servo/components/layout/flow/root.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 app_units::Au;
use atomic_refcell::AtomicRef;
use compositing_traits::display_list::AxesScrollSensitivity;
use euclid::Rect;
use euclid::default::Size2D as UntypedSize2D;
use malloc_size_of_derive::MallocSizeOf;
use script::layout_dom::ServoLayoutNode;
use script_layout_interface::wrapper_traits::{
    LayoutNode, ThreadSafeLayoutElement, ThreadSafeLayoutNode,
};
use script_layout_interface::{LayoutElementType, LayoutNodeType};
use servo_arc::Arc;
use style::dom::{NodeInfo, TNode};
use style::properties::ComputedValues;
use style::values::computed::Overflow;
use style_traits::CSSPixel;

use crate::cell::ArcRefCell;
use crate::context::LayoutContext;
use crate::dom::{LayoutBox, NodeExt};
use crate::dom_traversal::{Contents, NodeAndStyleInfo, NonReplacedContents, iter_child_nodes};
use crate::flexbox::FlexLevelBox;
use crate::flow::float::FloatBox;
use crate::flow::inline::InlineItem;
use crate::flow::{BlockContainer, BlockFormattingContext, BlockLevelBox};
use crate::formatting_contexts::IndependentFormattingContext;
use crate::fragment_tree::FragmentTree;
use crate::geom::{LogicalVec2, PhysicalRect, PhysicalSize};
use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
use crate::replaced::ReplacedContents;
use crate::style_ext::{Display, DisplayGeneratingBox, DisplayInside};
use crate::taffy::{TaffyItemBox, TaffyItemBoxInner};
use crate::{DefiniteContainingBlock, PropagatedBoxTreeData};

#[derive(MallocSizeOf)]
pub struct BoxTree {
    /// Contains typically exactly one block-level box, which was generated by the root element.
    /// There may be zero if that element has `display: none`.
    root: BlockFormattingContext,

    /// Whether or not the viewport should be sensitive to scrolling input events in two axes
    viewport_scroll_sensitivity: AxesScrollSensitivity,
}

impl BoxTree {
    pub fn construct(context: &LayoutContext, root_element: ServoLayoutNode<'_>) -> Self {
        let boxes = construct_for_root_element(context, root_element);

        // Zero box for `:root { display: none }`, one for the root element otherwise.
        assert!(boxes.len() <= 1);

        // From https://www.w3.org/TR/css-overflow-3/#overflow-propagation:
        // > UAs must apply the overflow-* values set on the root element to the viewport when the
        // > root element’s display value is not none. However, when the root element is an [HTML]
        // > html element (including XML syntax for HTML) whose overflow value is visible (in both
        // > axes), and that element has as a child a body element whose display value is also not
        // > none, user agents must instead apply the overflow-* values of the first such child
        // > element to the viewport. The element from which the value is propagated must then have a
        // > used overflow value of visible.
        let root_style = root_element.style(context.shared_context());

        let mut viewport_overflow_x = root_style.clone_overflow_x();
        let mut viewport_overflow_y = root_style.clone_overflow_y();
        if viewport_overflow_x == Overflow::Visible &&
            viewport_overflow_y == Overflow::Visible &&
            !root_style.get_box().display.is_none()
        {
            for child in iter_child_nodes(root_element) {
                if !child
                    .to_threadsafe()
                    .as_element()
                    .is_some_and(|element| element.is_body_element_of_html_element_root())
                {
                    continue;
                }

                let style = child.style(context.shared_context());
                if !style.get_box().display.is_none() {
                    viewport_overflow_x = style.clone_overflow_x();
                    viewport_overflow_y = style.clone_overflow_y();

                    break;
                }
            }
        }

        let contents = BlockContainer::BlockLevelBoxes(boxes);
        let contains_floats = contents.contains_floats();
        Self {
            root: BlockFormattingContext {
                contents,
                contains_floats,
            },
            // From https://www.w3.org/TR/css-overflow-3/#overflow-propagation:
            // > If visible is applied to the viewport, it must be interpreted as auto.
            // > If clip is applied to the viewport, it must be interpreted as hidden.
            viewport_scroll_sensitivity: AxesScrollSensitivity {
                x: viewport_overflow_x.to_scrollable().into(),
                y: viewport_overflow_y.to_scrollable().into(),
            },
        }
    }

    /// This method attempts to incrementally update the box tree from an
    /// arbitrary node that is not necessarily the document's root element.
    ///
    /// If the node is not a valid candidate for incremental update, the method
    /// loops over its parent. The only valid candidates for now are absolutely
    /// positioned boxes which don't change their outside display mode (i.e. it
    /// will not attempt to update from an absolutely positioned inline element
    /// which became an absolutely positioned block element). The value `true`
    /// is returned if an incremental update could be done, and `false`
    /// otherwise.
    ///
    /// There are various pain points that need to be taken care of to extend
    /// the set of valid candidates:
    /// * it is not obvious how to incrementally check whether a block
    ///   formatting context still contains floats or not;
    /// * the propagation of text decorations towards node descendants is
    ///   hard to do incrementally with our current representation of boxes
    /// * how intrinsic content sizes are computed eagerly makes it hard
    ///   to update those sizes for ancestors of the node from which we
    ///   made an incremental update.
    pub fn update(context: &LayoutContext, mut dirty_node: ServoLayoutNode<'_>) -> bool {
        #[allow(clippy::enum_variant_names)]
        enum UpdatePoint {
            AbsolutelyPositionedBlockLevelBox(ArcRefCell<BlockLevelBox>),
            AbsolutelyPositionedInlineLevelBox(ArcRefCell<InlineItem>, usize),
            AbsolutelyPositionedFlexLevelBox(ArcRefCell<FlexLevelBox>),
            AbsolutelyPositionedTaffyLevelBox(ArcRefCell<TaffyItemBox>),
        }

        fn update_point(
            node: ServoLayoutNode<'_>,
        ) -> Option<(Arc<ComputedValues>, DisplayInside, UpdatePoint)> {
            if !node.is_element() {
                return None;
            }

            if node.type_id() == LayoutNodeType::Element(LayoutElementType::HTMLBodyElement) {
                // This can require changes to the canvas background.
                return None;
            }

            // Don't update unstyled nodes or nodes that have pseudo-elements.
            let element_data = node.style_data()?.element_data.borrow();
            if !element_data.styles.pseudos.is_empty() {
                return None;
            }

            let layout_data = NodeExt::layout_data(&node)?;
            if layout_data.pseudo_before_box.borrow().is_some() {
                return None;
            }
            if layout_data.pseudo_after_box.borrow().is_some() {
                return None;
            }

            let primary_style = element_data.styles.primary();
            let box_style = primary_style.get_box();

            if !box_style.position.is_absolutely_positioned() {
                return None;
            }

            let display_inside = match Display::from(box_style.display) {
                Display::GeneratingBox(DisplayGeneratingBox::OutsideInside { inside, .. }) => {
                    inside
                },
                _ => return None,
            };

            let update_point =
                match &*AtomicRef::filter_map(layout_data.self_box.borrow(), Option::as_ref)? {
                    LayoutBox::DisplayContents(..) => return None,
                    LayoutBox::BlockLevel(block_level_box) => match &*block_level_box.borrow() {
                        BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(_)
                            if box_style.position.is_absolutely_positioned() =>
                        {
                            UpdatePoint::AbsolutelyPositionedBlockLevelBox(block_level_box.clone())
                        },
                        _ => return None,
                    },
                    LayoutBox::InlineLevel(inline_level_items) => {
                        let inline_level_box = inline_level_items.first()?;
                        let InlineItem::OutOfFlowAbsolutelyPositionedBox(_, text_offset_index) =
                            &*inline_level_box.borrow()
                        else {
                            return None;
                        };
                        UpdatePoint::AbsolutelyPositionedInlineLevelBox(
                            inline_level_box.clone(),
                            *text_offset_index,
                        )
                    },
                    LayoutBox::FlexLevel(flex_level_box) => match &*flex_level_box.borrow() {
                        FlexLevelBox::OutOfFlowAbsolutelyPositionedBox(_)
                            if box_style.position.is_absolutely_positioned() =>
                        {
                            UpdatePoint::AbsolutelyPositionedFlexLevelBox(flex_level_box.clone())
                        },
                        _ => return None,
                    },
                    LayoutBox::TableLevelBox(..) => return None,
                    LayoutBox::TaffyItemBox(taffy_level_box) => match &taffy_level_box
                        .borrow()
                        .taffy_level_box
                    {
                        TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(_)
                            if box_style.position.is_absolutely_positioned() =>
                        {
                            UpdatePoint::AbsolutelyPositionedTaffyLevelBox(taffy_level_box.clone())
                        },
                        _ => return None,
                    },
                };
            Some((primary_style.clone(), display_inside, update_point))
        }

        loop {
            let Some((primary_style, display_inside, update_point)) = update_point(dirty_node)
            else {
                dirty_node = match dirty_node.parent_node() {
                    Some(parent) => parent,
                    None => return false,
                };
                continue;
            };

            let contents = ReplacedContents::for_element(dirty_node, context)
                .map_or_else(|| NonReplacedContents::OfElement.into(), Contents::Replaced);
            let info = NodeAndStyleInfo::new(dirty_node, Arc::clone(&primary_style));
            let out_of_flow_absolutely_positioned_box = ArcRefCell::new(
                AbsolutelyPositionedBox::construct(context, &info, display_inside, contents),
            );
            match update_point {
                UpdatePoint::AbsolutelyPositionedBlockLevelBox(block_level_box) => {
                    *block_level_box.borrow_mut() = BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(
                        out_of_flow_absolutely_positioned_box,
                    );
                },
                UpdatePoint::AbsolutelyPositionedInlineLevelBox(
                    inline_level_box,
                    text_offset_index,
                ) => {
                    *inline_level_box.borrow_mut() = InlineItem::OutOfFlowAbsolutelyPositionedBox(
                        out_of_flow_absolutely_positioned_box,
                        text_offset_index,
                    );
                },
                UpdatePoint::AbsolutelyPositionedFlexLevelBox(flex_level_box) => {
                    *flex_level_box.borrow_mut() = FlexLevelBox::OutOfFlowAbsolutelyPositionedBox(
                        out_of_flow_absolutely_positioned_box,
                    );
                },
                UpdatePoint::AbsolutelyPositionedTaffyLevelBox(taffy_level_box) => {
                    taffy_level_box.borrow_mut().taffy_level_box =
                        TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(
                            out_of_flow_absolutely_positioned_box,
                        );
                },
            }
            break;
        }

        // We are going to rebuild the box tree from the update point downward, but this update
        // point is an absolute, which means that it needs to be laid out again in the containing
        // block for absolutes, which is established by one of its ancestors. In addition,
        // absolutes, when laid out, can produce more absolutes (either fixed or absolutely
        // positioned) elements, so there may be yet more layout that has to happen in this
        // ancestor.
        //
        // We do not know which ancestor is the one that established the containing block for this
        // update point, so just invalidate the fragment cache of all ancestors, meaning that even
        // though the box tree is preserved, the fragment tree from the root to the update point and
        // all of its descendants will need to be rebuilt. This isn't as bad as it seems, because
        // siblings and siblings of ancestors of this path through the tree will still have cached
        // fragments.
        //
        // TODO: Do better. This is still a very crude way to do incremental layout.
        while let Some(parent_node) = dirty_node.parent_node() {
            parent_node.invalidate_cached_fragment();
            dirty_node = parent_node;
        }

        true
    }
}

fn construct_for_root_element(
    context: &LayoutContext,
    root_element: ServoLayoutNode<'_>,
) -> Vec<ArcRefCell<BlockLevelBox>> {
    let info = NodeAndStyleInfo::new(root_element, root_element.style(context.shared_context()));
    let box_style = info.style.get_box();

    let display_inside = match Display::from(box_style.display) {
        Display::None => {
            root_element.unset_all_boxes();
            return Vec::new();
        },
        Display::Contents => {
            // Unreachable because the style crate adjusts the computed values:
            // https://drafts.csswg.org/css-display-3/#transformations
            // “'display' of 'contents' computes to 'block' on the root element”
            unreachable!()
        },
        // The root element is blockified, ignore DisplayOutside
        Display::GeneratingBox(display_generating_box) => display_generating_box.display_inside(),
    };

    let contents = ReplacedContents::for_element(root_element, context)
        .map_or_else(|| NonReplacedContents::OfElement.into(), Contents::Replaced);

    let propagated_data = PropagatedBoxTreeData::default();
    let root_box = if box_style.position.is_absolutely_positioned() {
        BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(ArcRefCell::new(
            AbsolutelyPositionedBox::construct(context, &info, display_inside, contents),
        ))
    } else if box_style.float.is_floating() {
        BlockLevelBox::OutOfFlowFloatBox(FloatBox::construct(
            context,
            &info,
            display_inside,
            contents,
            propagated_data,
        ))
    } else {
        BlockLevelBox::Independent(IndependentFormattingContext::construct(
            context,
            &info,
            display_inside,
            contents,
            propagated_data,
        ))
    };

    let root_box = ArcRefCell::new(root_box);
    root_element
        .element_box_slot()
        .set(LayoutBox::BlockLevel(root_box.clone()));
    vec![root_box]
}

impl BoxTree {
    pub fn layout(
        &self,
        layout_context: &LayoutContext,
        viewport: UntypedSize2D<Au>,
    ) -> FragmentTree {
        let style = layout_context
            .style_context
            .stylist
            .device()
            .default_computed_values();

        // FIXME: use the document’s mode:
        // https://drafts.csswg.org/css-writing-modes/#principal-flow
        let physical_containing_block: Rect<Au, CSSPixel> =
            PhysicalSize::from_untyped(viewport).into();
        let initial_containing_block = DefiniteContainingBlock {
            size: LogicalVec2 {
                inline: physical_containing_block.size.width,
                block: physical_containing_block.size.height,
            },
            style,
        };

        let mut positioning_context = PositioningContext::default();
        let independent_layout = self.root.layout(
            layout_context,
            &mut positioning_context,
            &(&initial_containing_block).into(),
            false, /* depends_on_block_constraints */
        );

        let mut root_fragments = independent_layout.fragments.into_iter().collect::<Vec<_>>();

        // Zero box for `:root { display: none }`, one for the root element otherwise.
        assert!(root_fragments.len() <= 1);

        // There may be more fragments at the top-level
        // (for positioned boxes whose containing is the initial containing block)
        // but only if there was one fragment for the root element.
        positioning_context.layout_initial_containing_block_children(
            layout_context,
            &initial_containing_block,
            &mut root_fragments,
        );

        let scrollable_overflow = root_fragments
            .iter()
            .fold(PhysicalRect::zero(), |acc, child| {
                let child_overflow = child.scrollable_overflow_for_parent();

                // https://drafts.csswg.org/css-overflow/#scrolling-direction
                // We want to clip scrollable overflow on box-start and inline-start
                // sides of the scroll container.
                //
                // FIXME(mrobinson, bug 25564): This should take into account writing
                // mode.
                let child_overflow = PhysicalRect::new(
                    euclid::Point2D::zero(),
                    euclid::Size2D::new(
                        child_overflow.size.width + child_overflow.origin.x,
                        child_overflow.size.height + child_overflow.origin.y,
                    ),
                );
                acc.union(&child_overflow)
            });

        FragmentTree::new(
            layout_context,
            root_fragments,
            scrollable_overflow,
            physical_containing_block,
            self.viewport_scroll_sensitivity,
        )
    }
}