layout/fragment_tree/

fragment.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::ops::Range;
use std::sync::Arc;
use std::sync::atomic::AtomicBool;

use app_units::Au;
use atomic_refcell::AtomicRef;
use euclid::{Point2D, Rect, Size2D};
use fonts::{FontMetrics, ShapedTextSlice};
use layout_api::BoxAreaType;
use malloc_size_of_derive::MallocSizeOf;
use servo_arc::Arc as ServoArc;
use servo_base::id::PipelineId;
use servo_base::print_tree::PrintTree;
use servo_base::text::Utf32CodeUnits;
use servo_url::ServoUrl;
use style::Zero;
use style::properties::ComputedValues;
use style_traits::CSSPixel;
use webrender_api::{FontInstanceKey, ImageKey};

use super::{
    BaseFragment, BoxFragment, ContainingBlockManager, HoistedSharedFragment, PositioningFragment,
    Tag,
};
use crate::SharedStyle;
use crate::cell::{ArcRefCell, RefOrAtomicRef};
use crate::flow::inline::text_run::SharedTextRunData;
use crate::fragment_tree::FragmentStatus;
use crate::geom::{LogicalSides, PhysicalPoint, PhysicalRect};
use crate::layout_impl::LayoutThread;
use crate::style_ext::ComputedValuesExt;

#[derive(Clone, MallocSizeOf)]
pub(crate) enum Fragment {
    LayoutRoot(LayoutRootFragment),
    Box(#[conditional_malloc_size_of] Arc<BoxFragment>),
    /// Floating content. A floated fragment is very similar to a normal
    /// [BoxFragment] but it isn't positioned using normal in block flow
    /// positioning rules (margin collapse, etc). Instead, they are laid
    /// out by the [crate::flow::float::SequentialLayoutState] of their
    /// float containing block formatting context.
    Float(#[conditional_malloc_size_of] Arc<BoxFragment>),
    Positioning(#[conditional_malloc_size_of] Arc<PositioningFragment>),
    /// Absolute and fixed position fragments are hoisted up so that they are children of the
    /// BoxFragment that establishes their containing blocks, so that they can be laid out properly.
    /// When this happens an `AbsoluteOrFixedPositionedPlaceholder` fragment is left at the original
    /// tree position. This allows these hoisted fragments to be painted with regard to their
    /// original tree order during stacking context tree / display list construction.
    AbsoluteOrFixedPositionedPlaceholder(ArcRefCell<HoistedSharedFragment>),
    Text(#[conditional_malloc_size_of] Arc<TextFragment>),
    Image(#[conditional_malloc_size_of] Arc<ImageFragment>),
    IFrame(#[conditional_malloc_size_of] Arc<IFrameFragment>),
}

#[derive(Clone, MallocSizeOf)]
pub(crate) struct CollapsedBlockMargins {
    pub collapsed_through: bool,
    pub start: CollapsedMargin,
    pub end: CollapsedMargin,
}

#[derive(Clone, Copy, Debug, MallocSizeOf)]
pub(crate) struct CollapsedMargin {
    max_positive: Au,
    min_negative: Au,
}

#[derive(Clone, MallocSizeOf)]
pub(crate) struct LayoutRootFragment {
    pub fragment: ArcRefCell<HoistedSharedFragment>,
}

impl LayoutRootFragment {
    pub(crate) fn inner(&self) -> AtomicRef<'_, Fragment> {
        AtomicRef::map(self.fragment.borrow(), |fragment| {
            fragment
                .fragment
                .as_ref()
                .expect("Should never create LayoutRoot without a Fragment")
        })
    }

    pub(crate) fn inner_box_fragment(&self) -> AtomicRef<'_, Arc<BoxFragment>> {
        AtomicRef::map(self.inner(), |fragment| match fragment {
            Fragment::Box(box_fragment) => box_fragment,
            _ => unreachable!("Layout root should always contain box fragment"),
        })
    }
}

#[derive(MallocSizeOf)]
pub(crate) struct TextFragment {
    pub base: BaseFragment,
    #[conditional_malloc_size_of]
    pub run_data: Arc<SharedTextRunData>,
    #[conditional_malloc_size_of]
    pub font_metrics: Arc<FontMetrics>,
    pub font_key: FontInstanceKey,
    #[conditional_malloc_size_of]
    pub glyphs: Vec<Arc<ShapedTextSlice>>,
    /// Extra space to add for each justification opportunity.
    pub justification_adjustment: Au,
    /// The range of characters this [`TextFragment`] represents within the text of its
    /// original DOM node (modified by text transformation).
    pub character_range_in_dom_node: Range<Utf32CodeUnits>,
    /// Whether or not this [`TextFragment`] is an empty fragment added for the
    /// benefit of placing a text cursor on an otherwise empty editable line.
    pub is_empty_for_text_cursor: bool,
}

#[derive(MallocSizeOf)]
pub(crate) struct ImageFragment {
    pub base: BaseFragment,
    pub style: SharedStyle,
    /// The style to use for the selection overlay if this [`ImageFragment`] is
    /// selected.
    pub selected_style: SharedStyle,
    pub clip: PhysicalRect<Au>,
    pub image_key: Option<ImageKey>,
    pub showing_broken_image_icon: bool,
    pub url: Option<ServoUrl>,
    /// The intrinsic (natural) width of the image, if known.
    pub natural_width: Option<Au>,
    /// The intrinsic (natural) height of the image, if known.
    pub natural_height: Option<Au>,
    /// Whether or not this image is selected.
    #[conditional_malloc_size_of]
    pub selected: Arc<AtomicBool>,
}

#[derive(MallocSizeOf)]
pub(crate) struct IFrameFragment {
    pub base: BaseFragment,
    pub style: SharedStyle,
    pub pipeline_id: PipelineId,
}

impl Fragment {
    pub fn base(&self) -> Option<RefOrAtomicRef<'_, BaseFragment>> {
        Some(match self {
            Fragment::LayoutRoot(fragment) => RefOrAtomicRef::AtomicRef(AtomicRef::map(
                fragment.inner_box_fragment(),
                |box_fragment| &box_fragment.base,
            )),
            Fragment::Box(fragment) => RefOrAtomicRef::Ref(&fragment.base),
            Fragment::Text(fragment) => RefOrAtomicRef::Ref(&fragment.base),
            Fragment::AbsoluteOrFixedPositionedPlaceholder(_) => return None,
            Fragment::Positioning(fragment) => RefOrAtomicRef::Ref(&fragment.base),
            Fragment::Image(fragment) => RefOrAtomicRef::Ref(&fragment.base),
            Fragment::IFrame(fragment) => RefOrAtomicRef::Ref(&fragment.base),
            Fragment::Float(fragment) => RefOrAtomicRef::Ref(&fragment.base),
        })
    }

    pub(crate) fn set_containing_block(&self, containing_block: &PhysicalRect<Au>) {
        match self {
            Fragment::LayoutRoot(layout_root_fragment) => layout_root_fragment
                .inner()
                .set_containing_block(containing_block),
            Fragment::Box(box_fragment) | Fragment::Float(box_fragment) => {
                box_fragment.set_containing_block(containing_block)
            },
            Fragment::Positioning(positioning_fragment) => {
                positioning_fragment.set_containing_block(containing_block)
            },
            Fragment::AbsoluteOrFixedPositionedPlaceholder(..) |
            Fragment::Text(..) |
            Fragment::Image(..) |
            Fragment::IFrame(..) => {},
        }
    }

    pub fn tag(&self) -> Option<Tag> {
        self.base().and_then(|base| base.tag)
    }

    pub fn print(&self, tree: &mut PrintTree) {
        match self {
            Fragment::LayoutRoot(layout_root_fragment) => layout_root_fragment.inner().print(tree),
            Fragment::Box(fragment) => fragment.print(tree),
            Fragment::Float(fragment) => {
                tree.new_level("Float".to_string());
                fragment.print(tree);
                tree.end_level();
            },
            Fragment::AbsoluteOrFixedPositionedPlaceholder(_) => {
                tree.add_item("AbsoluteOrFixedPositioned".to_string());
            },
            Fragment::Positioning(fragment) => fragment.print(tree),
            Fragment::Text(fragment) => fragment.print(tree),
            Fragment::Image(fragment) => fragment.print(tree),
            Fragment::IFrame(fragment) => fragment.print(tree),
        }
    }

    pub(crate) fn scrolling_area(&self, layout_thread: &LayoutThread) -> PhysicalRect<Au> {
        self.retrieve_box_fragment().map_or_else(
            || self.scrollable_overflow_for_parent(),
            |box_fragment| {
                box_fragment.offset_by_containing_block(
                    &box_fragment.with_style().scrollable_overflow(),
                    layout_thread.into(),
                )
            },
        )
    }

    /// Clear the scrollable overflow on this [`Fragment`]. This is called during damage
    /// propagation when a fragment is preserved, itself or one of its descendants has
    /// scrollable overflow damage.
    pub(crate) fn clear_scrollable_overflow(&self) {
        match self {
            Fragment::LayoutRoot(fragment) => {
                fragment.inner_box_fragment().clear_scrollable_overflow()
            },
            Fragment::Box(fragment) | Fragment::Float(fragment) => {
                fragment.clear_scrollable_overflow()
            },
            Fragment::Positioning(fragment) => fragment.clear_scrollable_overflow(),
            _ => {},
        }
    }

    pub(crate) fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
        match self {
            Fragment::LayoutRoot(layout_root) => {
                layout_root.inner().scrollable_overflow_for_parent()
            },
            Fragment::Box(fragment) | Fragment::Float(fragment) => {
                fragment.with_style().scrollable_overflow_for_parent()
            },
            Fragment::Positioning(fragment) => fragment.scrollable_overflow_for_parent(),
            Fragment::Text(fragment) => fragment.scrollable_overflow_for_parent(),
            Fragment::AbsoluteOrFixedPositionedPlaceholder(_) |
            Fragment::Image(..) |
            Fragment::IFrame(..) => self.base().map(|base| base.rect()).unwrap_or_default(),
        }
    }

    /// From <https://drafts.csswg.org/css-overflow-3/#scrollable>:
    /// > This padding represents, within the scrollable overflow rectangle, the box’s own padding
    /// > so that when its content is scrolled to its end, there is padding between the edge of its
    /// > in-flow (or floated) content and the border edge of the box. It typically ends up being
    /// > exactly the same size as the box’s own padding, except in a few cases—​such as when an
    /// > out-of-flow positioned element, or the visible overflow of a descendent, has already
    /// > increased the size of the scrollable overflow rectangle outside the conceptual “content
    /// > edge” of the scroll container’s content.
    pub(crate) fn scrollable_overflow_padding_contribution_for_parent(
        &self,
    ) -> Option<PhysicalRect<Au>> {
        match self {
            // TODO: This should consider the box in pre-relative-adjusted position state.
            Fragment::Box(fragment) | Fragment::Float(fragment)
                if !fragment
                    .style()
                    .get_box()
                    .position
                    .is_absolutely_positioned() =>
            {
                Some(fragment.margin_rect())
            },
            // Layout roots and absolutely positioned elements do not affect scrollable overflow
            // for parents.
            Fragment::Box(..) | Fragment::Float(..) | Fragment::LayoutRoot(..) => None,
            // TODO: This rectangle does not include extra size from overflowing inline items. As
            // this measurement is concerned with the actual fragments, it's quite likely that this
            // rectangle should include that.
            Fragment::Positioning(fragment) => Some(fragment.base.rect()),
            Fragment::AbsoluteOrFixedPositionedPlaceholder(_) => None,
            Fragment::Text(..) | Fragment::Image(..) | Fragment::IFrame(..) => {
                Some(self.base()?.rect())
            },
        }
    }

    pub(crate) fn cumulative_box_area_rect(
        &self,
        area: BoxAreaType,
        containing_block_computation: ContainingBlockCalculation<'_>,
    ) -> Option<PhysicalRect<Au>> {
        match self {
            Fragment::LayoutRoot(layout_root_fragment) => layout_root_fragment
                .inner()
                .cumulative_box_area_rect(area, containing_block_computation),
            Fragment::Box(fragment) | Fragment::Float(fragment) => Some(match area {
                BoxAreaType::Content => {
                    fragment.cumulative_content_box_rect(containing_block_computation)
                },
                BoxAreaType::Padding => {
                    fragment.cumulative_padding_box_rect(containing_block_computation)
                },
                BoxAreaType::Border => {
                    fragment.cumulative_border_box_rect(containing_block_computation)
                },
            }),
            Fragment::Positioning(fragment) => {
                Some(fragment.offset_by_containing_block(
                    &fragment.base.rect(),
                    containing_block_computation,
                ))
            },
            Fragment::Text(_) |
            Fragment::AbsoluteOrFixedPositionedPlaceholder(_) |
            Fragment::Image(_) |
            Fragment::IFrame(_) => None,
        }
    }

    pub(crate) fn client_rect(&self) -> Rect<i32, CSSPixel> {
        let Some(fragment) = self.retrieve_box_fragment() else {
            return Rect::zero();
        };
        let fragment = fragment.with_style();

        // https://drafts.csswg.org/cssom-view/#dom-element-clienttop
        // " If the element has no associated CSS layout box or if the
        //   CSS layout box is inline, return zero." For this check we
        // also explicitly ignore the list item portion of the display
        // style.
        if fragment.is_inline_box() {
            return Rect::zero();
        }

        let rect = if fragment.is_table_wrapper() {
            // For tables the border actually belongs to the table grid box,
            // so we need to include it in the dimension of the table wrapper box.
            let mut rect = fragment.border_rect();
            rect.origin = PhysicalPoint::zero();
            rect
        } else {
            let mut rect = fragment.padding_rect();
            rect.origin = PhysicalPoint::new(fragment.border.left, fragment.border.top);
            rect
        };

        let rect = Rect::new(
            Point2D::new(rect.origin.x.to_f32_px(), rect.origin.y.to_f32_px()),
            Size2D::new(rect.size.width.to_f32_px(), rect.size.height.to_f32_px()),
        );
        rect.round().to_i32()
    }

    pub(crate) fn children(&self) -> Option<RefOrAtomicRef<'_, Vec<Fragment>>> {
        match self {
            Fragment::LayoutRoot(fragment) => Some(RefOrAtomicRef::AtomicRef(AtomicRef::map(
                fragment.inner_box_fragment(),
                |fragment| &fragment.children,
            ))),
            Fragment::Box(fragment) | Fragment::Float(fragment) => {
                Some(RefOrAtomicRef::Ref(&fragment.children))
            },
            Fragment::Positioning(fragment) => Some(RefOrAtomicRef::Ref(&fragment.children)),
            _ => None,
        }
    }

    pub(crate) fn find<T>(
        &self,
        manager: &ContainingBlockManager<PhysicalRect<Au>>,
        level: usize,
        process_func: &mut impl FnMut(&Fragment, usize, &PhysicalRect<Au>) -> Option<T>,
    ) -> Option<T> {
        let containing_block = manager.get_containing_block_for_fragment(self);
        if let Some(result) = process_func(self, level, containing_block) {
            return Some(result);
        }

        match self {
            Fragment::LayoutRoot(layout_root_fragment) => {
                layout_root_fragment
                    .inner()
                    .find(manager, level, process_func)
            },
            Fragment::Box(fragment) | Fragment::Float(fragment) => {
                let style = fragment.style();
                let content_rect = fragment
                    .content_rect()
                    .translate(containing_block.origin.to_vector());
                let padding_rect = fragment
                    .padding_rect()
                    .translate(containing_block.origin.to_vector());
                let new_manager = if style
                    .establishes_containing_block_for_all_descendants(fragment.base.flags)
                {
                    manager.new_for_absolute_and_fixed_descendants(&content_rect, &padding_rect)
                } else if style
                    .establishes_containing_block_for_absolute_descendants(fragment.base.flags)
                {
                    manager.new_for_absolute_descendants(&content_rect, &padding_rect)
                } else {
                    manager.new_for_non_absolute_descendants(&content_rect)
                };

                fragment
                    .children
                    .iter()
                    .find_map(|child| child.find(&new_manager, level + 1, process_func))
            },
            Fragment::Positioning(fragment) => {
                let content_rect = fragment
                    .base
                    .rect()
                    .translate(containing_block.origin.to_vector());
                let new_manager = manager.new_for_non_absolute_descendants(&content_rect);
                fragment
                    .children
                    .iter()
                    .find_map(|child| child.find(&new_manager, level + 1, process_func))
            },
            _ => None,
        }
    }

    pub(crate) fn repair_style(&self, new_style: &ServoArc<ComputedValues>) {
        if let Some(base) = self.base() {
            base.set_status(FragmentStatus::StyleChanged);
        }

        let inner_box_fragment;
        let shared_style = match self {
            Fragment::LayoutRoot(fragment) => {
                inner_box_fragment = fragment.inner_box_fragment();
                &inner_box_fragment.style
            },
            Fragment::Box(fragment) => &fragment.style,
            Fragment::Text(fragment) => &fragment.run_data.inline_styles.style,
            Fragment::AbsoluteOrFixedPositionedPlaceholder(_) => return,
            Fragment::Positioning(fragment) => &fragment.style,
            Fragment::Image(fragment) => &fragment.style,
            Fragment::IFrame(fragment) => &fragment.style,
            Fragment::Float(fragment) => &fragment.style,
        };
        *shared_style.borrow_mut() = new_style.clone();
    }

    pub(crate) fn retrieve_box_fragment(&self) -> Option<RefOrAtomicRef<'_, Arc<BoxFragment>>> {
        match self {
            Fragment::LayoutRoot(layout_root_fragment) => Some(RefOrAtomicRef::AtomicRef(
                layout_root_fragment.inner_box_fragment(),
            )),
            Fragment::Box(box_fragment) | Fragment::Float(box_fragment) => {
                Some(RefOrAtomicRef::Ref(box_fragment))
            },
            _ => None,
        }
    }
}

impl TextFragment {
    pub(crate) fn style<'a>(&'a self) -> AtomicRef<'a, ServoArc<ComputedValues>> {
        self.run_data.inline_styles.style.borrow()
    }

    pub(crate) fn selected_style<'a>(&'a self) -> AtomicRef<'a, ServoArc<ComputedValues>> {
        self.run_data.inline_styles.selected.borrow()
    }

    pub fn print(&self, tree: &mut PrintTree) {
        tree.add_item(format!(
            "Text num_glyphs={} box={:?}",
            self.glyphs
                .iter()
                .map(|shaped_text_slice| shaped_text_slice.glyph_count())
                .sum::<usize>(),
            self.base.rect()
        ));
    }

    /// Whether or not the given point is within the vertical boundaries of this
    /// [`TextFragment`].
    pub(crate) fn point_is_within_vertical_boundaries(
        &self,
        point_in_fragment: Point2D<Au, CSSPixel>,
    ) -> bool {
        let rect = &self.base.rect();
        rect.min_y() <= point_in_fragment.y && rect.max_y() >= point_in_fragment.y
    }

    /// Find the distance between for point relative to a [`TextFragment`] for the
    /// purposes of finding a glyph offset. This is used to identify the most relevant
    /// fragment for glyph offset queries during click handling.
    pub(crate) fn distance_to_point_for_glyph_offset(
        &self,
        point_in_fragment: Point2D<Au, CSSPixel>,
    ) -> Au {
        // point_in_fragment is alerady in a coordinate space where the fragment origin is (0, 0)
        let rect = Rect::new(Point2D::origin(), self.base.rect().size);

        // This is the distance between the closest point on the edge of the rectangle and
        // the point. From <https://stackoverflow.com/a/18157551>.
        let dx = (rect.min_x() - point_in_fragment.x)
            .max(Au::zero())
            .max(point_in_fragment.x - rect.max_x());
        let dy = (rect.min_y() - point_in_fragment.y)
            .max(Au::zero())
            .max(point_in_fragment.y - rect.max_y());
        Au::from_f64_px(dx.to_f64_px().hypot(dy.to_f64_px()))
    }

    /// Given a point relative to this [`TextFragment`], find the most appropriate
    /// character offset.
    ///
    /// Note that the given point may be outside the [`TextFragment`]'s content rect:
    ///
    ///  - If the point is vertically above the [`TextFragment`] the first offset will be returned.
    ///  - If the point is vertically below the [`TextFragment`], `None` will be returned.
    pub(crate) fn character_offset(
        &self,
        point_in_fragment: Point2D<Au, CSSPixel>,
    ) -> Option<Utf32CodeUnits> {
        self.unmapped_character_offset(point_in_fragment)
            .map(|character_offset| {
                self.run_data
                    .map_transformed_offset_to_dom_offset(character_offset)
            })
    }

    /// Like [`Self::character_offset`], but returning the character offset in layout text (after
    /// applying white space collapse and the `text-transform` property), instead of the original DOM
    /// text.
    fn unmapped_character_offset(
        &self,
        point_in_fragment: Point2D<Au, CSSPixel>,
    ) -> Option<Utf32CodeUnits> {
        // If the click was far enough above the top of the fragment, then pick the first index.
        let max_vertical_offset = self.base.rect().height().scale_by(0.25);
        if point_in_fragment.y < -max_vertical_offset {
            return Some(self.character_range_in_dom_node.start);
        }

        // If the click was below the fragment, return `None`, which will cause the
        // caller to move the cursor to the end.
        //
        // TODO: It would be nice to just return the last offset here, but <textarea>
        // does not currently make a fragment for all selection indices.
        if point_in_fragment.y > self.base.rect().max_y() + max_vertical_offset {
            return None;
        }

        let mut current_character = self.character_range_in_dom_node.start;
        let mut current_offset = Au::zero();
        for glyph_store in &self.glyphs {
            for glyph in glyph_store.glyphs() {
                let mut advance = glyph.advance();
                if glyph.char_is_word_separator() {
                    advance += self.justification_adjustment;
                }
                if current_offset + advance.scale_by(0.5) >= point_in_fragment.x {
                    return Some(current_character);
                }
                current_offset += advance;
                current_character += glyph.character_count();
            }
        }

        Some(current_character)
    }

    fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
        let mut rect = self.base.rect();
        let mut hanging_advance = Au::zero();
        for slice in self.glyphs.iter().rev() {
            hanging_advance += slice.hangable_advance();
            if slice.has_non_hangable_non_removable_content() {
                break;
            }
        }

        // TODO: This is not correct for vertical and right-to-left writing modes.
        rect.size.width -= hanging_advance;

        rect
    }
}

impl ImageFragment {
    pub fn print(&self, tree: &mut PrintTree) {
        tree.add_item(format!(
            "Image\
                \nrect={:?}",
            self.base.rect()
        ));
    }
}

impl IFrameFragment {
    pub fn print(&self, tree: &mut PrintTree) {
        tree.add_item(format!(
            "IFrame\
                \npipeline={:?} rect={:?}",
            self.pipeline_id,
            self.base.rect()
        ));
    }
}

impl CollapsedBlockMargins {
    pub fn from_margin(margin: &LogicalSides<Au>) -> Self {
        Self {
            collapsed_through: false,
            start: CollapsedMargin::new(margin.block_start),
            end: CollapsedMargin::new(margin.block_end),
        }
    }

    pub fn zero() -> Self {
        Self {
            collapsed_through: false,
            start: CollapsedMargin::zero(),
            end: CollapsedMargin::zero(),
        }
    }
}

impl CollapsedMargin {
    pub fn zero() -> Self {
        Self {
            max_positive: Au::zero(),
            min_negative: Au::zero(),
        }
    }

    pub fn new(margin: Au) -> Self {
        Self {
            max_positive: margin.max(Au::zero()),
            min_negative: margin.min(Au::zero()),
        }
    }

    pub fn adjoin(&self, other: &Self) -> Self {
        Self {
            max_positive: self.max_positive.max(other.max_positive),
            min_negative: self.min_negative.min(other.min_negative),
        }
    }

    pub fn adjoin_assign(&mut self, other: &Self) {
        *self = self.adjoin(other);
    }

    pub fn solve(&self) -> Au {
        self.max_positive + self.min_negative
    }
}

/// A token which ensures the calculation and assignment of cumulative containing
/// blocks to fragments in the fragment tree. This is used because these cumulative
/// containing block offsets are set during stacking context tree construction, but
/// some queries might need them beforehand. If the query is executed before stacking
/// context tree construction, a quick traversal is performed to calculate them for
/// the purpose of the query.
pub(crate) enum ContainingBlockCalculation<'a> {
    /// This token variant is for the purpose of a layout query. In this case, if stacking
    /// context tree construction has not yet taken place, a cumulative containing block
    /// calculation traversal will be performed.
    Lazy { layout_thread: &'a LayoutThread },
    /// This token variant is used when the code can guarantee that stacking context
    /// tree construction has already taken place.
    ///
    /// Note: Using this before stacking context tree construction can lead
    /// to incorrect layout or layout query results!
    AlreadyDoneWithStackingContextTree,
}

impl ContainingBlockCalculation<'_> {
    pub(crate) fn ensure(&self) {
        match self {
            Self::Lazy { layout_thread } => layout_thread.ensure_containing_block_calculation(),
            Self::AlreadyDoneWithStackingContextTree => {},
        }
    }
}

impl<'a> From<&'a LayoutThread> for ContainingBlockCalculation<'a> {
    fn from(layout_thread: &'a LayoutThread) -> Self {
        Self::Lazy { layout_thread }
    }
}