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 servo_arc::Arc;
use style::dom::{NodeInfo, TNode};
use style::properties::ComputedValues;
use style::selector_parser::RestyleDamage;
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, PhysicalSize};
use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
use crate::replaced::ReplacedContents;
use crate::style_ext::{Display, DisplayInside};
use crate::taffy::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. Currently, the only valid candidates for now
    /// are absolutely positioned boxes and their originating nodes must have
    /// restyle damage that is `REPAIR_BOX` rather than `REBUILD_BOX`.  In the
    /// future, this may be extended to other types of boxes if the new restyle
    /// damage types for incremental layout is ready.
    ///
    /// There are various reasons why only absolutely positioned boxes can be
    /// selected as the update point:
    /// * it needs a bit of trick to incrementally update the `contains_floats`
    ///   for a block formatting context and `SameFormattingContext`, which make
    ///   the incremental update less eligible.
    /// * the propagation of box tree data towards node descendants is hard to do
    ///   incrementally with our current representation of boxes. To support
    ///   incremental update, we have to store it at the `LayoutBoxBase`.
    /// * We have to clear all layout caches for ancestors of the update point
    ///   when the incremental layout is not fully ready. However, it is really hard
    ///   to do that incrementally because a box does not hold a reference to its
    ///   parent box with current representation of boxes. The anonymous ancestor
    ///   boxes can not be visited. Thus, select the absolutely positioned boxes
    ///   as the update point, this is not a problem.
    pub fn update(context: &LayoutContext, dirty_node: ServoLayoutNode<'_>) -> bool {
        let mut helper = BoxTreeUpdateHelper::new(context, dirty_node);
        helper.try_update_box_tree()
    }
}

fn construct_for_root_element(
    context: &LayoutContext,
    root_element: ServoLayoutNode<'_>,
) -> Vec<ArcRefCell<BlockLevelBox>> {
    root_element.clear_restyle_damage();

    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,
        );

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

struct BoxTreeUpdateHelper<'style, 'dom> {
    context: &'style LayoutContext<'style>,
    dirty_node: ServoLayoutNode<'dom>,
    primary_style: Option<Arc<ComputedValues>>,
    display_inside: Option<DisplayInside>,
}

enum UpdatePoint {
    DoNotNeedUpdateTree,
    RootElementPrimaryBox(ArcRefCell<BlockLevelBox>),
    AbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
}

impl<'style, 'dom> BoxTreeUpdateHelper<'style, 'dom> {
    fn new(context: &'style LayoutContext<'style>, dirty_node: ServoLayoutNode<'dom>) -> Self {
        Self {
            context,
            dirty_node,
            primary_style: None,
            display_inside: None,
        }
    }

    fn try_update_box_tree(&mut self) -> bool {
        loop {
            let Some(update_point) = self.update_point() else {
                self.dirty_node = match self.dirty_node.parent_node() {
                    Some(parent) => parent,
                    None => return false,
                };
                continue;
            };

            match update_point {
                UpdatePoint::DoNotNeedUpdateTree => {},
                _ => {
                    self.update_box_tree(update_point);
                    self.invalidate_layout_cache();
                    self.clear_ancestor_restyle_damage();
                },
            };

            break;
        }

        true
    }

    fn update_point(&mut self) -> Option<UpdatePoint> {
        if !self.dirty_node.is_element() {
            return None;
        }

        let element_data = self.dirty_node.style_data()?.element_data.borrow();

        if !element_data.damage.contains(RestyleDamage::REPAIR_BOX) {
            return Some(UpdatePoint::DoNotNeedUpdateTree);
        }

        if element_data.damage.contains(RestyleDamage::REBUILD_BOX) {
            return None;
        }

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

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

        let layout_data = NodeExt::layout_data(&self.dirty_node)?;
        let layout_box = &*AtomicRef::filter_map(layout_data.self_box.borrow(), Option::as_ref)?;
        if self.dirty_node.to_threadsafe().as_element()?.is_root() {
            match layout_box {
                LayoutBox::BlockLevel(block_level_box) => {
                    self.primary_style = Some(primary_style.clone());
                    self.display_inside = Some(display_inside);
                    return Some(UpdatePoint::RootElementPrimaryBox(block_level_box.clone()));
                },
                // 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!("Root element should only has display: block"),
            }
        }

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

        let update_point = match layout_box {
            LayoutBox::DisplayContents(..) => return None,
            LayoutBox::BlockLevel(block_level_box) => match &*block_level_box.borrow() {
                BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(positioned_box) => {
                    UpdatePoint::AbsolutelyPositionedBox(positioned_box.clone())
                },
                _ => return None,
            },
            LayoutBox::InlineLevel(inline_level_items) => {
                let inline_level_item = inline_level_items.first()?;
                match &*inline_level_item.borrow() {
                    InlineItem::OutOfFlowAbsolutelyPositionedBox(positioned_box, _) => {
                        UpdatePoint::AbsolutelyPositionedBox(positioned_box.clone())
                    },
                    _ => return None,
                }
            },
            LayoutBox::FlexLevel(flex_level_box) => match &*flex_level_box.borrow() {
                FlexLevelBox::OutOfFlowAbsolutelyPositionedBox(positioned_box) => {
                    UpdatePoint::AbsolutelyPositionedBox(positioned_box.clone())
                },
                _ => return None,
            },
            LayoutBox::TableLevelBox(..) => return None,
            LayoutBox::TaffyItemBox(taffy_item_box) => {
                match &taffy_item_box.borrow().taffy_level_box {
                    TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(positioned_box) => {
                        UpdatePoint::AbsolutelyPositionedBox(positioned_box.clone())
                    },
                    _ => return None,
                }
            },
        };

        self.primary_style = Some(primary_style.clone());
        self.display_inside = Some(display_inside);

        Some(update_point)
    }

    /// We are going to repair 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 on if 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
    /// slibings and slibings 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.
    fn invalidate_layout_cache(&self) {
        let mut invalidation_start_point = self.dirty_node;
        while let Some(parent_node) = invalidation_start_point.parent_node() {
            parent_node.invalidate_cached_fragment();
            invalidation_start_point = parent_node;
        }
    }

    /// We have already propagate up some restyle damage from descendants to
    /// the ancestors of update point, but we are just going to traverse the
    /// subtree from the update point rather than the root element during
    /// incremental update. Thus, clear all ancestor's restyle damage now.
    fn clear_ancestor_restyle_damage(&self) {
        let mut inclusive_ancestor = Some(self.dirty_node);
        while let Some(node) = inclusive_ancestor {
            node.clear_restyle_damage();
            inclusive_ancestor = node.parent_node();
        }
    }

    fn update_box_tree(&self, update_point: UpdatePoint) {
        let context = self.context;
        let display_inside = self.display_inside.unwrap();

        let info = NodeAndStyleInfo::new(self.dirty_node, self.primary_style.clone().unwrap());
        let contents = ReplacedContents::for_element(self.dirty_node, context)
            .map_or_else(|| NonReplacedContents::OfElement.into(), Contents::Replaced);

        match update_point {
            UpdatePoint::DoNotNeedUpdateTree => unreachable!("Should have been filtered out"),
            UpdatePoint::RootElementPrimaryBox(block_level_box) => {
                block_level_box.borrow_mut().repair(
                    context,
                    &info,
                    contents,
                    display_inside,
                    PropagatedBoxTreeData::default(),
                );
            },
            UpdatePoint::AbsolutelyPositionedBox(positioned_box) => {
                positioned_box.borrow_mut().context.repair(
                    context,
                    &info,
                    contents,
                    display_inside,
                    PropagatedBoxTreeData::default(),
                );
            },
        }
    }
}