBlockSizes {
        Self::get_block_sizes_with_style(
            &self.style,
            font_metrics,
            line_height(&self.style, &font_metrics),
        )
    }

    fn get_cumulative_baseline_offset_for_child(
        &self,
        child_vertical_align: GenericVerticalAlign<LengthPercentage>,
        child_block_size: &LineBlockSizes,
    ) -> Au {
        let block_size = self.get_block_size_contribution(&self.font_metrics);
        self.baseline_offset +
            match child_vertical_align {
                // `top` and `bottom are not actually relative to the baseline, but this value is unused
                // in those cases.
                // TODO: We should distinguish these from `baseline` in order to implement "aligned subtrees" properly.
                // See https://drafts.csswg.org/css2/#aligned-subtree.
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::Baseline) |
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::Top) |
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::Bottom) => Au::zero(),
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::Sub) => Au::from_f32_px(
                    block_size
                        .resolve()
                        .scale_by(FONT_SUBSCRIPT_OFFSET_RATIO)
                        .px(),
                ),
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::Super) => -Au::from_f32_px(
                    block_size
                        .resolve()
                        .scale_by(FONT_SUPERSCRIPT_OFFSET_RATIO)
                        .px(),
                ),
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::TextTop) => {
                    child_block_size.size_for_baseline_positioning.ascent - self.font_metrics.ascent
                },
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::Middle) => {
                    // "Align the vertical midpoint of the box with the baseline of the parent
                    // box plus half the x-height of the parent."
                    (child_block_size.size_for_baseline_positioning.ascent -
                        child_block_size.size_for_baseline_positioning.descent -
                        self.font_metrics.x_height)
                        .scale_by(0.5)
                },
                GenericVerticalAlign::Keyword(VerticalAlignKeyword::TextBottom) => {
                    (self.font_metrics.descent -
                        child_block_size.size_for_baseline_positioning.descent)
                        .into()
                },
                GenericVerticalAlign::Length(length_percentage) => {
                    Au::from_f32_px(-length_percentage.resolve(child_block_size.line_height).px())
                },
            }
    }
}

impl InlineBoxContainerState {
    fn new(
        inline_box: &InlineBox,
        containing_block: &ContainingBlock,
        layout_context: &LayoutContext,
        parent_container: &InlineContainerState,
        is_last_fragment: bool,
    ) -> Self {
        let style = inline_box.style.clone();
        let pbm = style.padding_border_margin(containing_block);
        let create_strut = inline_container_needs_strut(&style, layout_context, Some(&pbm));
        Self {
            base: InlineContainerState::new(
                style,
                layout_context,
                Some(parent_container),
                parent_container.text_decoration_line,
                create_strut,
            ),
            base_fragment_info: inline_box.base_fragment_info,
            pbm,
            is_last_fragment,
        }
    }

    fn layout_into_line_item(
        &mut self,
        is_first_fragment: bool,
        is_last_fragment_of_ib_split: bool,
    ) -> InlineBoxLineItem {
        InlineBoxLineItem {
            base_fragment_info: self.base_fragment_info,
            style: self.base.style.clone(),
            pbm: self.pbm.clone(),
            is_first_fragment,
            is_last_fragment_of_ib_split,
            font_metrics: self.base.font_metrics.clone(),
            baseline_offset: self.base.baseline_offset,
        }
    }
}

impl IndependentFormattingContext {
    fn layout_into_line_items(
        &mut self,
        layout_context: &LayoutContext,
        ifc: &mut InlineFormattingContextState,
    ) {
        let style = self.style();
        let pbm = style.padding_border_margin(&ifc.containing_block);
        let margin = pbm.margin.auto_is(Length::zero);
        let pbm_sums = &(&pbm.padding + &pbm.border) + &margin;
        let mut child_positioning_context = None;

        // We need to know the inline size of the atomic before deciding whether to do the line break.
        let fragment = match self {
            IndependentFormattingContext::Replaced(replaced) => {
                let size = replaced.contents.used_size_as_if_inline_element(
                    ifc.containing_block,
                    &replaced.style,
                    None,
                    &pbm,
                );
                let fragments = replaced
                    .contents
                    .make_fragments(&replaced.style, size.clone());
                let content_rect = LogicalRect {
                    start_corner: pbm_sums.start_offset(),
                    size,
                };

                BoxFragment::new(
                    replaced.base_fragment_info,
                    replaced.style.clone(),
                    fragments,
                    content_rect,
                    pbm.padding,
                    pbm.border,
                    margin,
                    None, /* clearance */
                    None, /* last_inflow_baseline_offset */
                    CollapsedBlockMargins::zero(),
                )
            },
            IndependentFormattingContext::NonReplaced(non_replaced) => {
                let box_size = non_replaced
                    .style
                    .content_box_size(&ifc.containing_block, &pbm);
                let max_box_size = non_replaced
                    .style
                    .content_max_box_size(&ifc.containing_block, &pbm);
                let min_box_size = non_replaced
                    .style
                    .content_min_box_size(&ifc.containing_block, &pbm)
                    .auto_is(Length::zero);

                // https://drafts.csswg.org/css2/visudet.html#inlineblock-width
                let tentative_inline_size = box_size.inline.auto_is(|| {
                    let available_size = ifc.containing_block.inline_size - pbm_sums.inline_sum();
                    non_replaced
                        .inline_content_sizes(layout_context)
                        .shrink_to_fit(available_size)
                });

                // https://drafts.csswg.org/css2/visudet.html#min-max-widths
                // In this case “applying the rules above again” with a non-auto inline-size
                // always results in that size.
                let inline_size = tentative_inline_size
                    .clamp_between_extremums(min_box_size.inline, max_box_size.inline);

                let containing_block_for_children = ContainingBlock {
                    inline_size,
                    block_size: box_size.block,
                    style: &non_replaced.style,
                };
                assert_eq!(
                    ifc.containing_block.style.writing_mode,
                    containing_block_for_children.style.writing_mode,
                    "Mixed writing modes are not supported yet"
                );

                // This always collects for the nearest positioned ancestor even if the parent positioning
                // context doesn't. The thing is we haven't kept track up to this point and there isn't
                // any harm in keeping the hoisted boxes separate.
                child_positioning_context = Some(PositioningContext::new_for_subtree(
                    true, /* collects_for_nearest_positioned_ancestor */
                ));
                let independent_layout = non_replaced.layout(
                    layout_context,
                    child_positioning_context.as_mut().unwrap(),
                    &containing_block_for_children,
                );

                // https://drafts.csswg.org/css2/visudet.html#block-root-margin
                let tentative_block_size = box_size
                    .block
                    .auto_is(|| independent_layout.content_block_size);

                // https://drafts.csswg.org/css2/visudet.html#min-max-heights
                // In this case “applying the rules above again” with a non-auto block-size
                // always results in that size.
                let block_size = tentative_block_size
                    .clamp_between_extremums(min_box_size.block, max_box_size.block);

                let content_rect = LogicalRect {
                    start_corner: pbm_sums.start_offset(),
                    size: LogicalVec2 {
                        block: block_size,
                        inline: inline_size,
                    },
                };

                BoxFragment::new(
                    non_replaced.base_fragment_info,
                    non_replaced.style.clone(),
                    independent_layout.fragments,
                    content_rect,
                    pbm.padding,
                    pbm.border,
                    margin,
                    None,
                    independent_layout.last_inflow_baseline_offset,
                    CollapsedBlockMargins::zero(),
                )
            },
        };

        let soft_wrap_opportunity_prevented = mem::replace(
            &mut ifc.prevent_soft_wrap_opportunity_before_next_atomic,
            false,
        );
        if ifc.white_space.allow_wrap() && !soft_wrap_opportunity_prevented {
            ifc.process_soft_wrap_opportunity();
        }

        let size = &pbm_sums.sum() + &fragment.content_rect.size;
        let baseline_offset = fragment
            .last_baseline_offset
            .map(|baseline_offset| pbm_sums.block_start + baseline_offset)
            .unwrap_or(size.block);
        let baseline_offset = Au::from_f32_px(baseline_offset.px());

        let (block_sizes, baseline_offset_in_parent) =
            self.get_block_sizes_and_baseline_offset(ifc, size.block, baseline_offset);
        ifc.update_unbreakable_segment_for_new_content(&block_sizes, size.inline, false);
        ifc.push_line_item_to_unbreakable_segment(LineItem::Atomic(AtomicLineItem {
            fragment,
            size,
            positioning_context: child_positioning_context,
            baseline_offset_in_parent,
            baseline_offset_in_item: baseline_offset,
        }));

        // Defer a soft wrap opportunity for when we next process text content.
        ifc.have_deferred_soft_wrap_opportunity = true;
    }

    fn get_block_sizes_and_baseline_offset(
        &self,
        ifc: &InlineFormattingContextState,
        block_size: Length,
        baseline_offset_in_content_area: Au,
    ) -> (LineBlockSizes, Au) {
        let mut contribution = if !is_baseline_relative(self.style().clone_vertical_align()) {
            LineBlockSizes {
                line_height: block_size,
                baseline_relative_size_for_line_height: None,
                size_for_baseline_positioning: BaselineRelativeSize::zero(),
            }
        } else {
            let baseline_relative_size = BaselineRelativeSize {
                ascent: baseline_offset_in_content_area,
                descent: Au::from_f32_px(block_size.px()) - baseline_offset_in_content_area,
            };
            LineBlockSizes {
                line_height: block_size,
                baseline_relative_size_for_line_height: Some(baseline_relative_size.clone()),
                size_for_baseline_positioning: baseline_relative_size,
            }
        };

        let baseline_offset = ifc
            .current_inline_container_state()
            .get_cumulative_baseline_offset_for_child(
                self.style().clone_vertical_align(),
                &contribution,
            );
        contribution.adjust_for_baseline_offset(baseline_offset);

        (contribution, baseline_offset)
    }
}

struct BreakAndShapeResult {
    font_metrics: FontMetrics,
    font_key: FontInstanceKey,
    runs: Vec<GlyphRun>,
    break_at_start: bool,
}

impl TextRun {
    fn break_and_shape(
        &self,
        layout_context: &LayoutContext,
        linebreaker: &mut Option<LineBreakLeafIter>,
    ) -> Result<BreakAndShapeResult, &'static str> {
        use gfx::font::ShapingFlags;
        use style::computed_values::text_rendering::T as TextRendering;
        use style::computed_values::word_break::T as WordBreak;

        let font_style = self.parent_style.clone_font();
        let inherited_text_style = self.parent_style.get_inherited_text();
        let letter_spacing = if inherited_text_style.letter_spacing.0.px() != 0. {
            Some(app_units::Au::from(inherited_text_style.letter_spacing.0))
        } else {
            None
        };

        let mut flags = ShapingFlags::empty();
        if letter_spacing.is_some() {
            flags.insert(ShapingFlags::IGNORE_LIGATURES_SHAPING_FLAG);
        }
        if inherited_text_style.text_rendering == TextRendering::Optimizespeed {
            flags.insert(ShapingFlags::IGNORE_LIGATURES_SHAPING_FLAG);
            flags.insert(ShapingFlags::DISABLE_KERNING_SHAPING_FLAG)
        }
        if inherited_text_style.word_break == WordBreak::KeepAll {
            flags.insert(ShapingFlags::KEEP_ALL_FLAG);
        }

        crate::context::with_thread_local_font_context(layout_context, |font_context| {
            let font_group = font_context.font_group(font_style);
            let font = match font_group.borrow_mut().first(font_context) {
                Some(font) => font,
                None => return Err("Could not find find for TextRun."),
            };
            let mut font = font.borrow_mut();

            let word_spacing = &inherited_text_style.word_spacing;
            let word_spacing = word_spacing
                .to_length()
                .map(|l| l.into())
                .unwrap_or_else(|| {
                    let space_width = font
                        .glyph_index(' ')
                        .map(|glyph_id| font.glyph_h_advance(glyph_id))
                        .unwrap_or(gfx::font::LAST_RESORT_GLYPH_ADVANCE);
                    word_spacing.to_used_value(Au::from_f64_px(space_width))
                });

            let shaping_options = gfx::font::ShapingOptions {
                letter_spacing,
                word_spacing,
                script: unicode_script::Script::Common,
                flags,
            };

            let (runs, break_at_start) = gfx::text::text_run::TextRun::break_and_shape(
                &mut font,
                &self.text,
                &shaping_options,
                linebreaker,
            );

            Ok(BreakAndShapeResult {
                font_metrics: font.metrics.clone(),
                font_key: font.font_key,
                runs,
                break_at_start,
            })
        })
    }

    fn glyph_run_is_whitespace_ending_with_preserved_newline(&self, run: &GlyphRun) -> bool {
        if !run.glyph_store.is_whitespace() {
            return false;
        }
        if !self
            .parent_style
            .get_inherited_text()
            .white_space
            .preserve_newlines()
        {
            return false;
        }

        let last_byte = self.text.as_bytes().get(run.range.end().to_usize() - 1);
        last_byte == Some(&b'\n')
    }

    fn layout_into_line_items(
        &self,
        layout_context: &LayoutContext,
        ifc: &mut InlineFormattingContextState,
    ) {
        let result = self.break_and_shape(layout_context, &mut ifc.linebreaker);
        let BreakAndShapeResult {
            font_metrics,
            font_key,
            runs,
            break_at_start,
        } = match result {
            Ok(result) => result,
            Err(string) => {
                warn!("Could not render TextRun: {string}");
                return;
            },
        };

        // We either have a soft wrap opportunity if specified by the breaker or if we are
        // following replaced content.
        let have_deferred_soft_wrap_opportunity =
            mem::replace(&mut ifc.have_deferred_soft_wrap_opportunity, false);
        let mut break_at_start = break_at_start || have_deferred_soft_wrap_opportunity;

        if have_deferred_soft_wrap_opportunity {
            if let Some(first_character) = self.text.chars().nth(0) {
                break_at_start = break_at_start &&
                    !char_prevents_soft_wrap_opportunity_when_before_or_after_atomic(
                        first_character,
                    )
            }
        }

        if let Some(last_character) = self.text.chars().last() {
            ifc.prevent_soft_wrap_opportunity_before_next_atomic =
                char_prevents_soft_wrap_opportunity_when_before_or_after_atomic(last_character);
        }

        for (run_index, run) in runs.into_iter().enumerate() {
            ifc.possibly_flush_deferred_forced_line_break();

            // If this whitespace forces a line break, queue up a hard line break the next time we
            // see any content. We don't line break immediately, because we'd like to finish processing
            // any ongoing inline boxes before ending the line.
            if self.glyph_run_is_whitespace_ending_with_preserved_newline(&run) {
                ifc.defer_forced_line_break();
                continue;
            }

            // Break before each unbrekable run in this TextRun, except the first unless the
            // linebreaker was set to break before the first run.
            if run_index != 0 || break_at_start {
                ifc.process_soft_wrap_opportunity();
            }

            ifc.push_glyph_store_to_unbreakable_segment(
                run.glyph_store,
                self.base_fragment_info,
                &self.parent_style,
                &font_metrics,
                font_key,
            );
        }
    }
}

impl FloatBox {
    fn layout_into_line_items(
        &mut self,
        layout_context: &LayoutContext,
        ifc: &mut InlineFormattingContextState,
    ) {
        let fragment = self.layout(
            layout_context,
            ifc.positioning_context,
            ifc.containing_block,
        );
        ifc.push_line_item_to_unbreakable_segment(LineItem::Float(FloatLineItem {
            fragment,
            needs_placement: true,
        }));
    }
}

enum InlineBoxChildIter<'box_tree> {
    InlineFormattingContext(std::slice::Iter<'box_tree, ArcRefCell<InlineLevelBox>>),
    InlineBox {
        inline_level_box: ArcRefCell<InlineLevelBox>,
        child_index: usize,
    },
}

impl<'box_tree> InlineBoxChildIter<'box_tree> {
    fn from_formatting_context(
        inline_formatting_context: &'box_tree InlineFormattingContext,
    ) -> InlineBoxChildIter<'box_tree> {
        InlineBoxChildIter::InlineFormattingContext(
            inline_formatting_context.inline_level_boxes.iter(),
        )
    }

    fn from_inline_level_box(
        inline_level_box: ArcRefCell<InlineLevelBox>,
    ) -> InlineBoxChildIter<'box_tree> {
        InlineBoxChildIter::InlineBox {
            inline_level_box,
            child_index: 0,
        }
    }
}

impl<'box_tree> Iterator for InlineBoxChildIter<'box_tree> {
    type Item = ArcRefCell<InlineLevelBox>;
    fn next(&mut self) -> Option<ArcRefCell<InlineLevelBox>> {
        match *self {
            InlineBoxChildIter::InlineFormattingContext(ref mut iter) => iter.next().cloned(),
            InlineBoxChildIter::InlineBox {
                ref inline_level_box,
                ref mut child_index,
            } => match *inline_level_box.borrow() {
                InlineLevelBox::InlineBox(ref inline_box) => {
                    if *child_index >= inline_box.children.len() {
                        return None;
                    }

                    let kid = inline_box.children[*child_index].clone();
                    *child_index += 1;
                    Some(kid)
                },
                _ => unreachable!(),
            },
        }
    }
}

struct LineMetrics {
    /// The block offset of the line start in the containing [`InlineFormattingContext`].
    block_offset: Length,

    /// The block size of this line.
    block_size: Length,

    /// The block offset of this line's baseline from [`Self:block_offset`].
    baseline_block_offset: Length,
}

/// State used when laying out the [`LineItem`]s collected for the line currently being
/// laid out.
struct LineItemLayoutState<'a> {
    inline_position: Length,

    /// The offset of the parent, relative to the start position of the line.
    parent_offset: LogicalVec2<Length>,

    /// The block offset of the parent's baseline relative to the block start of the line. This
    /// is often the same as [`Self::block_offset_of_parent`], but can be different for the root
    /// element.
    baseline_offset: Length,

    ifc_containing_block: &'a ContainingBlock<'a>,
    positioning_context: &'a mut PositioningContext,

    /// The amount of space to add to each justification opportunity in order to implement
    /// `text-align: justify`.
    justification_adjustment: Length,

    /// The metrics of this line, which should remain constant throughout the
    /// layout process.
    line_metrics: &'a LineMetrics,
}

fn layout_line_items(
    iterator: &mut IntoIter<LineItem>,
    layout_context: &LayoutContext,
    state: &mut LineItemLayoutState,
    saw_end: &mut bool,
) -> Vec<Fragment> {
    let mut fragments = vec![];
    while let Some(item) = iterator.next() {
        match item {
            LineItem::TextRun(text_line_item) => {
                if let Some(fragment) = text_line_item.layout(state) {
                    fragments.push(Fragment::Text(fragment));
                }
            },
            LineItem::StartInlineBox(box_line_item) => {
                if let Some(fragment) = box_line_item.layout(iterator, layout_context, state) {
                    fragments.push(Fragment::Box(fragment))
                }
            },
            LineItem::EndInlineBox => {
                *saw_end = true;
                break;
            },
            LineItem::Atomic(atomic_line_item) => {
                fragments.push(Fragment::Box(atomic_line_item.layout(state)));
            },
            LineItem::AbsolutelyPositioned(absolute_line_item) => {
                fragments.push(Fragment::AbsoluteOrFixedPositioned(
                    absolute_line_item.layout(state),
                ));
            },
            LineItem::Float(float_line_item) => {
                fragments.push(Fragment::Float(float_line_item.layout(state)));
            },
        }
    }
    fragments
}

fn place_pending_floats(ifc: &mut InlineFormattingContextState, line_items: &mut Vec<LineItem>) {
    for item in line_items.into_iter() {
        match item {
            LineItem::Float(float_line_item) => {
                if float_line_item.needs_placement {
                    ifc.place_float_fragment(&mut float_line_item.fragment);
                }
            },
            _ => {},
        }
    }
}

enum LineItem {
    TextRun(TextRunLineItem),
    StartInlineBox(InlineBoxLineItem),
    EndInlineBox,
    Atomic(AtomicLineItem),
    AbsolutelyPositioned(AbsolutelyPositionedLineItem),
    Float(FloatLineItem),
}

impl LineItem {
    fn trim_whitespace_at_end(
        &mut self,
        whitespace_trimmed: &mut Length,
        spaces_trimmed: &mut usize,
    ) -> bool {
        match self {
            LineItem::TextRun(ref mut item) => {
                item.trim_whitespace_at_end(whitespace_trimmed, spaces_trimmed)
            },
            LineItem::StartInlineBox(_) => true,
            LineItem::EndInlineBox => true,
            LineItem::Atomic(_) => false,
            LineItem::AbsolutelyPositioned(_) => true,
            LineItem::Float(_) => true,
        }
    }

    fn trim_whitespace_at_start(
        &mut self,
        whitespace_trimmed: &mut Length,
        spaces_trimmed: &mut usize,
    ) -> bool {
        match self {
            LineItem::TextRun(ref mut item) => {
                item.trim_whitespace_at_start(whitespace_trimmed, spaces_trimmed)
            },
            LineItem::StartInlineBox(_) => true,
            LineItem::EndInlineBox => true,
            LineItem::Atomic(_) => false,
            LineItem::AbsolutelyPositioned(_) => true,
            LineItem::Float(_) => true,
        }
    }
}

struct TextRunLineItem {
    base_fragment_info: BaseFragmentInfo,
    parent_style: Arc<ComputedValues>,
    text: Vec<std::sync::Arc<GlyphStore>>,
    font_metrics: FontMetrics,
    font_key: FontInstanceKey,
    text_decoration_line: TextDecorationLine,
}

fn line_height(parent_style: &ComputedValues, font_metrics: &FontMetrics) -> Length {
    let font_size = parent_style.get_font().font_size.computed_size();
    match parent_style.get_inherited_text().line_height {
        LineHeight::Normal => Length::from(font_metrics.line_gap),
        LineHeight::Number(number) => font_size * number.0,
        LineHeight::Length(length) => length.0,
    }
}

fn font_metrics_from_style(layout_context: &LayoutContext, style: &ComputedValues) -> FontMetrics {
    crate::context::with_thread_local_font_context(layout_context, |font_context| {
        let font_group = font_context.font_group(style.clone_font());
        let font = match font_group.borrow_mut().first(font_context) {
            Some(font) => font,
            None => {
                warn!("Could not find find for TextRun.");
                return FontMetrics::empty();
            },
        };
        let font = font.borrow();
        font.metrics.clone()
    })
}

impl TextRunLineItem {
    fn trim_whitespace_at_end(
        &mut self,
        whitespace_trimmed: &mut Length,
        spaces_trimmed: &mut usize,
    ) -> bool {
        if self
            .parent_style
            .get_inherited_text()
            .white_space
            .preserve_spaces()
        {
            return false;
        }

        let index_of_last_non_whitespace = self
            .text
            .iter()
            .rev()
            .position(|glyph| !glyph.is_whitespace())
            .map(|offset_from_end| self.text.len() - offset_from_end);

        let first_whitespace_index = index_of_last_non_whitespace.unwrap_or(0);
        *spaces_trimmed += self.text.len() - first_whitespace_index;
        *whitespace_trimmed += self
            .text
            .drain(first_whitespace_index..)
            .map(|glyph| Length::from(glyph.total_advance()))
            .sum();

        // Only keep going if we only encountered whitespace.
        index_of_last_non_whitespace.is_none()
    }

    fn trim_whitespace_at_start(
        &mut self,
        whitespace_trimmed: &mut Length,
        spaces_trimmed: &mut usize,
    ) -> bool {
        if self
            .parent_style
            .get_inherited_text()
            .white_space
            .preserve_spaces()
        {
            return false;
        }

        let index_of_first_non_whitespace = self
            .text
            .iter()
            .position(|glyph| !glyph.is_whitespace())
            .unwrap_or(self.text.len());

        *spaces_trimmed += index_of_first_non_whitespace;
        *whitespace_trimmed += self
            .text
            .drain(0..index_of_first_non_whitespace)
            .map(|glyph| Length::from(glyph.total_advance()))
            .sum();

        // Only keep going if we only encountered whitespace.
        self.text.is_empty()
    }

    fn layout(self, state: &mut LineItemLayoutState) -> Option<TextFragment> {
        if self.text.is_empty() {
            return None;
        }

        let mut number_of_justification_opportunities = 0;
        let mut inline_advance: Length = self
            .text
            .iter()
            .map(|glyph_store| {
                number_of_justification_opportunities += glyph_store.total_word_separators();
                Length::from(glyph_store.total_advance())
            })
            .sum();

        if !state.justification_adjustment.is_zero() {
            inline_advance +=
                state.justification_adjustment * number_of_justification_opportunities as f32;
        }

        // The block start of the TextRun is often zero (meaning it has the same font metrics as the
        // inline box's strut), but for children of the inline formatting context root or for
        // fallback fonts that use baseline relatve alignment, it might be different.
        let mut start_corner = &LogicalVec2 {
            inline: state.inline_position,
            block: state.baseline_offset - self.font_metrics.ascent.into(),
        } - &state.parent_offset;
        if !is_baseline_relative(effective_vertical_for_inline_container(&self.parent_style)) {
            start_corner.block = Length::zero();
        }

        let rect = LogicalRect {
            start_corner,
            size: LogicalVec2 {
                block: self.font_metrics.line_gap.into(),
                inline: inline_advance,
            },
        };

        state.inline_position += inline_advance;
        Some(TextFragment {
            base: self.base_fragment_info.into(),
            parent_style: self.parent_style,
            rect,
            font_metrics: self.font_metrics,
            font_key: self.font_key,
            glyphs: self.text,
            text_decoration_line: self.text_decoration_line,
            justification_adjustment: state.justification_adjustment,
        })
    }
}

#[derive(Clone)]
struct InlineBoxLineItem {
    base_fragment_info: BaseFragmentInfo,
    style: Arc<ComputedValues>,
    pbm: PaddingBorderMargin,

    /// Whether this is the first fragment for this inline box. This means that it's the
    /// first potentially split box of a block-in-inline-split (or only if there's no
    /// split) and also the first appearance of this fragment on any line.
    is_first_fragment: bool,

    /// Whether this is the last fragment for this inline box. This means that it's the
    /// last potentially split box of a block-in-inline-split (or the only fragment if
    /// there's no split).
    is_last_fragment_of_ib_split: bool,

    /// The FontMetrics for the default font used in this inline box.
    font_metrics: FontMetrics,

    /// The block offset of this baseline relative to the baseline of the line. This will be
    /// zero for boxes with `vertical-align: top` and `vertical-align: bottom` since their
    /// baselines are calculated late in layout.
    baseline_offset: Au,
}

impl InlineBoxLineItem {
    fn layout(
        self,
        iterator: &mut IntoIter<LineItem>,
        layout_context: &LayoutContext,
        state: &mut LineItemLayoutState,
    ) -> Option<BoxFragment> {
        let style = self.style.clone();
        let mut padding = self.pbm.padding.clone();
        let mut border = self.pbm.border.clone();
        let mut margin = self.pbm.margin.auto_is(Length::zero);

        if !self.is_first_fragment {
            padding.inline_start = Length::zero();
            border.inline_start = Length::zero();
            margin.inline_start = Length::zero();
        }
        if !self.is_last_fragment_of_ib_split {
            padding.inline_end = Length::zero();
            border.inline_end = Length::zero();
            margin.inline_end = Length::zero();
        }
        let pbm_sums = &(&padding + &border) + &margin;
        state.inline_position += pbm_sums.inline_start;

        let space_above_baseline = self.calculate_space_above_baseline();
        let block_start_offset = self.calculate_block_start(state, space_above_baseline);

        let mut positioning_context = PositioningContext::new_for_style(&style);
        let nested_positioning_context = match positioning_context.as_mut() {
            Some(positioning_context) => positioning_context,
            None => &mut state.positioning_context,
        };
        let original_nested_positioning_context_length = nested_positioning_context.len();

        let mut nested_state = LineItemLayoutState {
            inline_position: state.inline_position,
            parent_offset: LogicalVec2 {
                inline: state.inline_position,
                block: block_start_offset,
            },
            ifc_containing_block: state.ifc_containing_block,
            positioning_context: nested_positioning_context,
            justification_adjustment: state.justification_adjustment,
            line_metrics: state.line_metrics,
            baseline_offset: block_start_offset + space_above_baseline,
        };

        let mut saw_end = false;
        let fragments =
            layout_line_items(iterator, layout_context, &mut nested_state, &mut saw_end);

        // Only add ending padding, border, margin if this is the last fragment of a
        // potential block-in-inline split and this line included the actual end of this
        // fragment (it doesn't continue on the next line).
        if !self.is_last_fragment_of_ib_split || !saw_end {
            padding.inline_end = Length::zero();
            border.inline_end = Length::zero();
            margin.inline_end = Length::zero();
        }
        let pbm_sums = &(&padding + &border) + &margin;

        // If the inline box didn't have any content at all, don't add a Fragment for it.
        let box_has_padding_border_or_margin = pbm_sums.inline_sum() > Length::zero();
        let box_had_absolutes =
            original_nested_positioning_context_length != nested_state.positioning_context.len();
        if !self.is_first_fragment &&
            fragments.is_empty() &&
            !box_has_padding_border_or_margin &&
            !box_had_absolutes
        {
            return None;
        }

        let mut content_rect = LogicalRect {
            start_corner: LogicalVec2 {
                inline: state.inline_position,
                block: block_start_offset,
            },
            size: LogicalVec2 {
                inline: nested_state.inline_position - state.inline_position,
                block: self.font_metrics.line_gap.into(),
            },
        };

        // Make `content_rect` relative to the parent Fragment.
        content_rect.start_corner = &content_rect.start_corner - &state.parent_offset;

        // Relative adjustment should not affect the rest of line layout, so we can
        // do it right before creating the Fragment.
        if style.clone_position().is_relative() {
            content_rect.start_corner += &relative_adjustement(&style, state.ifc_containing_block);
        }

        let mut fragment = BoxFragment::new(
            self.base_fragment_info,
            self.style.clone(),
            fragments,
            content_rect,
            padding,
            border,
            margin,
            None, /* clearance */
            // There is no need to set a baseline offset for this BoxFragment, because
            // the last baseline of this InlineFormattingContext is what will propagate
            // to `display: inline-block` ancestors.
            None, /* last_inflow_baseline_offset */
            CollapsedBlockMargins::zero(),
        );

        state.inline_position = nested_state.inline_position + pbm_sums.inline_end;

        if let Some(mut positioning_context) = positioning_context.take() {
            assert!(original_nested_positioning_context_length == PositioningContextLength::zero());
            positioning_context.layout_collected_children(layout_context, &mut fragment);
            positioning_context.adjust_static_position_of_hoisted_fragments_with_offset(
                &fragment.content_rect.start_corner,
                PositioningContextLength::zero(),
            );
            state.positioning_context.append(positioning_context);
        } else {
            state
                .positioning_context
                .adjust_static_position_of_hoisted_fragments_with_offset(
                    &fragment.content_rect.start_corner,
                    original_nested_positioning_context_length,
                );
        }

        Some(fragment)
    }

    /// Given our font metrics, calculate the space above the baseline we need for our content.
    /// Note that this space does not include space for any content in child inline boxes, as
    /// they are not included in our content rect.
    fn calculate_space_above_baseline(&self) -> Length {
        let (ascent, descent, line_gap) = (
            self.font_metrics.ascent,
            self.font_metrics.descent,
            self.font_metrics.line_gap,
        );
        let leading = line_gap - (ascent + descent);
        (leading.scale_by(0.5) + ascent).into()
    }

    /// Given the state for a line item layout and the space above the baseline for this inline
    /// box, find the block start position relative to the line block start position.
    fn calculate_block_start(
        &self,
        state: &LineItemLayoutState,
        space_above_baseline: Length,
    ) -> Length {
        let vertical_align = effective_vertical_for_inline_container(&self.style);
        let line_gap = self.font_metrics.line_gap;

        // The baseline offset that we have in `Self::baseline_offset` is relative to the line
        // baseline, so we need to make it relative to the line block start.
        match vertical_align {
            GenericVerticalAlign::Keyword(VerticalAlignKeyword::Top) => {
                let line_height = line_height(&self.style, &self.font_metrics);
                (line_height - line_gap.into()).scale_by(0.5)
            },
            GenericVerticalAlign::Keyword(VerticalAlignKeyword::Bottom) => {
                let line_height = line_height(&self.style, &self.font_metrics);
                let half_leading = (line_height - line_gap.into()).scale_by(0.5);
                state.line_metrics.block_size - line_height + half_leading
            },
            _ => {
                state.line_metrics.baseline_block_offset + Length::from(self.baseline_offset) -
                    space_above_baseline
            },
        }
    }
}

struct AtomicLineItem {
    fragment: BoxFragment,
    size: LogicalVec2<Length>,
    positioning_context: Option<PositioningContext>,

    /// The block offset of this items' baseline relative to the baseline of the line.
    /// This will be zero for boxes with `vertical-align: top` and `vertical-align:
    /// bottom` since their baselines are calculated late in layout.
    baseline_offset_in_parent: Au,

    /// The offset of the baseline inside this item.
    baseline_offset_in_item: Au,
}

impl AtomicLineItem {
    fn layout(mut self, state: &mut LineItemLayoutState) -> BoxFragment {
        // The initial `start_corner` of the Fragment is only the PaddingBorderMargin sum start
        // offset, which is the sum of the start component of the padding, border, and margin.
        // This needs to be added to the calculated block and inline positions.
        self.fragment.content_rect.start_corner.inline += state.inline_position;
        self.fragment.content_rect.start_corner.block +=
            self.calculate_block_start(&state.line_metrics);

        // Make the final result relative to the parent box.
        self.fragment.content_rect.start_corner =
            &self.fragment.content_rect.start_corner - &state.parent_offset;

        if self.fragment.style.clone_position().is_relative() {
            self.fragment.content_rect.start_corner +=
                &relative_adjustement(&self.fragment.style, state.ifc_containing_block);
        }

        state.inline_position += self.size.inline;

        if let Some(mut positioning_context) = self.positioning_context {
            positioning_context.adjust_static_position_of_hoisted_fragments_with_offset(
                &self.fragment.content_rect.start_corner,
                PositioningContextLength::zero(),
            );
            state.positioning_context.append(positioning_context);
        }

        self.fragment
    }

    /// Given the metrics for a line, our vertical alignment, and our block size, find a block start
    /// position relative to the top of the line.
    fn calculate_block_start(&self, line_metrics: &LineMetrics) -> Length {
        match self.fragment.style.clone_vertical_align() {
            GenericVerticalAlign::Keyword(VerticalAlignKeyword::Top) => Length::zero(),
            GenericVerticalAlign::Keyword(VerticalAlignKeyword::Bottom) => {
                line_metrics.block_size - self.size.block
            },

            // This covers all baseline-relative vertical alignment.
            _ => {
                let baseline = line_metrics.baseline_block_offset +
                    Length::from(self.baseline_offset_in_parent);
                baseline - Length::from(self.baseline_offset_in_item)
            },
        }
    }
}

struct AbsolutelyPositionedLineItem {
    absolutely_positioned_box: ArcRefCell<AbsolutelyPositionedBox>,
}

impl AbsolutelyPositionedLineItem {
    fn layout(self, state: &mut LineItemLayoutState) -> ArcRefCell<HoistedSharedFragment> {
        let box_ = self.absolutely_positioned_box;
        let style = AtomicRef::map(box_.borrow(), |box_| box_.context.style());
        let initial_start_corner = match Display::from(style.get_box().original_display) {
            Display::GeneratingBox(DisplayGeneratingBox::OutsideInside { outside, inside: _ }) => {
                LogicalVec2 {
                    inline: match outside {
                        DisplayOutside::Inline => {
                            state.inline_position - state.parent_offset.inline
                        },
                        DisplayOutside::Block => Length::zero(),
                    },
                    // The blocks start position of the absolute should be at the top of the line.
                    block: -state.parent_offset.block,
                }
            },
            Display::GeneratingBox(DisplayGeneratingBox::LayoutInternal(_)) => {
                unreachable!(
                    "The result of blockification should never be a layout-internal value."
                );
            },
            Display::Contents => {
                panic!("display:contents does not generate an abspos box")
            },
            Display::None => {
                panic!("display:none does not generate an abspos box")
            },
        };
        let hoisted_box = AbsolutelyPositionedBox::to_hoisted(
            box_.clone(),
            initial_start_corner,
            state.ifc_containing_block,
        );
        let hoisted_fragment = hoisted_box.fragment.clone();
        state.positioning_context.push(hoisted_box);
        hoisted_fragment
    }
}

struct FloatLineItem {
    fragment: BoxFragment,
    /// Whether or not this float Fragment has been placed yet. Fragments that
    /// do not fit on a line need to be placed after the hypothetical block start
    /// of the next line.
    needs_placement: bool,
}

impl FloatLineItem {
    fn layout(mut self, state: &mut LineItemLayoutState<'_>) -> BoxFragment {
        // The `BoxFragment` for this float is positioned relative to the IFC, so we need
        // to move it to be positioned relative to our parent InlineBox line item. Floats
        // fragments are children of these InlineBoxes and not children of the inline
        // formatting context, so that they are parented properly for StackingContext
        // properties such as opacity & filters.
        let distance_from_parent_to_ifc = LogicalVec2 {
            inline: state.parent_offset.inline,
            block: state.line_metrics.block_offset + state.parent_offset.block,
        };
        self.fragment.content_rect.start_corner =
            &self.fragment.content_rect.start_corner - &distance_from_parent_to_ifc;
        self.fragment
    }
}

/// Whether or not this character prevents a soft line wrap opportunity when it
/// comes before or after an atomic inline element.
///
/// From https://www.w3.org/TR/css-text-3/#line-break-details:
///
/// > For Web-compatibility there is a soft wrap opportunity before and after each
/// > replaced element or other atomic inline, even when adjacent to a character that
/// > would normally suppress them, including U+00A0 NO-BREAK SPACE. However, with
/// > the exception of U+00A0 NO-BREAK SPACE, there must be no soft wrap opportunity
/// > between atomic inlines and adjacent characters belonging to the Unicode GL, WJ,
/// > or ZWJ line breaking classes.
fn char_prevents_soft_wrap_opportunity_when_before_or_after_atomic(character: char) -> bool {
    if character == '\u{00A0}' {
        return false;
    }
    let class = linebreak_property(character);
    class == XI_LINE_BREAKING_CLASS_GL ||
        class == XI_LINE_BREAKING_CLASS_WJ ||
        class == XI_LINE_BREAKING_CLASS_ZWJ
}

fn is_baseline_relative(vertical_align: GenericVerticalAlign<LengthPercentage>) -> bool {
    match vertical_align {
        GenericVerticalAlign::Keyword(VerticalAlignKeyword::Top) |
        GenericVerticalAlign::Keyword(VerticalAlignKeyword::Bottom) => false,
        _ => true,
    }
}

fn effective_vertical_for_inline_container(
    style: &ComputedValues,
) -> GenericVerticalAlign<LengthPercentage> {
    match style.clone_display().outside() {
        StyloDisplayOutside::Block => GenericVerticalAlign::Keyword(VerticalAlignKeyword::Baseline),
        _ => style.clone_vertical_align(),
    }
}

/// Whether or not a strut should be created for an inline container. Normally
/// all inline containers get struts. In quirks mode this isn't always the case
/// though.
///
/// From https://quirks.spec.whatwg.org/#the-line-height-calculation-quirk
///
/// > ### § 3.3. The line height calculation quirk
/// > In quirks mode and limited-quirks mode, an inline box that matches the following
/// > conditions, must, for the purpose of line height calculation, act as if the box had a
/// > line-height of zero.
/// >
/// >  - The border-top-width, border-bottom-width, padding-top and padding-bottom
/// >    properties have a used value of zero and the box has a vertical writing mode, or the
/// >    border-right-width, border-left-width, padding-right and padding-left properties have
/// >    a used value of zero and the box has a horizontal writing mode.
/// >  - It either contains no text or it contains only collapsed whitespace.
/// >
/// > ### § 3.4. The blocks ignore line-height quirk
/// > In quirks mode and limited-quirks mode, for a block container element whose content is
/// > composed of inline-level elements, the element’s line-height must be ignored for the
/// > purpose of calculating the minimal height of line boxes within the element.
///
/// Since we incorporate the size of the strut into the line-height calculation when
/// adding text, we can simply not incorporate the strut at the start of inline box
/// processing. This also works the same for the root of the IFC.
fn inline_container_needs_strut(
    style: &ComputedValues,
    layout_context: &LayoutContext,
    pbm: Option<&PaddingBorderMargin>,
) -> bool {
    if layout_context.style_context.quirks_mode() == QuirksMode::NoQuirks {
        return true;
    }

    // This is not in a standard yet, but all browsers disable this quirk for list items.
    // See https://github.com/whatwg/quirks/issues/38.
    if style.get_box().display.is_list_item() {
        return true;
    }

    pbm.map(|pbm| !pbm.padding_border_sums.inline.is_zero())
        .unwrap_or(false)
}