script/dom/

selection.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::{Cell, LazyCell};
use std::cmp::Ordering;

use atomic_refcell::{AtomicRef, AtomicRefCell};
use bitflags::bitflags;
use dom_struct::dom_struct;
use js::context::{JSContext, NoGC};
use layout_api::QueryMsg;
use rustc_hash::{FxHashMap, FxHashSet};
use script_bindings::cell::DomRefCell;
use script_bindings::codegen::GenericBindings::ShadowRootBinding::ShadowRootMethods;
use script_bindings::dom::UnrootedDom;
use script_bindings::reflector::{Reflector, reflect_dom_object};
use servo_base::text::{
    AssumeUnder4GB, RangeAny, Utf16CodeUnits, Utf32CodeUnits, Utf32CodeUnitsOrNodeOffset,
};

use crate::dom::abstractrange::bp_position;
use crate::dom::bindings::cell::AtomicSafeBorrowMut;
use crate::dom::bindings::codegen::Bindings::NodeBinding::{GetRootNodeOptions, NodeMethods};
use crate::dom::bindings::codegen::Bindings::RangeBinding::RangeMethods;
use crate::dom::bindings::codegen::Bindings::SelectionBinding::{
    GetComposedRangesOptions, SelectionMethods,
};
use crate::dom::bindings::error::{Error, ErrorResult, Fallible};
use crate::dom::bindings::inheritance::Castable;
use crate::dom::bindings::refcounted::Trusted;
use crate::dom::bindings::reflector::DomGlobal;
use crate::dom::bindings::root::{Dom, DomRoot, LayoutDom, MutNullableDom};
use crate::dom::bindings::str::DOMString;
use crate::dom::comparator::compare_dom_positions;
use crate::dom::document::Document;
use crate::dom::eventtarget::EventTarget;
use crate::dom::iterators::{PrePostIteration, UnrootedFollowingFlatTreeNodesTraversal};
use crate::dom::node::{Node, NodeTraits};
use crate::dom::range::Range;
use crate::dom::selection_range::{RootedSelectionBoundary, SelectionBoundary, SelectionRange};
use crate::dom::staticrange::StaticRange;
use crate::dom::traversal::FlatTreeForSelectionNoGcTraversal;
use crate::dom::types::ShadowRoot;
use crate::dom::{CharacterData, FlatTreeParent, NodeDamage, NodeFlags, StartOrEnd};

/// Used value of [`user-select`](https://drafts.csswg.org/css-ui-4/#propdef-user-select):
/// like the computed value but excludes `auto`.
#[derive(Copy, Clone, PartialEq)]
pub(crate) enum UsedUserSelect {
    Text,
    None,
    Contain,
    All,
}

#[derive(Clone, Copy, JSTraceable, MallocSizeOf)]
pub(crate) enum Direction {
    Forwards,
    Backwards,
    Directionless,
}

/// <https://w3c.github.io/selection-api/#dfn-selection>
#[dom_struct]
pub(crate) struct Selection {
    reflector_: Reflector,
    document: Dom<Document>,
    /// A range that holds the start and end of this selection, which may potentially
    /// cross shadow roots.
    range: DomRefCell<Option<SelectionRange>>,
    /// The live range version of this selection, which will never cross shadow roots.
    live_range: MutNullableDom<Range>,
    /// This is like [`Range`], but in the flat tree, which means that:
    ///
    /// * It is `None` if the selection is unrenderable.
    /// * Boundaries are in flat tree order.
    #[custom_trace]
    visible_range: AtomicRefCell<Option<SelectionRange>>,
    /// The [`Direction`] of this [`Selection`] which determines which endpoint of
    /// [`Self::range`] is the anchor and which is the focus.
    direction: Cell<Direction>,
    /// <https://w3c.github.io/selection-api/#dfn-has-scheduled-selectionchange-event>
    has_scheduled_selectionchange_event: Cell<bool>,
    /// Whether or not this [`Selection`] needs to remark DOM nodes with selection flags
    /// after a change to its underlying [`Range`].
    visible_selection_dirty: Cell<bool>,
}

impl Selection {
    fn new_inherited(document: &Document) -> Selection {
        Selection {
            reflector_: Reflector::new(),
            document: Dom::from_ref(document),
            range: Default::default(),
            live_range: MutNullableDom::new(None),
            visible_range: Default::default(),
            direction: Cell::new(Direction::Directionless),
            has_scheduled_selectionchange_event: Cell::new(false),
            visible_selection_dirty: Cell::new(false),
        }
    }

    pub(crate) fn new(cx: &mut JSContext, document: &Document) -> DomRoot<Selection> {
        reflect_dom_object(
            cx,
            Box::new(Selection::new_inherited(document)),
            &*document.global(),
        )
    }

    pub(crate) fn visible_selection_dirty(&self) -> bool {
        self.visible_selection_dirty.get()
    }

    pub(crate) fn collapsed(&self) -> bool {
        self.range
            .borrow()
            .as_ref()
            .is_none_or(|range| range.collapsed())
    }

    fn clear_cached_live_range(&self) {
        if let Some(old_range) = self.live_range.take() {
            old_range.disassociate_selection(self);
        }
    }

    /// Clear this entire [`Selection`] if the [`Document`] of its live range has changed
    /// or its boundaries are no longer connected. Returns true if the selection was
    /// cleared or false otherwise.
    pub(crate) fn clear_selection_if_live_range_document_changed(
        &self,
        no_gc: &NoGC,
        range: &Range,
    ) -> bool {
        // The live range might be in an intermediate state where its boundaries
        // are in different documents, so this must check both ends.
        let range_start_container = range.start_container();
        let range_end_container = range.end_container();
        if self.is_in_composed_tree_of_document_and_is_not_ua_widget(&range_start_container) &&
            self.is_in_composed_tree_of_document_and_is_not_ua_widget(&range_end_container)
        {
            return false;
        }

        self.set_range(no_gc, None);
        true
    }

    /// Update the start of end boundary of this [`Selection`] based on a change to the
    /// given associated [`Range`]. While the behavior here isn't totally specified yet,
    /// it follows other browsers. If the change would change the direction of the
    /// [`Selection`] it is collapsed at the newly-set boundary.
    pub(crate) fn set_start_or_end_from_live_range(
        &self,
        no_gc: &NoGC,
        start_or_end: StartOrEnd,
        live_range: &Range,
    ) -> bool {
        let mut range = self.range.borrow_mut();
        let range = range
            .as_mut()
            .expect("A live range implies a selection range");

        let (container, offset) = match start_or_end {
            StartOrEnd::Start if range.start != *live_range.start() => {
                (live_range.start_container(), live_range.start_offset())
            },
            StartOrEnd::End if range.end != *live_range.end() => {
                (live_range.end_container(), live_range.end_offset())
            },
            _ => return false,
        };

        let would_invert = match start_or_end {
            StartOrEnd::Start => {
                compare_shadow_including_dom_positions(
                    no_gc,
                    &container,
                    offset,
                    &range.end.container,
                    range.end.offset,
                ) == Ordering::Greater
            },
            StartOrEnd::End => {
                compare_shadow_including_dom_positions(
                    no_gc,
                    &range.start.container,
                    range.start.offset,
                    &container,
                    offset,
                ) == Ordering::Greater
            },
        };

        if would_invert {
            *range = SelectionRange::collapsed_at(SelectionBoundary::new(&container, offset));
        } else {
            match start_or_end {
                StartOrEnd::Start => range.start = SelectionBoundary::new(&container, offset),
                StartOrEnd::End => range.end = SelectionBoundary::new(&container, offset),
            }
        }
        true
    }

    pub(crate) fn update_from_live_range(
        &self,
        no_gc: &NoGC,
        live_range: &Range,
        notification: SelectionLiveRangeNotification,
    ) {
        debug_assert!(Some(live_range) == self.live_range.get().as_deref());
        let start_changed = notification.contains(SelectionLiveRangeNotification::Start) &&
            self.set_start_or_end_from_live_range(no_gc, StartOrEnd::Start, live_range);
        let end_changed = notification.contains(SelectionLiveRangeNotification::End) &&
            self.set_start_or_end_from_live_range(no_gc, StartOrEnd::End, live_range);
        if start_changed || end_changed {
            self.selection_boundaries_changed();
        }
    }

    #[cfg_attr(crown, expect(crown::unrooted_must_root))]
    fn set_range(&self, _no_gc: &NoGC, new_range: Option<SelectionRange>) -> bool {
        let changed;
        {
            let mut range = self.range.borrow_mut();
            changed = *range != new_range;
            *range = new_range;

            if range.is_none() {
                self.direction.set(Direction::Directionless);
            }
        }

        // Any changes must unconditionally install a new live range.
        self.clear_cached_live_range();

        if changed {
            self.selection_boundaries_changed();
            #[cfg(debug_assertions)]
            self.assert_valid_selection(_no_gc);
        }

        changed
    }

    pub(crate) fn set_live_range(&self, no_gc: &NoGC, new_range: Option<&Range>) {
        if new_range == self.live_range.get().as_deref() {
            return;
        }

        let boundaries_changed = self.set_range(no_gc, new_range.map(|new_range| new_range.into()));

        // It's possible that `set_range` was a no-op, but still in that case we need to
        // replace the live range per-specification.
        if let Some(old_range) = self.live_range.take() {
            old_range.disassociate_selection(self);
        }
        if let Some(new_range) = new_range {
            self.live_range.set(Some(new_range));
            new_range.associate_selection(self);
        }

        // From <https://w3c.github.io/selection-api/#selectionchange-event>:
        // > When the selection is dissociated with its range, associated with a new
        // > range, or the associated range's boundary point is mutated either by the user
        // > or the content script, the user agent must schedule a selectionchange event on
        // > document.
        //
        // This means we should fire the event even if the boundaries themselves did not change. A
        // change to the range object is enough. Normally, this happens in `set_range`, but
        // only when the boundaries changed. In this case the call to `set_range` above did
        // not queue the task.
        if !boundaries_changed {
            self.queue_selectionchange_task();
        }
    }

    fn selection_boundaries_changed(&self) {
        self.set_visible_selection_dirty();
        self.queue_selectionchange_task();

        // See:
        //  - <https://w3c.github.io/editing/docs/execCommand/#state-override> and
        //  - <https://w3c.github.io/editing/docs/execCommand/#value-override>
        //
        // > Whenever the number of ranges in the selection changes to something
        // > different, and whenever a boundary point of the range at a given index in the
        // > selection changes to something different, the state override and value
        // > override must be unset for every command.
        self.document.clear_command_overrides();
    }

    fn iter_nodes_with_overlaps_document_selection_flag<'no_gc>(
        &self,
        no_gc: &'no_gc NoGC,
    ) -> impl Iterator<Item = UnrootedDom<'no_gc, Node>> {
        let mut traversal = self
            .document
            .upcast::<Node>()
            .following_flat_tree_nodes_unrooted(no_gc);
        let mut next = traversal.next();
        std::iter::from_fn(move || {
            while let Some(node) = next.take() {
                match node {
                    PrePostIteration::Enter(node) => {
                        if node.get_flag(NodeFlags::OVERLAPS_DOCUMENT_SELECTION) {
                            next = traversal.next();
                            return Some(node);
                        } else {
                            // This relies on flags being set consistently: this node
                            // with the flag unset claims that no part of it overlaps selection,
                            // which implies that none of its descendant either have any part
                            // of them overlapping selection, meaning none of them have the flag
                            next = traversal.next_skipping_subtree();
                        }
                    },
                    PrePostIteration::Leave(_) => next = traversal.next(),
                }
            }
            None
        })
    }

    fn set_visible_range(&self, no_gc: &NoGC, flat_tree_selection: Option<FlatTreeSelection>) {
        *self.visible_range.safe_borrow_mut(no_gc) = flat_tree_selection.map(|selection| {
            SelectionRange::new(
                SelectionBoundary::new(&selection.start.container, selection.start.offset),
                SelectionBoundary::new(&selection.end.container, selection.end.offset),
            )
        });
    }

    pub(crate) fn update_overlaps_document_selection_flags(&self, no_gc: &NoGC) {
        if !self.visible_selection_dirty.take() {
            return;
        }

        let previously_flagged_nodes = self
            .iter_nodes_with_overlaps_document_selection_flag(no_gc)
            .collect();
        let flat_tree_selection = FlatTreeSelection::from_selection_if_renderable(no_gc, self);
        VisibleSelectionFlagUpdate::run(
            no_gc,
            previously_flagged_nodes,
            flat_tree_selection.as_ref(),
            &self.document,
        );
        self.set_visible_range(no_gc, flat_tree_selection);
    }

    /// <https://w3c.github.io/selection-api/#dfn-schedule-a-selectionchange-event>
    pub(crate) fn queue_selectionchange_task(&self) {
        // Step 1. If target's has scheduled selectionchange event is true, abort these steps.
        if self.has_scheduled_selectionchange_event.get() {
            return;
        }
        // Step 2. Set target's has scheduled selectionchange event to true.
        self.has_scheduled_selectionchange_event.set(true);
        // Step 3. Queue a task on the user interaction task source to fire a
        // selectionchange event on target.
        let this = Trusted::new(self);
        self.document
            .owner_global()
            .task_manager()
            .user_interaction_task_source() // w3c/selection-api#117
            .queue(
                // https://w3c.github.io/selection-api/#firing-selectionchange-event
                task!(selectionchange_task_steps: move |cx| {
                    let this = this.root();
                    // Step 1. Set target's has scheduled selectionchange event to false.
                    this.has_scheduled_selectionchange_event.set(false);
                    // Step 2. If target is an element, fire an event named
                    // selectionchange, which bubbles and not cancelable, at target.
                    //
                    // n/a

                    // Step 3. Otherwise, if target is a document, fire an event named
                    // selectionchange, which does not bubble and not cancelable, at
                    // target.
                    this.document.upcast::<EventTarget>().fire_event(cx, atom!("selectionchange"));
                }),
            );
    }

    fn is_in_composed_tree_of_document_and_is_not_ua_widget(&self, node: &Node) -> bool {
        &*node.GetRootNode(&GetRootNodeOptions { composed: true }) == self.document.upcast::<Node>() &&
            !node.is_in_ua_widget()
    }

    pub(crate) fn start_boundary(&self, cx: &mut JSContext) -> (DomRoot<Node>, u32) {
        let range = self.expect_active_range(cx);
        (range.start_container(), range.start_offset())
    }

    pub(crate) fn end_boundary(&self, cx: &mut JSContext) -> (DomRoot<Node>, u32) {
        let range = self.expect_active_range(cx);
        (range.end_container(), range.end_offset())
    }

    #[cfg(debug_assertions)]
    fn assert_valid_selection(&self, no_gc: &NoGC) {
        let range_borrow = self.range.borrow();
        let Some(range) = range_borrow.as_ref() else {
            return;
        };
        debug_assert_eq!(
            range
                .start
                .container
                .GetRootNode(&GetRootNodeOptions { composed: true }),
            range
                .end
                .container
                .GetRootNode(&GetRootNodeOptions { composed: true })
        );
        debug_assert!(
            compare_shadow_including_dom_positions(
                no_gc,
                &range.start.container,
                range.start.offset,
                &range.end.container,
                range.end.offset
            ) != Ordering::Greater
        );
    }

    #[cfg(debug_assertions)]
    fn assert_valid_selection_and_live_range(&self, no_gc: &NoGC) {
        self.assert_valid_selection(no_gc);

        let Some(active_range) = self.live_range.get() else {
            return;
        };
        debug_assert!(
            bp_position(
                no_gc,
                &active_range.start_container(),
                active_range.start_offset(),
                &active_range.end_container(),
                active_range.end_offset()
            ) != Ordering::Greater
        );
    }

    /// <https://w3c.github.io/editing/docs/execCommand/#active-range>
    ///
    /// > The active range is the range of the selection given by calling
    /// > getSelection() on the context object. (Thus the active range may be null.)
    pub(crate) fn active_range(&self, cx: &mut JSContext) -> Option<DomRoot<Range>> {
        #[cfg(debug_assertions)]
        self.assert_valid_selection_and_live_range(cx.no_gc());

        if let Some(active_range) = self.live_range.get() {
            return Some(active_range);
        }

        let live_range = {
            let range = self.range.borrow();
            let range = range.as_ref()?;
            let live_range = Range::new(
                cx,
                &self.document,
                &range.start.container,
                range.start.offset,
                &range.start.container,
                range.start.offset,
            );
            live_range
                .SetEnd(cx.no_gc(), &range.end.container, range.end.offset)
                .expect("New end boundary should always be valid");
            live_range
        };

        self.live_range.set(Some(&live_range));
        live_range.associate_selection(self);
        Some(live_range)
    }

    pub(crate) fn expect_active_range(&self, cx: &mut JSContext) -> DomRoot<Range> {
        self.active_range(cx)
            .expect("Should always have an active range")
    }

    pub(crate) fn set_visible_selection_dirty(&self) {
        self.visible_selection_dirty.set(true);
    }

    pub(crate) fn composed_anchor_position(&self) -> Option<RootedSelectionBoundary> {
        let range = self.range.borrow();
        let range = range.as_ref()?;
        let (container, offset) = match self.direction.get() {
            Direction::Forwards => (range.start.container.as_rooted(), range.start.offset),
            _ => (range.end.container.as_rooted(), range.end.offset),
        };
        Some(RootedSelectionBoundary::new_with_utf16_offset(
            container, offset,
        ))
    }

    /// <https://w3c.github.io/selection-api/#dfn-anchor>
    fn live_anchor_node(&self, cx: &mut JSContext) -> Option<DomRoot<Node>> {
        self.active_range(cx)
            .map(|range| match self.direction.get() {
                Direction::Forwards => range.start_container(),
                _ => range.end_container(),
            })
    }

    /// <https://w3c.github.io/selection-api/#dfn-anchor>
    fn live_anchor_offset(&self, cx: &mut JSContext) -> u32 {
        self.active_range(cx)
            .map_or(0, |range| match self.direction.get() {
                Direction::Forwards => range.start_offset(),
                _ => range.end_offset(),
            })
    }

    /// <https://w3c.github.io/selection-api/#dfn-focus>
    fn live_focus_node(&self, cx: &mut JSContext) -> Option<DomRoot<Node>> {
        self.active_range(cx)
            .map(|range| match self.direction.get() {
                Direction::Forwards => range.end_container(),
                _ => range.start_container(),
            })
    }

    /// <https://w3c.github.io/selection-api/#dfn-focus>
    fn live_focus_offset(&self, cx: &mut JSContext) -> u32 {
        self.active_range(cx)
            .map_or(0, |range| match self.direction.get() {
                Direction::Forwards => range.end_offset(),
                _ => range.start_offset(),
            })
    }

    /// <https://dom.spec.whatwg.org/#concept-node-insert> steps 5.1-5.2
    /// and
    /// <https://dom.spec.whatwg.org/#move> steps 17.1-17.2
    /// adapted for selections.
    pub(crate) fn insert_steps(&self, parent: &Node, child: &Node, count: u32) {
        let mut range_borrow = self.range.borrow_mut();
        let Some(range) = &mut *range_borrow else {
            return;
        };
        let child_index = LazyCell::new(|| child.index());
        // Step 5.1: For each live range whose start node is parent and start offset is
        // greater than child’s index: increase its start offset by count.
        if range.start.container == parent && range.start.offset > *child_index {
            range.start.offset += count;
            self.selection_boundaries_changed();
        }
        // Step 5.2: For each live range whose end node is parent and end offset is
        // greater than child’s index: increase its end offset by count.
        if range.end.container == parent && range.end.offset > *child_index {
            range.end.offset += count;
            self.selection_boundaries_changed();
        }
    }

    /// <https://dom.spec.whatwg.org/#live-range-pre-remove-steps> steps 4-7
    /// adapted for selections.
    pub(crate) fn pre_remove_steps(
        &self,
        removed_node: &Node,           // "node" in the specification
        parent_of_removed_node: &Node, // "parent" in the specification
        index_of_removed_node: &mut dyn FnMut() -> u32, // "index" in the specification
    ) {
        let mut range_borrow = self.range.borrow_mut();
        let Some(range) = &mut *range_borrow else {
            return;
        };

        // Step 4: For each live range whose start node is an inclusive descendant of
        // node, set its start to (parent, index).
        if removed_node.is_shadow_including_inclusive_ancestor_of(&range.start.container) {
            range.start = SelectionBoundary::new(parent_of_removed_node, index_of_removed_node());
            self.selection_boundaries_changed();
        }
        // Step 5: For each live range whose end node is an inclusive descendant of node,
        // set its end to (parent, index).
        if removed_node.is_shadow_including_inclusive_ancestor_of(&range.end.container) {
            range.end = SelectionBoundary::new(parent_of_removed_node, index_of_removed_node());
            self.selection_boundaries_changed();
        }
        // Step 6: For each live range whose start node is parent and start offset is
        // greater than index, decrease its start offset by 1.
        if range.start.container == parent_of_removed_node &&
            range.start.offset > index_of_removed_node()
        {
            range.start.offset -= 1;
            self.selection_boundaries_changed();
        }
        // Step 7: For each live range whose end node is parent and end offset is greater than
        // index, decrease its end offset by 1.
        if range.end.container == parent_of_removed_node &&
            range.end.offset > index_of_removed_node()
        {
            range.end.offset -= 1;
            self.selection_boundaries_changed();
        }
    }

    /// <https://dom.spec.whatwg.org/#dom-node-normalize> Steps 6.1-6.4 adapted for selections.
    ///
    /// - `parent`: The parent of both other node arguments.
    /// - `node`: The node that text is being merged into.
    /// - `current_node`: The node which has text being merged into `node` and will be
    ///   removed from the DOM.
    /// - `length`: The length of the text content that was merged into `node` from
    ///   siblings before `current_node`.
    pub(crate) fn normalization_steps(
        &self,
        parent: &Node,
        node: &Node,
        current_node: &Node,
        current_node_index: &dyn Fn() -> u32,
        length: u32,
    ) {
        let mut range_borrow = self.range.borrow_mut();
        let Some(range) = &mut *range_borrow else {
            return;
        };
        // Step 6.1: For each live range whose start node is currentNode: add length to its start
        // offset and set its start node to node.
        if range.start.container == current_node {
            range.start.offset += length;
            range.start.container = Dom::from_ref(node);
            self.selection_boundaries_changed();
        }
        // Step 6.2 For each live range whose end node is currentNode: add length to its end offset
        // and set its end node to node.
        if range.end.container == current_node {
            range.end.offset += length;
            range.end.container = Dom::from_ref(node);
            self.selection_boundaries_changed();
        }
        // Step 6.3: For each live range whose start node is currentNode’s parent and start
        // offset is currentNode’s index: set its start node to node and its start offset
        // to length.
        if range.start.container == parent && range.start.offset == current_node_index() {
            range.start.container = Dom::from_ref(node);
            range.start.offset = length;
            self.selection_boundaries_changed();
        }
        // Step 6.4: For each live range whose end node is currentNode’s parent and end offset is
        // currentNode’s index: set its end node to node and its end offset to length.
        if range.end.container == parent && range.end.offset == current_node_index() {
            range.end.container = Dom::from_ref(node);
            range.end.offset = length;
            self.selection_boundaries_changed();
        }
    }

    /// <https://dom.spec.whatwg.org/#concept-cd-replace> steps 8-11
    /// adapted for selections.
    pub(crate) fn replace_data_steps(
        &self,
        node: &Node,
        offset: u32,
        removed_code_units: u32,
        added_code_units: &mut dyn FnMut() -> u32,
    ) {
        let mut range_borrow = self.range.borrow_mut();
        let Some(range) = &mut *range_borrow else {
            return;
        };
        // Step 8: For each live range whose start node is node and start offset is
        // greater than offset but less than or equal to offset + count: set its start
        // offset to offset.
        let start_container = &range.start.container;
        let start_offset = range.start.offset;
        if &**start_container == node &&
            start_offset > offset &&
            start_offset <= offset + removed_code_units
        {
            range.start.offset = offset;
            self.selection_boundaries_changed();
        }
        // Step 9: For each live range whose end node is node and end offset is
        // greater than offset but less than or equal to offset + count: set its end
        // offset to offset.
        let end_container = &range.end.container;
        let end_offset = range.end.offset;
        if &**end_container == node &&
            end_offset > offset &&
            end_offset <= offset + removed_code_units
        {
            range.end.offset = offset;
            self.selection_boundaries_changed();
        }
        // Step 10: For each live range whose start node is node and start offset is
        // greater than offset + count: increase its start offset by data’s length and
        // decrease it by count.
        if &**start_container == node && start_offset > offset + removed_code_units {
            range.start.offset = start_offset + added_code_units() - removed_code_units;
            self.selection_boundaries_changed();
        }
        // Step 11: For each live range whose end node is node and end offset is
        // greater than offset + count: increase its end offset by data’s length and
        // decrease it by count.
        if &**end_container == node && end_offset > offset + removed_code_units {
            range.end.offset = end_offset + added_code_units() - removed_code_units;
            self.selection_boundaries_changed();
        }
    }

    /// <https://dom.spec.whatwg.org/#concept-text-split> steps 7.2-7.3
    /// adapted for selections.
    pub(crate) fn text_split_steps(
        &self,
        node: &Node,
        offset: u32,
        parent_node: &Node,
        new_node: &Node,
    ) {
        let mut range_borrow = self.range.borrow_mut();
        let Some(range) = &mut *range_borrow else {
            return;
        };
        // Step 7.2: For each live range whose start node is node and start offset is
        // greater than offset, set its start node to newNode and decrease its start
        // offset by offset.
        if range.start.container == node && range.start.offset > offset {
            range.start.container = Dom::from_ref(new_node);
            range.start.offset -= offset;
            self.selection_boundaries_changed();
        }
        // Step 7.3: For each live range whose end node is node and end offset is greater
        // than offset, set its end node to newNode and decrease its end offset by offset.
        if range.end.container == node && range.end.offset > offset {
            range.end.container = Dom::from_ref(new_node);
            range.end.offset -= offset;
            self.selection_boundaries_changed();
        }
        // Step 7.4: For each live range whose start node is parent and start offset is
        // equal to the index of node plus 1, increase its start offset by 1.
        let node_index = LazyCell::new(|| node.index());
        if range.start.container == parent_node && range.start.offset == *node_index + 1 {
            range.start.offset += 1;
            self.selection_boundaries_changed();
        }
        // Step 7.5: For each live range whose end node is parent and end offset is equal
        // to the index of node plus 1, increase its end offset by 1.
        if range.end.container == parent_node && range.end.offset == *node_index + 1 {
            range.end.offset += 1;
            self.selection_boundaries_changed();
        }
    }

    pub(crate) fn collapse_to_dom_position(
        &self,
        cx: &mut JSContext,
        boundary: &RootedSelectionBoundary,
    ) {
        let _ = self.Collapse(cx, Some(&boundary.container), boundary.offset);
    }

    pub(crate) fn collapse_or_extend_to_dom_position(
        &self,
        cx: &mut JSContext,
        boundary: &RootedSelectionBoundary,
    ) {
        let is_anchor = self
            .composed_anchor_position()
            .is_some_and(|anchor| anchor == *boundary);

        if self.range.borrow().is_none() || is_anchor {
            let _ = self.Collapse(cx, Some(&boundary.container), boundary.offset);
        } else {
            let _ = self.Extend(cx, &boundary.container, boundary.offset);
        }
    }

    /// Call [`Selection::SetBaseAndExtent`] with the given `anchor` and `focus`
    /// arguments. Like `SetBaseAndExtent` the selection direction is determined by the
    /// order of `anchor` and `focus` in the DOM.
    pub(crate) fn set_anchor_and_focus(
        &self,
        cx: &mut JSContext,
        anchor: &RootedSelectionBoundary,
        focus: &RootedSelectionBoundary,
    ) {
        let _ = self.SetBaseAndExtent(
            cx,
            &anchor.container,
            anchor.offset,
            &focus.container,
            focus.offset,
        );
    }
}

impl SelectionMethods<crate::DomTypeHolder> for Selection {
    /// <https://w3c.github.io/selection-api/#dom-selection-anchornode>
    fn GetAnchorNode(&self, cx: &mut JSContext) -> Option<DomRoot<Node>> {
        // > The attribute must return the anchor node of this, or null if the anchor is
        // > null or anchor is not in the document tree.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        self.live_anchor_node(cx)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-anchoroffset>
    fn AnchorOffset(&self, cx: &mut JSContext) -> u32 {
        // > The attribute must return the anchor offset of this, or 0 if the anchor is null
        // > or anchor is not in the document tree.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        self.live_anchor_offset(cx)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-focusnode>
    fn GetFocusNode(&self, cx: &mut JSContext) -> Option<DomRoot<Node>> {
        // > The attribute must return the focus node of this, or null if the focus is
        // > null or focus is not in the document tree.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        self.live_focus_node(cx)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-focusoffset>
    fn FocusOffset(&self, cx: &mut JSContext) -> u32 {
        // > The attribute must return the focus offset of this, or 0 if the focus is null
        // > or focus is not in the document tree.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        self.live_focus_offset(cx)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-iscollapsed>
    fn IsCollapsed(&self, cx: &mut JSContext) -> bool {
        // > The attribute must return true if and only if the anchor and focus are the
        // > same (including if both are null). Otherwise it must return false.
        self.active_range(cx).is_none_or(|range| range.collapsed())
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-rangecount>
    fn RangeCount(&self) -> u32 {
        // > The attribute must return 0 if this is empty or either focus or anchor is not
        // > in the document tree, and must return 1 otherwise.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        if self.range.borrow().is_none() {
            return 0;
        }
        1
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-type>
    fn Type(&self) -> DOMString {
        // > The attribute must return "None" if this is empty or either focus or anchor
        // > is not in the document tree, "Caret" if this's range is collapsed, and "Range"
        // > otherwise.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        let range = self.range.borrow();
        let Some(range) = range.as_ref() else {
            return DOMString::from_static("None");
        };
        if range.collapsed() {
            DOMString::from_static("Caret")
        } else {
            DOMString::from_static("Range")
        }
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-direction>
    fn Direction(&self) -> DOMString {
        // > The attribute must return "none" if this is empty or this selection is
        // > directionless. "forward" if this selection's direction is forwards and
        // > "backward" if this selection's direction is backwards.
        match self.direction.get() {
            Direction::Directionless => DOMString::from_static("none"),
            Direction::Forwards => DOMString::from_static("forward"),
            Direction::Backwards => DOMString::from_static("backward"),
        }
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-getrangeat>
    fn GetRangeAt(&self, cx: &mut JSContext, index: u32) -> Fallible<DomRoot<Range>> {
        // > The method must throw an IndexSizeError exception if index is not 0, or if this
        // > is empty or either focus or anchor is not in the document tree. Otherwise, it
        // > must return a reference to (not a copy of) this's range.
        //
        // See <https://github.com/w3c/selection-api/issues/361> for why we don't
        // do the document tree check (other browsers don't either).
        if index != 0 {
            return Err(Error::IndexSize(Some("Index must be zero".into())));
        }

        let Some(range) = self.active_range(cx) else {
            return Err(Error::IndexSize(Some("Selection is empty".into())));
        };

        Ok(range)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-addrange>
    fn AddRange(&self, no_gc: &NoGC, range: &Range) {
        // Step 1. If the root of the range's boundary points are not the document
        // associated with this, abort these steps.
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(&range.start_container()) {
            return;
        }

        // Step 2. If rangeCount is not 0, abort these steps.
        if self.RangeCount() != 0 {
            return;
        }

        // Step 3. Set this's range to range by a strong reference (not by making a copy).
        self.set_live_range(no_gc, Some(range));

        // Are we supposed to set Direction here? w3c/selection-api#116
        self.direction.set(Direction::Forwards);
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-removerange>
    fn RemoveRange(&self, no_gc: &NoGC, range: &Range) -> ErrorResult {
        // > The method must make this empty by disassociating its range if this's range
        // > is range. Otherwise, it must throw a NotFoundError.
        if let Some(own_range) = self.live_range.get() &&
            &*own_range == range
        {
            self.set_range(no_gc, None);
            return Ok(());
        }
        Err(Error::NotFound(Some(
            "Range is not the selection's range".into(),
        )))
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-removeallranges>
    fn RemoveAllRanges(&self, no_gc: &NoGC) {
        // > The method must make this empty by disassociating its range if this has an
        // > associated range.
        self.set_range(no_gc, None);
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-empty>
    fn Empty(&self, no_gc: &NoGC) {
        // > The method must be an alias, and behave identically, to removeAllRanges().
        self.RemoveAllRanges(no_gc);
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-getcomposedranges>
    fn GetComposedRanges(
        &self,
        cx: &mut JSContext,
        options: &GetComposedRangesOptions,
    ) -> Vec<DomRoot<StaticRange>> {
        // Step 1. If this is empty, return an empty array.
        let borrowed_range = self.range.borrow();
        let Some(range) = borrowed_range.as_ref() else {
            return Vec::new();
        };

        // Step 2. Otherwise, let startNode be start node of the range associated with
        // this, and let startOffset be start offset of the range.
        let mut start_node = range.start.container.as_rooted();
        let mut start_offset = range.start.offset;

        let is_ancestor_of_provided_shadow_roots = |shadow_root: &ShadowRoot| {
            let shadow_root_node = shadow_root.upcast::<Node>();
            options.shadowRoots.iter().any(|option_shadow_root| {
                shadow_root_node
                    .is_shadow_including_inclusive_ancestor_of(option_shadow_root.upcast())
            })
        };

        // Step 3. While startNode is a node, startNode's root is a shadow root, and
        // startNode's root is not a shadow-including inclusive ancestor of any of
        // options["shadowRoots"], repeat these steps:
        while let Some(containing_shadow_root) = start_node.containing_shadow_root() &&
            !is_ancestor_of_provided_shadow_roots(&containing_shadow_root)
        {
            // Step 3.1. Set startOffset to index of startNode's root's host.
            let host = DomRoot::upcast::<Node>(containing_shadow_root.Host());
            start_offset = host.index();

            // Step 3.2. Set startNode to startNode's root's host's parent.
            // See <https://github.com/w3c/selection-api/issues/161> for why
            // we always know that the start_node is a node.
            let Some(new_start_node) = host.GetParentNode() else {
                return Vec::new();
            };
            start_node = new_start_node;
        }

        // Step 4. Let endNode be end node of the range associated with this, and let
        // endOffset be end offset of the range.
        let mut end_node = range.end.container.as_rooted();
        let mut end_offset = range.end.offset;

        // Step 5. While endNode is a node, endNode's root is a shadow root, and endNode's
        // root is not a shadow-including inclusive ancestor of any of
        // options["shadowRoots"], repeat these steps:
        while let Some(containing_shadow_root) = end_node.containing_shadow_root() &&
            !is_ancestor_of_provided_shadow_roots(&containing_shadow_root)
        {
            // Step 5.1. Set endOffset to index of endNode's root's host plus 1.
            let host = DomRoot::upcast::<Node>(containing_shadow_root.Host());
            end_offset = host.index() + 1;

            // Step 5.2. Set endNode to endNode's root's host's parent.
            // See <https://github.com/w3c/selection-api/issues/161> for why
            // we always know that the end_node is a node.
            let Some(new_end_node) = host.GetParentNode() else {
                return Vec::new();
            };
            end_node = new_end_node;
        }

        drop(borrowed_range);

        // Step 6. Return an array consisting of new StaticRange whose start node is
        // startNode, start offset is startOffset, end node is endNode, and end offset is
        // endOffset.
        vec![DomRoot::from_ref(&StaticRange::new(
            cx,
            &self.document,
            &start_node,
            start_offset,
            &end_node,
            end_offset,
        ))]
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-collapse>
    fn Collapse(&self, cx: &mut JSContext, node: Option<&Node>, offset: u32) -> ErrorResult {
        // Step 1. If node is null, this method must behave identically as
        // removeAllRanges() and abort these steps.
        let Some(node) = node else {
            self.set_range(cx.no_gc(), None);
            return Ok(());
        };

        // Step 2. If node is a DocumentType, throw an InvalidNodeTypeError exception and
        // abort these steps.
        if node.is_doctype() {
            return Err(Error::InvalidNodeType(Some("Node is a doctype".into())));
        }

        // Step 3. The method must throw an IndexSizeError exception if offset is longer
        // than node's length and abort these steps.
        if offset > node.len() {
            return Err(Error::IndexSize(Some(
                "Offset is greater than the node's length".into(),
            )));
        }

        // Step 4. If document associated with this is not a shadow-including inclusive
        // ancestor of node, abort these steps.
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(node) {
            return Ok(());
        }

        // Step 5. Otherwise, let newRange be a new range.
        // Step 6. Set the start the start and the end of newRange to (node, offset).
        // Step 7. Set this's range to newRange.
        self.set_range(
            cx.no_gc(),
            Some(SelectionRange::collapsed_at(SelectionBoundary::new(
                node, offset,
            ))),
        );

        // Are we supposed to set Direction here? w3c/selection-api#116
        self.direction.set(Direction::Forwards);

        Ok(())
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-setposition>
    fn SetPosition(&self, cx: &mut JSContext, node: Option<&Node>, offset: u32) -> ErrorResult {
        // > The method must be an alias, and behave identically, to collapse().
        self.Collapse(cx, node, offset)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-collapsetostart>
    fn CollapseToStart(&self, cx: &mut JSContext) -> ErrorResult {
        // > The method must throw InvalidStateError exception if the this is empty.
        // > Otherwise, it must create a new range, set the start both its start and end to
        // > the start of this's range, and then set this's range to the newly-created
        // > range.
        let Some((start_container, start_offset)) = self
            .range
            .borrow()
            .as_ref()
            .map(|range| (range.start.container.as_rooted(), range.start.offset))
        else {
            return Err(Error::InvalidState(Some("Selection is empty".into())));
        };
        self.Collapse(cx, Some(&*start_container), start_offset)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-collapsetoend>
    fn CollapseToEnd(&self, cx: &mut JSContext) -> ErrorResult {
        // > The method must throw InvalidStateError exception if the this is empty.
        // > Otherwise, it must create a new range, set the start both its start and end to
        // > the end of this's range, and then set this's range to the newly-created range.
        let Some((end_container, end_offset)) = self
            .range
            .borrow()
            .as_ref()
            .map(|range| (range.end.container.as_rooted(), range.end.offset))
        else {
            return Err(Error::InvalidState(Some("Selection is empty".into())));
        };
        self.Collapse(cx, Some(&*end_container), end_offset)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-extend>
    fn Extend(&self, cx: &mut JSContext, node: &Node, offset: u32) -> ErrorResult {
        // Step 1. If the document associated with this is not a shadow-including
        // inclusive ancestor of node, abort these steps.
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(node) {
            return Ok(());
        }

        // Step 2. If this is empty, throw an InvalidStateError exception and abort these steps.
        let range_borrow = self.range.borrow();
        let Some(range) = range_borrow.as_ref() else {
            return Err(Error::InvalidState(Some("Selection is empty".into())));
        };

        // This isn't specified, but it appears to be implementation behavior of other
        // browsers. See w3c/selection-api#118.
        if node.is_doctype() {
            return Err(Error::InvalidNodeType(Some("Node is a doctype".into())));
        }

        // As with is_doctype, this is not explicit in the selection specification steps
        // here but implied by which exceptions are thrown in WPT tests.
        if offset > node.len() {
            return Err(Error::IndexSize(Some(
                "Offset is greater than the node's length".into(),
            )));
        }

        // Step 3. Let oldAnchor and oldFocus be the this's anchor and focus, and let
        // newFocus be the boundary point (node, offset).
        //
        // Note: oldFocus is unused, so we do not set it here.
        let old_anchor = self
            .composed_anchor_position()
            .expect("has range, therefore has anchor node");

        // Step 4. Let newRange be a new range.
        // Note: Set directly to satisfy crown.
        let direction;

        // Step 5. If node's root is not the same as the this's range's root, set the
        // start newRange's start and end to newFocus.
        let in_different_roots = !nodes_have_same_shadow_root(&range.start.container, node);
        drop(range_borrow);

        if in_different_roots {
            self.set_range(
                cx.no_gc(),
                Some(SelectionRange::collapsed_at(SelectionBoundary::new(
                    node, offset,
                ))),
            );
            direction = Direction::Forwards;
        } else {
            let is_old_anchor_before_or_equal = matches!(
                compare_shadow_including_dom_positions(
                    cx.no_gc(),
                    &old_anchor.container,
                    old_anchor.offset,
                    node,
                    offset
                ),
                Ordering::Less | Ordering::Equal
            );
            if is_old_anchor_before_or_equal {
                // Step 6. Otherwise, if oldAnchor is before or equal to newFocus, set the start
                // newRange's start to oldAnchor, then set its end to newFocus.
                self.set_range(
                    cx.no_gc(),
                    Some(SelectionRange::new(
                        SelectionBoundary::new(&old_anchor.container, old_anchor.offset),
                        SelectionBoundary::new(node, offset),
                    )),
                );
                direction = Direction::Forwards;
            } else {
                // Step 7. Otherwise, set the start newRange's start to newFocus, then set
                // its end to oldAnchor.
                self.set_range(
                    cx.no_gc(),
                    Some(SelectionRange::new(
                        SelectionBoundary::new(node, offset),
                        SelectionBoundary::new(&old_anchor.container, old_anchor.offset),
                    )),
                );
                direction = Direction::Backwards;
            }
        }

        // Step 8. Set this's range to newRange.
        // Note: Done above to satisfy crown.

        // Step 9. If newFocus is before oldAnchor, set this's direction to backwards.
        // Otherwise, set it to forwards.
        self.direction.set(direction);

        Ok(())
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-setbaseandextent>
    fn SetBaseAndExtent(
        &self,
        cx: &mut JSContext,
        anchor_node: &Node,
        anchor_offset: u32,
        focus_node: &Node,
        focus_offset: u32,
    ) -> ErrorResult {
        // This isn't specified, but it appears to be implementation behavior of other
        // browsers. See w3c/selection-api#118.
        if anchor_node.is_doctype() || focus_node.is_doctype() {
            return Err(Error::InvalidNodeType(Some("Node is a doctype".into())));
        }

        // Step 1. If anchorOffset is longer than anchorNode's length or if focusOffset is
        // longer than focusNode's length, throw an IndexSizeError exception and abort
        // these steps.
        if anchor_offset > anchor_node.len() || focus_offset > focus_node.len() {
            return Err(Error::IndexSize(Some(
                "Offset is greater than the node's length".into(),
            )));
        }

        // Step 2. If document associated with this is not a shadow-including inclusive
        // ancestor of anchorNode or focusNode, abort these steps.
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(anchor_node) ||
            !self.is_in_composed_tree_of_document_and_is_not_ua_widget(focus_node)
        {
            return Ok(());
        }

        // Step 3. Let anchor be the boundary point (anchorNode, anchorOffset) and let
        // focus be the boundary point (focusNode, focusOffset).
        //
        // Note: We do not model the boundary point in this way.

        // Step 4. Let newRange be a new range.
        // Note: We set the range directly to satisfy crown.

        // Step 5. If anchor is before focus, set the start the newRange's start to anchor
        // and its end to focus. Otherwise, set the start them to focus and anchor
        // respectively.
        let ordering = compare_shadow_including_dom_positions(
            cx.no_gc(),
            anchor_node,
            anchor_offset,
            focus_node,
            focus_offset,
        );
        if ordering == Ordering::Less {
            self.set_range(
                cx.no_gc(),
                Some(SelectionRange::new(
                    SelectionBoundary::new(anchor_node, anchor_offset),
                    SelectionBoundary::new(focus_node, focus_offset),
                )),
            );
        } else {
            self.set_range(
                cx.no_gc(),
                Some(SelectionRange::new(
                    SelectionBoundary::new(focus_node, focus_offset),
                    SelectionBoundary::new(anchor_node, anchor_offset),
                )),
            );
        };

        // Step 6. Set this's range to newRange.
        // Note: Done above to satisfy crown.

        // Step 7. If focus is before anchor, set this's direction to backwards.
        // Otherwise, set it to forwards
        if ordering == Ordering::Greater {
            self.direction.set(Direction::Backwards);
        } else {
            self.direction.set(Direction::Forwards);
        }

        Ok(())
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-selectallchildren>
    fn SelectAllChildren(&self, cx: &mut JSContext, node: &Node) -> ErrorResult {
        // Step 1. If node is a DocumentType, throw an InvalidNodeTypeError exception and
        // abort these steps.
        if node.is_doctype() {
            return Err(Error::InvalidNodeType(Some("Node is a doctype".into())));
        }

        // Step 2. If node's root is not the document associated with this, abort these
        // steps.
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(node) {
            return Ok(());
        }

        // Let newRange be a new range and childCount be the number of children of node.
        let child_count = node.children_count();

        // Step 4. Set newRange's start to (node, 0).
        // Step 5. Set newRange's end to (node, childCount).
        // Step 6. Set this's range to newRange.
        self.set_range(
            cx.no_gc(),
            Some(SelectionRange::new(
                SelectionBoundary::new(node, 0),
                SelectionBoundary::new(node, child_count),
            )),
        );

        // Step 7. Set this's direction to forwards.
        self.direction.set(Direction::Forwards);

        Ok(())
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-deletecontents>
    fn DeleteFromDocument(&self, cx: &mut JSContext) -> ErrorResult {
        // > The method must invoke deleteContents() on this's range if this is not empty
        // > and both focus and anchor are in the document tree. Otherwise the method must
        // > do nothing.
        let Some(active_range) = self
            .active_range(cx)
            .filter(|range| range.start_and_end_are_in_document_tree())
        else {
            return Ok(());
        };

        active_range.DeleteContents(cx)
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-containsnode>
    fn ContainsNode(&self, no_gc: &NoGC, node: &Node, allow_partial_containment: bool) -> bool {
        // > The method must return false if this is empty or if node's root is not the document
        // > associated with this.
        // >
        // > Otherwise, if allowPartialContainment is false, the method must return true if and only
        // > if start of its range is before or visually equivalent to the first boundary point in
        // > the node *and* end of its range is after or visually equivalent to the last boundary
        // > point in the node.
        // >
        // > If allowPartialContainment is true, the method must return true if and only if start of
        // > its range is before or visually equivalent to the last boundary point in the node *and*
        // > end of its range is after or visually equivalent to the first boundary point in the
        // > node.
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(node) {
            return false;
        }
        let range = self.range.borrow();
        let Some(range) = range.as_ref() else {
            return false;
        };
        let start_node = &*range.start.container;
        if !self.is_in_composed_tree_of_document_and_is_not_ua_widget(start_node) {
            return false;
        }
        let end_node = &*range.end.container;

        let first_offset = 0;
        let last_offset = node.len();
        let (compare_start_to, compare_end_to) = if allow_partial_containment {
            (last_offset, first_offset)
        } else {
            (first_offset, last_offset)
        };

        // TODO: find out what "visually equivalent" means for boundary points and implement it.
        // https://github.com/w3c/selection-api/issues/6
        //
        // For now it is simplified to "node is in the flat tree" and "position is equal".
        if !node.is_in_flat_tree(no_gc) {
            return false;
        }

        matches!(
            compare_shadow_including_dom_positions(
                no_gc,
                start_node,
                range.start.offset,
                node,
                compare_start_to
            ),
            Ordering::Less | Ordering::Equal
        ) && matches!(
            compare_shadow_including_dom_positions(
                no_gc,
                end_node,
                range.end.offset,
                node,
                compare_end_to
            ),
            Ordering::Greater | Ordering::Equal
        )
    }

    /// <https://w3c.github.io/selection-api/#dom-selection-stringifier>
    fn Stringifier(&self, cx: &mut JSContext) -> DOMString {
        // Flush all layout before stringifying so that rendered text is up-to-date.
        self.document
            .window()
            .layout_reflow(cx, QueryMsg::StyleQuery);

        // > The stringification must return the string, which is the concatenation of the
        // > rendered text if there is a range associated with this.
        // >
        // > If the selection is within a textarea or input element, it must return the
        // > selected substring in its value.
        let Some(visible_selection) =
            FlatTreeSelection::from_selection_if_renderable(cx.no_gc(), self)
        else {
            return DOMString::new();
        };

        let mut user_select_cache = Default::default();
        let mut string = DOMString::new();
        for node in visible_selection.traversal() {
            let Some(character_data) = node.downcast::<CharacterData>() else {
                continue;
            };

            if node.used_user_select(cx.no_gc(), &mut user_select_cache) == UsedUserSelect::None {
                continue;
            }

            let range = visible_selection.range_for_character_data(character_data);
            let Some(text) = character_data.rendered_text(range) else {
                continue;
            };
            string.push_str(&text);
        }

        string
    }
}

impl<'dom> LayoutDom<'dom, Selection> {
    pub(crate) fn range_for_layout(&self) -> Option<AtomicRef<'dom, SelectionRange>> {
        AtomicRef::filter_map(self.unsafe_get().visible_range.borrow(), Option::as_ref)
    }
}

enum FlatTreeNodePosition {
    Before(DomRoot<Node>),
    Inside(DomRoot<Node>),
    After(DomRoot<Node>),
}

impl FlatTreeNodePosition {
    fn node(&self) -> &Node {
        match self {
            FlatTreeNodePosition::Before(node) => node,
            FlatTreeNodePosition::Inside(node) => node,
            FlatTreeNodePosition::After(node) => node,
        }
    }
}

/// Find the position of a node and offset in the flat tree for the purposes of selection
/// boundaries. This projects the given position onto the flat tree, accounting for origin
/// nodes that may not actually be in the flat tree at all.
fn position_in_flat_tree_for_selection(
    no_gc: &NoGC,
    boundary: &FlatTreeBoundary,
) -> FlatTreeNodePosition {
    if boundary.container.is::<CharacterData>() {
        return FlatTreeNodePosition::Inside(boundary.container.as_rooted());
    }

    let shadow_host_or_node = |node: &Node| {
        boundary
            .container
            .downcast::<ShadowRoot>()
            .map(|shadow_root| DomRoot::upcast(shadow_root.Host()))
            .unwrap_or(DomRoot::from_ref(node))
    };

    if let Some(child) = boundary.container.children().nth(boundary.offset as usize) {
        if let FlatTreeParent::Parent(_) = child.parent_in_flat_tree(no_gc) {
            return FlatTreeNodePosition::Before(child);
        }
    } else if let Some(last_child) = boundary.container.GetLastChild() &&
        let FlatTreeParent::Parent(_) = last_child.parent_in_flat_tree(no_gc)
    {
        return FlatTreeNodePosition::After(shadow_host_or_node(&boundary.container.as_rooted()));
    }

    // The container has no child in the flat tree or the child indicated by the index
    // isn't in the flat tree, so just return a position inside that container.
    FlatTreeNodePosition::Inside(shadow_host_or_node(&boundary.container.as_rooted()))
}

impl Node {
    /// Get the `Utf16CodeUnits` offset for the given offset if `self` is a
    /// `CharacterData` or else return the offset in the child list.
    pub(crate) fn to_sibling_or_utf16_offset(&self, offset: Utf32CodeUnitsOrNodeOffset) -> u32 {
        if let Some(character_data) = self.downcast::<CharacterData>() {
            // TODO: ensure that each `CharacterData` holds no more than 4 GiB?
            offset
                .to_utf16_code_units_in(AssumeUnder4GB, &character_data.data())
                .0
        } else {
            offset.0
        }
    }

    /// Get the `Utf32CodeUnitsOrNodeOffset` for `self` and the given UTF-16 offset if
    /// `self` is `CharacterData` or else return the offset in the child list.
    pub(crate) fn to_sibling_or_utf32_offset(&self, offset: u32) -> Utf32CodeUnitsOrNodeOffset {
        Utf32CodeUnitsOrNodeOffset(
            if let Some(character_data) = self.downcast::<CharacterData>() {
                // TODO: ensure that each `CharacterData` holds no more than 4 GiB?
                Utf16CodeUnits(offset)
                    .to_utf32_code_units_in(&character_data.data())
                    .0
            } else {
                offset
            },
        )
    }
}

bitflags! {
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub(crate) struct SelectionLiveRangeNotification: u8 {
        const Start = 1 << 0;
        const End = 1 << 1;
    }
}

fn nodes_have_same_shadow_root(a: &Node, b: &Node) -> bool {
    a.is_connected() && b.is_connected() && a.containing_shadow_root() == b.containing_shadow_root()
}

/// Project the position given by `container_a` and `offset_a` into a root that also
/// contains `target`. If `target` is not supplied then project the position into the
/// `Document` root.
///
/// This does something very similar to GetComposedRanges, but doesn't have special
/// handling for start and end nodes.
fn project_into_shared_tree<'a>(
    no_gc: &'a NoGC,
    container_a: &Node,
    mut offset_a: u32,
    target: Option<&Node>,
) -> (UnrootedDom<'a, Node>, u32) {
    let mut start_node = UnrootedDom::from_ref(container_a, no_gc);
    while let Some(containing_shadow_root) = start_node.containing_shadow_root_unrooted(no_gc) &&
        target.is_none_or(|target| {
            !containing_shadow_root
                .upcast::<Node>()
                .is_shadow_including_inclusive_ancestor_of(target)
        })
    {
        let host = DomRoot::upcast::<Node>(containing_shadow_root.Host());
        offset_a = host.index();
        start_node = host
            .get_parent_node_unrooted(no_gc)
            .expect("Should only be called on nodes in the same tree");
    }

    (start_node, offset_a)
}

/// Compare two shadow-including DOM positions by first projecting them into
/// a shared root and then comparing the two positions. This function assumes
/// that the two positions share a [`Document`].
pub(crate) fn compare_shadow_including_dom_positions(
    no_gc: &NoGC,
    container_a: &Node,
    offset_a: u32,
    container_b: &Node,
    offset_b: u32,
) -> Ordering {
    let (projected_container_a, projected_offset_a) =
        project_into_shared_tree(no_gc, container_a, offset_a, Some(container_b));
    let (projected_container_b, projected_offset_b) =
        project_into_shared_tree(no_gc, container_b, offset_b, Some(container_a));

    let a_was_projected = &**projected_container_a != container_a;
    let b_was_projected = &**projected_container_b != container_b;

    match bp_position(
        no_gc,
        &projected_container_a,
        projected_offset_a,
        &projected_container_b,
        projected_offset_b,
    ) {
        Ordering::Equal if a_was_projected && !b_was_projected => Ordering::Greater,
        Ordering::Equal if !a_was_projected && b_was_projected => Ordering::Less,
        ordering => ordering,
    }
}

struct FlatTreeBoundary<'no_gc> {
    container: UnrootedDom<'no_gc, Node>,
    offset: u32,
}

struct FlatTreeSelection<'no_gc> {
    no_gc: &'no_gc NoGC,
    start: FlatTreeBoundary<'no_gc>,
    end: FlatTreeBoundary<'no_gc>,
}

impl<'no_gc> FlatTreeSelection<'no_gc> {
    /// Get the [`FlatTreeSelection`] for this [`Selection`], returning `None` if the [`Selection`]
    /// doesn't have a range or if that flat tree range is unrenderable (for instance, because one
    /// or both of the endpoints is not in the flat tree).
    fn from_selection_if_renderable(no_gc: &'no_gc NoGC, selection: &Selection) -> Option<Self> {
        let range = selection.range.borrow();
        let range = range.as_ref()?;

        let mut start_container = range.start.container.as_unrooted(no_gc);
        let mut start_offset = range.start.offset;
        let mut end_container = range.end.container.as_unrooted(no_gc);
        let mut end_offset = range.end.offset;

        if !start_container.is_in_flat_tree(no_gc) {
            return None;
        }
        if !end_container.is_in_flat_tree(no_gc) {
            return None;
        }

        // Compare the position of the two nodes in the flat tree, considering the
        // condition when they don't share a common ancestor to be an unrenderable
        // selection.
        let ordering = compare_dom_positions::<FlatTreeForSelectionNoGcTraversal>(
            no_gc,
            &range.start.container,
            range.start.offset,
            &range.end.container,
            range.end.offset,
        )
        .0?;

        // If the visible selection is inverted relative to the composed tree range, swap
        // the boundaries.
        if ordering == Ordering::Greater {
            std::mem::swap(&mut start_container, &mut end_container);
            std::mem::swap(&mut start_offset, &mut end_offset);
        }

        Some(FlatTreeSelection {
            no_gc,
            start: FlatTreeBoundary {
                container: start_container,
                offset: start_offset,
            },
            end: FlatTreeBoundary {
                container: end_container,
                offset: end_offset,
            },
        })
    }

    fn range_for_character_data(&self, character_data: &CharacterData) -> RangeAny<Utf32CodeUnits> {
        let text = character_data.data();
        let node: &Node = character_data.upcast();
        RangeAny::new(
            (node == &**self.start.container)
                .then(|| Utf16CodeUnits(self.start.offset).to_utf32_code_units_in(&text)),
            (node == &**self.end.container)
                .then(|| Utf16CodeUnits(self.end.offset).to_utf32_code_units_in(&text)),
        )
    }

    fn traversal(&self) -> VisibleSelectionTraversal<'no_gc> {
        let start_position = position_in_flat_tree_for_selection(self.no_gc, &self.start);
        let end_position = position_in_flat_tree_for_selection(self.no_gc, &self.end);
        VisibleSelectionTraversal {
            following: start_position
                .node()
                .following_flat_tree_nodes_unrooted(self.no_gc),
            start: start_position,
            end: end_position,
            finished: false,
            skip_subtree: false,
        }
    }
}

struct VisibleSelectionTraversal<'no_gc> {
    following: UnrootedFollowingFlatTreeNodesTraversal<'no_gc>,
    start: FlatTreeNodePosition,
    end: FlatTreeNodePosition,
    finished: bool,
    skip_subtree: bool,
}

impl<'no_gc> Iterator for VisibleSelectionTraversal<'no_gc> {
    type Item = UnrootedDom<'no_gc, Node>;

    fn next(&mut self) -> Option<Self::Item> {
        while !self.finished {
            let following = if std::mem::take(&mut self.skip_subtree) {
                self.following.next_skipping_subtree()?
            } else {
                self.following.next()?
            };

            match following {
                PrePostIteration::Enter(node) => {
                    // If the traversal ends right before the final node and this is the
                    // final node, just finish now.
                    if &**node == self.end.node() &&
                        matches!(self.end, FlatTreeNodePosition::Before(_))
                    {
                        self.finished = true;
                        break;
                    }
                    // If the selection starts after the first node, do not set any flags
                    // on that nodes descendants.
                    if &**node == self.start.node() &&
                        matches!(self.start, FlatTreeNodePosition::After(_))
                    {
                        self.skip_subtree = true;
                    }
                    return Some(node);
                },
                PrePostIteration::Leave(node) => {
                    self.finished = &**node == self.end.node();
                },
            }
        }

        None
    }
}

struct VisibleSelectionFlagUpdate<'no_gc> {
    no_gc: &'no_gc NoGC,
    /// The nodes that previously had the OVERLAPS_DOCUMENT_SELECTION set on them before
    /// this flag update.
    ///
    /// Hash keys are pointer addresses which are not directly controlled by web content
    /// so we don’t need HashDoS resistance and can use a faster hasher than `std`’s default
    previously_flagged_nodes: FxHashSet<UnrootedDom<'no_gc, Node>>,
    /// Cache shared between calls to [`Node::used_user_select`]
    used_user_select_cache: FxHashMap<UnrootedDom<'no_gc, Node>, UsedUserSelect>,
    /// Whether or not this update requires a display list update.
    needs_new_display_list: bool,
}

impl<'no_gc> VisibleSelectionFlagUpdate<'no_gc> {
    fn run(
        no_gc: &'no_gc NoGC,
        previously_flagged_nodes: FxHashSet<UnrootedDom<'no_gc, Node>>,
        flat_tree_selection: Option<&FlatTreeSelection<'_>>,
        document: &Document,
    ) {
        let mut update = Self {
            no_gc,
            previously_flagged_nodes,
            used_user_select_cache: Default::default(),
            needs_new_display_list: false,
        };

        if let Some(flat_tree_selection) = flat_tree_selection {
            let traversal = flat_tree_selection.traversal();
            for ancestor in traversal
                .start
                .node()
                .ancestors_in_flat_tree_unrooted(no_gc)
            {
                update.set(&ancestor, flat_tree_selection);
            }
            for node in traversal {
                update.set(&node, flat_tree_selection);
            }
        }

        update.finish(document);
    }

    fn set(&mut self, node: &UnrootedDom<'no_gc, Node>, flat_tree_selection: &FlatTreeSelection) {
        if !node.get_flag(NodeFlags::OVERLAPS_DOCUMENT_SELECTION) {
            node.set_flag(NodeFlags::OVERLAPS_DOCUMENT_SELECTION, true);
            debug_assert!(!self.previously_flagged_nodes.contains(node));
        } else {
            self.previously_flagged_nodes.remove(node);
        }

        // TODO: We should ensure that the style is up-to-date before reading the
        // `user-select` property and changes to `user-select` should trigger new visual
        // selection updates. Not doing this means that the calculations here are one
        // layout old and are never run again until the selection changes.
        let user_select = node.used_user_select(self.no_gc, &mut self.used_user_select_cache);
        let inhibited = user_select == UsedUserSelect::None;
        node.set_flag(NodeFlags::SELECTION_INHIBITED, inhibited);

        self.set_node_selection(node, (!inhibited).then_some(flat_tree_selection));
    }

    fn clear(&mut self, node: &Node) {
        node.set_flag(NodeFlags::OVERLAPS_DOCUMENT_SELECTION, false);
        node.set_flag(NodeFlags::SELECTION_INHIBITED, false);
        self.set_node_selection(node, None);
    }

    fn set_character_data_selection(
        &mut self,
        character_data: &CharacterData,
        range: Option<RangeAny<Utf32CodeUnits>>,
    ) {
        if character_data.set_text_run_selection(range) {
            self.needs_new_display_list = true;
        } else {
            // Note: This isn't just necessary when the `CharacterData` doesn't have a
            // corresponding box tree node, but *also* for properly handling `:first-letter`
            // at the moment. This cannot be removed until `:first-letter` is represented
            // properly in the box tree.
            character_data
                .upcast::<Node>()
                .dirty(self.no_gc, NodeDamage::ContentOrHeritage);
        }
    }

    fn set_node_selection(&mut self, node: &Node, flat_tree_selection: Option<&FlatTreeSelection>) {
        if let Some(character_data) = node.downcast::<CharacterData>() {
            let range = flat_tree_selection.map(|flat_tree_selection| {
                flat_tree_selection.range_for_character_data(character_data)
            });
            self.set_character_data_selection(character_data, range);
        } else if node.set_element_selection(flat_tree_selection.is_some()) {
            self.needs_new_display_list = true;
        }
    }

    fn finish(mut self, document: &Document) {
        for node in std::mem::take(&mut self.previously_flagged_nodes) {
            self.clear(&node);
        }
        if self.needs_new_display_list {
            document.window().layout().set_needs_new_display_list();
        }
    }
}