Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1075 insertions, 0 deletions

diff --git a/gfx/wr/webrender/src/spatial_tree.rs b/gfx/wr/webrender/src/spatial_tree.rs
new file mode 100644
index 0000000000..04da9d9ea9
--- /dev/null
+++ b/gfx/wr/webrender/src/spatial_tree.rs
@@ -0,0 +1,1075 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+use api::{ExternalScrollId, PropertyBinding, ReferenceFrameKind, TransformStyle};
+use api::{PipelineId, ScrollClamping, ScrollNodeState, ScrollSensitivity};
+use api::units::*;
+use euclid::Transform3D;
+use crate::gpu_types::TransformPalette;
+use crate::internal_types::{FastHashMap, FastHashSet};
+use crate::print_tree::{PrintableTree, PrintTree, PrintTreePrinter};
+use crate::scene::SceneProperties;
+use crate::spatial_node::{ScrollFrameInfo, SpatialNode, SpatialNodeType, StickyFrameInfo, ScrollFrameKind};
+use std::{ops, u32};
+use crate::util::{FastTransform, LayoutToWorldFastTransform, MatrixHelpers, ScaleOffset, scale_factors};
+
+pub type ScrollStates = FastHashMap<ExternalScrollId, ScrollFrameInfo>;
+
+/// An id that identifies coordinate systems in the SpatialTree. Each
+/// coordinate system has an id and those ids will be shared when the coordinates
+/// system are the same or are in the same axis-aligned space. This allows
+/// for optimizing mask generation.
+#[derive(Debug, Copy, Clone, PartialEq)]
+#[cfg_attr(feature = "capture", derive(Serialize))]
+#[cfg_attr(feature = "replay", derive(Deserialize))]
+pub struct CoordinateSystemId(pub u32);
+
+/// A node in the hierarchy of coordinate system
+/// transforms.
+#[derive(Debug)]
+pub struct CoordinateSystem {
+    pub transform: LayoutTransform,
+    pub world_transform: LayoutToWorldTransform,
+    pub should_flatten: bool,
+    pub parent: Option<CoordinateSystemId>,
+}
+
+impl CoordinateSystem {
+    fn root() -> Self {
+        CoordinateSystem {
+            transform: LayoutTransform::identity(),
+            world_transform: LayoutToWorldTransform::identity(),
+            should_flatten: false,
+            parent: None,
+        }
+    }
+}
+
+#[derive(Debug, Copy, Clone, Eq, Hash, MallocSizeOf, PartialEq, PartialOrd, Ord)]
+#[cfg_attr(feature = "capture", derive(Serialize))]
+#[cfg_attr(feature = "replay", derive(Deserialize))]
+pub struct SpatialNodeIndex(pub u32);
+
+impl SpatialNodeIndex {
+    pub const INVALID: SpatialNodeIndex = SpatialNodeIndex(u32::MAX);
+}
+
+//Note: these have to match ROOT_REFERENCE_FRAME_SPATIAL_ID and ROOT_SCROLL_NODE_SPATIAL_ID
+pub const ROOT_SPATIAL_NODE_INDEX: SpatialNodeIndex = SpatialNodeIndex(0);
+const TOPMOST_SCROLL_NODE_INDEX: SpatialNodeIndex = SpatialNodeIndex(1);
+
+// In some cases, the conversion from CSS pixels to device pixels can result in small
+// rounding errors when calculating the scrollable distance of a scroll frame. Apply
+// a small epsilon so that we don't detect these frames as "real" scroll frames.
+const MIN_SCROLLABLE_AMOUNT: f32 = 0.01;
+
+impl SpatialNodeIndex {
+    pub fn new(index: usize) -> Self {
+        debug_assert!(index < ::std::u32::MAX as usize);
+        SpatialNodeIndex(index as u32)
+    }
+}
+
+impl CoordinateSystemId {
+    pub fn root() -> Self {
+        CoordinateSystemId(0)
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub enum VisibleFace {
+    Front,
+    Back,
+}
+
+impl Default for VisibleFace {
+    fn default() -> Self {
+        VisibleFace::Front
+    }
+}
+
+impl ops::Not for VisibleFace {
+    type Output = Self;
+    fn not(self) -> Self {
+        match self {
+            VisibleFace::Front => VisibleFace::Back,
+            VisibleFace::Back => VisibleFace::Front,
+        }
+    }
+}
+
+pub struct SpatialTree {
+    /// Nodes which determine the positions (offsets and transforms) for primitives
+    /// and clips.
+    pub spatial_nodes: Vec<SpatialNode>,
+
+    /// A list of transforms that establish new coordinate systems.
+    /// Spatial nodes only establish a new coordinate system when
+    /// they have a transform that is not a simple 2d translation.
+    coord_systems: Vec<CoordinateSystem>,
+
+    pub pending_scroll_offsets: FastHashMap<ExternalScrollId, (LayoutPoint, ScrollClamping)>,
+
+    /// A set of pipelines which should be discarded the next time this
+    /// tree is drained.
+    pub pipelines_to_discard: FastHashSet<PipelineId>,
+
+    /// Temporary stack of nodes to update when traversing the tree.
+    nodes_to_update: Vec<(SpatialNodeIndex, TransformUpdateState)>,
+}
+
+#[derive(Clone)]
+pub struct TransformUpdateState {
+    pub parent_reference_frame_transform: LayoutToWorldFastTransform,
+    pub parent_accumulated_scroll_offset: LayoutVector2D,
+    pub nearest_scrolling_ancestor_offset: LayoutVector2D,
+    pub nearest_scrolling_ancestor_viewport: LayoutRect,
+
+    /// An id for keeping track of the axis-aligned space of this node. This is used in
+    /// order to to track what kinds of clip optimizations can be done for a particular
+    /// display list item, since optimizations can usually only be done among
+    /// coordinate systems which are relatively axis aligned.
+    pub current_coordinate_system_id: CoordinateSystemId,
+
+    /// Scale and offset from the coordinate system that started this compatible coordinate system.
+    pub coordinate_system_relative_scale_offset: ScaleOffset,
+
+    /// True if this node is transformed by an invertible transform.  If not, display items
+    /// transformed by this node will not be displayed and display items not transformed by this
+    /// node will not be clipped by clips that are transformed by this node.
+    pub invertible: bool,
+
+    /// True if this node is a part of Preserve3D hierarchy.
+    pub preserves_3d: bool,
+}
+
+
+/// Transformation between two nodes in the spatial tree that can sometimes be
+/// encoded more efficiently than with a full matrix.
+#[derive(Debug, Clone)]
+pub enum CoordinateSpaceMapping<Src, Dst> {
+    Local,
+    ScaleOffset(ScaleOffset),
+    Transform(Transform3D<f32, Src, Dst>),
+}
+
+impl<Src, Dst> CoordinateSpaceMapping<Src, Dst> {
+    pub fn into_transform(self) -> Transform3D<f32, Src, Dst> {
+        match self {
+            CoordinateSpaceMapping::Local => Transform3D::identity(),
+            CoordinateSpaceMapping::ScaleOffset(scale_offset) => scale_offset.to_transform(),
+            CoordinateSpaceMapping::Transform(transform) => transform,
+        }
+    }
+
+    pub fn into_fast_transform(self) -> FastTransform<Src, Dst> {
+        match self {
+            CoordinateSpaceMapping::Local => FastTransform::identity(),
+            CoordinateSpaceMapping::ScaleOffset(scale_offset) => FastTransform::with_scale_offset(scale_offset),
+            CoordinateSpaceMapping::Transform(transform) => FastTransform::with_transform(transform),
+        }
+    }
+
+    pub fn is_perspective(&self) -> bool {
+        match *self {
+            CoordinateSpaceMapping::Local |
+            CoordinateSpaceMapping::ScaleOffset(_) => false,
+            CoordinateSpaceMapping::Transform(ref transform) => transform.has_perspective_component(),
+        }
+    }
+
+    pub fn is_2d_axis_aligned(&self) -> bool {
+        match *self {
+            CoordinateSpaceMapping::Local |
+            CoordinateSpaceMapping::ScaleOffset(_) => true,
+            CoordinateSpaceMapping::Transform(ref transform) => transform.preserves_2d_axis_alignment(),
+        }
+    }
+
+    pub fn scale_factors(&self) -> (f32, f32) {
+        match *self {
+            CoordinateSpaceMapping::Local => (1.0, 1.0),
+            CoordinateSpaceMapping::ScaleOffset(ref scale_offset) => (scale_offset.scale.x, scale_offset.scale.y),
+            CoordinateSpaceMapping::Transform(ref transform) => scale_factors(transform),
+        }
+    }
+
+    pub fn inverse(&self) -> Option<CoordinateSpaceMapping<Dst, Src>> {
+        match *self {
+            CoordinateSpaceMapping::Local => Some(CoordinateSpaceMapping::Local),
+            CoordinateSpaceMapping::ScaleOffset(ref scale_offset) => {
+                Some(CoordinateSpaceMapping::ScaleOffset(scale_offset.inverse()))
+            }
+            CoordinateSpaceMapping::Transform(ref transform) => {
+                transform.inverse().map(CoordinateSpaceMapping::Transform)
+            }
+        }
+    }
+}
+
+enum TransformScroll {
+    Scrolled,
+    Unscrolled,
+}
+
+impl SpatialTree {
+    pub fn new() -> Self {
+        SpatialTree {
+            spatial_nodes: Vec::new(),
+            coord_systems: Vec::new(),
+            pending_scroll_offsets: FastHashMap::default(),
+            pipelines_to_discard: FastHashSet::default(),
+            nodes_to_update: Vec::new(),
+        }
+    }
+
+    /// Calculate the accumulated external scroll offset for
+    /// a given spatial node.
+    pub fn external_scroll_offset(&self, node_index: SpatialNodeIndex) -> LayoutVector2D {
+        let mut offset = LayoutVector2D::zero();
+        let mut current_node = Some(node_index);
+
+        while let Some(node_index) = current_node {
+            let node = &self.spatial_nodes[node_index.0 as usize];
+
+            match node.node_type {
+                SpatialNodeType::ScrollFrame(ref scrolling) => {
+                    offset += scrolling.external_scroll_offset;
+                }
+                SpatialNodeType::StickyFrame(..) => {
+                    // Doesn't provide any external scroll offset
+                }
+                SpatialNodeType::ReferenceFrame(..) => {
+                    // External scroll offsets are not propagated across
+                    // reference frames.
+                    break;
+                }
+            }
+
+            current_node = node.parent;
+        }
+
+        offset
+    }
+
+    /// Calculate the relative transform from `child_index` to `parent_index`.
+    /// This method will panic if the nodes are not connected!
+    pub fn get_relative_transform(
+        &self,
+        child_index: SpatialNodeIndex,
+        parent_index: SpatialNodeIndex,
+    ) -> CoordinateSpaceMapping<LayoutPixel, LayoutPixel> {
+        self.get_relative_transform_with_face(child_index, parent_index, None)
+    }
+
+    /// Calculate the relative transform from `child_index` to `parent_index`.
+    /// This method will panic if the nodes are not connected!
+    /// Also, switch the visible face to `Back` if at any stage where the
+    /// combined transform is flattened, we see the back face.
+    pub fn get_relative_transform_with_face(
+        &self,
+        child_index: SpatialNodeIndex,
+        parent_index: SpatialNodeIndex,
+        mut visible_face: Option<&mut VisibleFace>,
+    ) -> CoordinateSpaceMapping<LayoutPixel, LayoutPixel> {
+        if child_index == parent_index {
+            return CoordinateSpaceMapping::Local;
+        }
+
+        let child = &self.spatial_nodes[child_index.0 as usize];
+        let parent = &self.spatial_nodes[parent_index.0 as usize];
+
+        if child.coordinate_system_id == parent.coordinate_system_id {
+            let scale_offset = parent.content_transform
+                .inverse()
+                .accumulate(&child.content_transform);
+            return CoordinateSpaceMapping::ScaleOffset(scale_offset);
+        }
+
+        if child_index.0 < parent_index.0 {
+            warn!("Unexpected transform queried from {:?} to {:?}, please call the graphics team!", child_index, parent_index);
+            let child_cs = &self.coord_systems[child.coordinate_system_id.0 as usize];
+            let child_transform = child.content_transform
+                .to_transform::<LayoutPixel, LayoutPixel>()
+                .then(&child_cs.world_transform);
+            let parent_cs = &self.coord_systems[parent.coordinate_system_id.0 as usize];
+            let parent_transform = parent.content_transform
+                .to_transform()
+                .then(&parent_cs.world_transform);
+
+            let result = parent_transform
+                .inverse()
+                .unwrap_or_default()
+                .then(&child_transform)
+                .with_source::<LayoutPixel>()
+                .with_destination::<LayoutPixel>();
+
+            if let Some(face) = visible_face {
+                if result.is_backface_visible() {
+                    *face = VisibleFace::Back;
+                }
+            }
+            return CoordinateSpaceMapping::Transform(result);
+        }
+
+        let mut coordinate_system_id = child.coordinate_system_id;
+        let mut transform = child.content_transform.to_transform();
+
+        // we need to update the associated parameters of a transform in two cases:
+        // 1) when the flattening happens, so that we don't lose that original 3D aspects
+        // 2) when we reach the end of iteration, so that our result is up to date
+
+        while coordinate_system_id != parent.coordinate_system_id {
+            let coord_system = &self.coord_systems[coordinate_system_id.0 as usize];
+
+            if coord_system.should_flatten {
+                if let Some(ref mut face) = visible_face {
+                    if transform.is_backface_visible() {
+                        **face = VisibleFace::Back;
+                    }
+                }
+                transform.flatten_z_output();
+            }
+
+            coordinate_system_id = coord_system.parent.expect("invalid parent!");
+            transform = transform.then(&coord_system.transform);
+        }
+
+        transform = transform.then(
+            &parent.content_transform
+                .inverse()
+                .to_transform(),
+        );
+        if let Some(face) = visible_face {
+            if transform.is_backface_visible() {
+                *face = VisibleFace::Back;
+            }
+        }
+
+        CoordinateSpaceMapping::Transform(transform)
+    }
+
+    pub fn is_relative_transform_complex(
+        &self,
+        child_index: SpatialNodeIndex,
+        parent_index: SpatialNodeIndex,
+    ) -> bool {
+        if child_index == parent_index {
+            return false;
+        }
+
+        let child = &self.spatial_nodes[child_index.0 as usize];
+        let parent = &self.spatial_nodes[parent_index.0 as usize];
+
+        child.coordinate_system_id != parent.coordinate_system_id
+    }
+
+    fn get_world_transform_impl(
+        &self,
+        index: SpatialNodeIndex,
+        scroll: TransformScroll,
+    ) -> CoordinateSpaceMapping<LayoutPixel, WorldPixel> {
+        let child = &self.spatial_nodes[index.0 as usize];
+
+        if child.coordinate_system_id.0 == 0 {
+            if index == ROOT_SPATIAL_NODE_INDEX {
+                CoordinateSpaceMapping::Local
+            } else {
+                CoordinateSpaceMapping::ScaleOffset(child.content_transform)
+            }
+        } else {
+            let system = &self.coord_systems[child.coordinate_system_id.0 as usize];
+            let scale_offset = match scroll {
+                TransformScroll::Scrolled => &child.content_transform,
+                TransformScroll::Unscrolled => &child.viewport_transform,
+            };
+            let transform = scale_offset
+                .to_transform()
+                .then(&system.world_transform);
+
+            CoordinateSpaceMapping::Transform(transform)
+        }
+    }
+
+    /// Calculate the relative transform from `index` to the root.
+    pub fn get_world_transform(
+        &self,
+        index: SpatialNodeIndex,
+    ) -> CoordinateSpaceMapping<LayoutPixel, WorldPixel> {
+        self.get_world_transform_impl(index, TransformScroll::Scrolled)
+    }
+
+    /// Calculate the relative transform from `index` to the root.
+    /// Unlike `get_world_transform`, this variant doesn't account for the local scroll offset.
+    pub fn get_world_viewport_transform(
+        &self,
+        index: SpatialNodeIndex,
+    ) -> CoordinateSpaceMapping<LayoutPixel, WorldPixel> {
+        self.get_world_transform_impl(index, TransformScroll::Unscrolled)
+    }
+
+    /// The root reference frame, which is the true root of the SpatialTree. Initially
+    /// this ID is not valid, which is indicated by ```spatial_nodes``` being empty.
+    pub fn root_reference_frame_index(&self) -> SpatialNodeIndex {
+        // TODO(mrobinson): We should eventually make this impossible to misuse.
+        debug_assert!(!self.spatial_nodes.is_empty());
+        ROOT_SPATIAL_NODE_INDEX
+    }
+
+    /// The root scroll node which is the first child of the root reference frame.
+    /// Initially this ID is not valid, which is indicated by ```spatial_nodes``` being empty.
+    pub fn topmost_scroll_node_index(&self) -> SpatialNodeIndex {
+        // TODO(mrobinson): We should eventually make this impossible to misuse.
+        debug_assert!(self.spatial_nodes.len() >= 1);
+        TOPMOST_SCROLL_NODE_INDEX
+    }
+
+    pub fn get_scroll_node_state(&self) -> Vec<ScrollNodeState> {
+        let mut result = vec![];
+        for node in &self.spatial_nodes {
+            if let SpatialNodeType::ScrollFrame(info) = node.node_type {
+                result.push(ScrollNodeState {
+                    id: info.external_id,
+                    scroll_offset: info.offset - info.external_scroll_offset,
+                })
+            }
+        }
+        result
+    }
+
+    pub fn drain(&mut self) -> ScrollStates {
+        let mut scroll_states = FastHashMap::default();
+        for old_node in &mut self.spatial_nodes.drain(..) {
+            if self.pipelines_to_discard.contains(&old_node.pipeline_id) {
+                continue;
+            }
+
+            match old_node.node_type {
+                SpatialNodeType::ScrollFrame(info) => {
+                    scroll_states.insert(info.external_id, info);
+                }
+                _ => {}
+            }
+        }
+
+        self.coord_systems.clear();
+        self.pipelines_to_discard.clear();
+        scroll_states
+    }
+
+    pub fn scroll_node(
+        &mut self,
+        origin: LayoutPoint,
+        id: ExternalScrollId,
+        clamp: ScrollClamping
+    ) -> bool {
+        for node in &mut self.spatial_nodes {
+            if node.matches_external_id(id) {
+                return node.set_scroll_origin(&origin, clamp);
+            }
+        }
+
+        self.pending_scroll_offsets.insert(id, (origin, clamp));
+        false
+    }
+
+    pub fn update_tree(
+        &mut self,
+        pan: WorldPoint,
+        global_device_pixel_scale: DevicePixelScale,
+        scene_properties: &SceneProperties,
+    ) {
+        if self.spatial_nodes.is_empty() {
+            return;
+        }
+
+        profile_scope!("update_tree");
+        self.coord_systems.clear();
+        self.coord_systems.push(CoordinateSystem::root());
+
+        let root_node_index = self.root_reference_frame_index();
+        let state = TransformUpdateState {
+            parent_reference_frame_transform: LayoutVector2D::new(pan.x, pan.y).into(),
+            parent_accumulated_scroll_offset: LayoutVector2D::zero(),
+            nearest_scrolling_ancestor_offset: LayoutVector2D::zero(),
+            nearest_scrolling_ancestor_viewport: LayoutRect::zero(),
+            current_coordinate_system_id: CoordinateSystemId::root(),
+            coordinate_system_relative_scale_offset: ScaleOffset::identity(),
+            invertible: true,
+            preserves_3d: false,
+        };
+        debug_assert!(self.nodes_to_update.is_empty());
+        self.nodes_to_update.push((root_node_index, state));
+
+        while let Some((node_index, mut state)) = self.nodes_to_update.pop() {
+            let (previous, following) = self.spatial_nodes.split_at_mut(node_index.0 as usize);
+            let node = match following.get_mut(0) {
+                Some(node) => node,
+                None => continue,
+            };
+
+            node.update(&mut state, &mut self.coord_systems, global_device_pixel_scale, scene_properties, &*previous);
+
+            if !node.children.is_empty() {
+                node.prepare_state_for_children(&mut state);
+                self.nodes_to_update.extend(node.children
+                    .iter()
+                    .rev()
+                    .map(|child_index| (*child_index, state.clone()))
+                );
+            }
+        }
+    }
+
+    pub fn build_transform_palette(&self) -> TransformPalette {
+        profile_scope!("build_transform_palette");
+        let mut palette = TransformPalette::new(self.spatial_nodes.len());
+        //Note: getting the world transform of a node is O(1) operation
+        for i in 0 .. self.spatial_nodes.len() {
+            let index = SpatialNodeIndex(i as u32);
+            let world_transform = self.get_world_transform(index).into_transform();
+            palette.set_world_transform(index, world_transform);
+        }
+        palette
+    }
+
+    pub fn finalize_and_apply_pending_scroll_offsets(&mut self, old_states: ScrollStates) {
+        for node in &mut self.spatial_nodes {
+            let external_id = match node.node_type {
+                SpatialNodeType::ScrollFrame(ScrollFrameInfo { external_id, ..}) => external_id,
+                _ => continue,
+            };
+
+            if let Some(scrolling_state) = old_states.get(&external_id) {
+                node.apply_old_scrolling_state(scrolling_state);
+            }
+
+            if let Some((offset, clamping)) = self.pending_scroll_offsets.remove(&external_id) {
+                node.set_scroll_origin(&offset, clamping);
+            }
+        }
+    }
+
+    pub fn add_scroll_frame(
+        &mut self,
+        parent_index: SpatialNodeIndex,
+        external_id: ExternalScrollId,
+        pipeline_id: PipelineId,
+        frame_rect: &LayoutRect,
+        content_size: &LayoutSize,
+        scroll_sensitivity: ScrollSensitivity,
+        frame_kind: ScrollFrameKind,
+        external_scroll_offset: LayoutVector2D,
+    ) -> SpatialNodeIndex {
+        let node = SpatialNode::new_scroll_frame(
+            pipeline_id,
+            parent_index,
+            external_id,
+            frame_rect,
+            content_size,
+            scroll_sensitivity,
+            frame_kind,
+            external_scroll_offset,
+        );
+        self.add_spatial_node(node)
+    }
+
+    pub fn add_reference_frame(
+        &mut self,
+        parent_index: Option<SpatialNodeIndex>,
+        transform_style: TransformStyle,
+        source_transform: PropertyBinding<LayoutTransform>,
+        kind: ReferenceFrameKind,
+        origin_in_parent_reference_frame: LayoutVector2D,
+        pipeline_id: PipelineId,
+    ) -> SpatialNodeIndex {
+        let node = SpatialNode::new_reference_frame(
+            parent_index,
+            transform_style,
+            source_transform,
+            kind,
+            origin_in_parent_reference_frame,
+            pipeline_id,
+        );
+        self.add_spatial_node(node)
+    }
+
+    pub fn add_sticky_frame(
+        &mut self,
+        parent_index: SpatialNodeIndex,
+        sticky_frame_info: StickyFrameInfo,
+        pipeline_id: PipelineId,
+    ) -> SpatialNodeIndex {
+        let node = SpatialNode::new_sticky_frame(
+            parent_index,
+            sticky_frame_info,
+            pipeline_id,
+        );
+        self.add_spatial_node(node)
+    }
+
+    pub fn add_spatial_node(&mut self, mut node: SpatialNode) -> SpatialNodeIndex {
+        let index = SpatialNodeIndex::new(self.spatial_nodes.len());
+
+        // When the parent node is None this means we are adding the root.
+        if let Some(parent_index) = node.parent {
+            let parent_node = &mut self.spatial_nodes[parent_index.0 as usize];
+            parent_node.add_child(index);
+            node.update_snapping(Some(parent_node));
+        } else {
+            node.update_snapping(None);
+        }
+
+        self.spatial_nodes.push(node);
+        index
+    }
+
+    pub fn discard_frame_state_for_pipeline(&mut self, pipeline_id: PipelineId) {
+        self.pipelines_to_discard.insert(pipeline_id);
+    }
+
+    /// Check if a given spatial node is an ancestor of another spatial node.
+    pub fn is_ancestor(
+        &self,
+        maybe_parent: SpatialNodeIndex,
+        maybe_child: SpatialNodeIndex,
+    ) -> bool {
+        // Early out if same node
+        if maybe_parent == maybe_child {
+            return false;
+        }
+
+        let mut current_node = maybe_child;
+
+        while current_node != ROOT_SPATIAL_NODE_INDEX {
+            let node = &self.spatial_nodes[current_node.0 as usize];
+            current_node = node.parent.expect("bug: no parent");
+
+            if current_node == maybe_parent {
+                return true;
+            }
+        }
+
+        false
+    }
+
+    /// Find the spatial node that is the scroll root for a given spatial node.
+    /// A scroll root is the first spatial node when found travelling up the
+    /// spatial node tree that is an explicit scroll frame.
+    pub fn find_scroll_root(
+        &self,
+        spatial_node_index: SpatialNodeIndex,
+    ) -> SpatialNodeIndex {
+        let mut real_scroll_root = ROOT_SPATIAL_NODE_INDEX;
+        let mut outermost_scroll_root = ROOT_SPATIAL_NODE_INDEX;
+        let mut node_index = spatial_node_index;
+
+        while node_index != ROOT_SPATIAL_NODE_INDEX {
+            let node = &self.spatial_nodes[node_index.0 as usize];
+            match node.node_type {
+                SpatialNodeType::ReferenceFrame(ref info) => {
+                    match info.kind {
+                        ReferenceFrameKind::Zoom => {
+                            // We can handle scroll nodes that pass through a zoom node
+                        }
+                        ReferenceFrameKind::Transform |
+                        ReferenceFrameKind::Perspective { .. } => {
+                            // When a reference frame is encountered, forget any scroll roots
+                            // we have encountered, as they may end up with a non-axis-aligned transform.
+                            real_scroll_root = ROOT_SPATIAL_NODE_INDEX;
+                            outermost_scroll_root = ROOT_SPATIAL_NODE_INDEX;
+                        }
+                    }
+                }
+                SpatialNodeType::StickyFrame(..) => {}
+                SpatialNodeType::ScrollFrame(ref info) => {
+                    match info.frame_kind {
+                        ScrollFrameKind::PipelineRoot { is_root_pipeline } => {
+                            // Once we encounter a pipeline root, there is no need to look further
+                            if is_root_pipeline {
+                                break;
+                            }
+                        }
+                        ScrollFrameKind::Explicit => {
+                            // Store the closest scroll root we find to the root, for use
+                            // later on, even if it's not actually scrollable.
+                            outermost_scroll_root = node_index;
+
+                            // If the scroll root has no scrollable area, we don't want to
+                            // consider it. This helps pages that have a nested scroll root
+                            // within a redundant scroll root to avoid selecting the wrong
+                            // reference spatial node for a picture cache.
+                            if info.scrollable_size.width > MIN_SCROLLABLE_AMOUNT ||
+                               info.scrollable_size.height > MIN_SCROLLABLE_AMOUNT {
+                                // Since we are skipping redundant scroll roots, we may end up
+                                // selecting inner scroll roots that are very small. There is
+                                // no performance benefit to creating a slice for these roots,
+                                // as they are cheap to rasterize. The size comparison is in
+                                // local-space, but makes for a reasonable estimate. The value
+                                // is arbitrary, but is generally small enough to ignore things
+                                // like scroll roots around text input elements.
+                                if info.viewport_rect.size.width > 128.0 &&
+                                   info.viewport_rect.size.height > 128.0 {
+                                    // If we've found a root that is scrollable, and a reasonable
+                                    // size, select that as the current root for this node
+                                    real_scroll_root = node_index;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            node_index = node.parent.expect("unable to find parent node");
+        }
+
+        // If we didn't find any real (scrollable) frames, then return the outermost
+        // redundant scroll frame. This is important so that we can correctly find
+        // the clips defined on the content which should be handled when drawing the
+        // picture cache tiles (by definition these clips are ancestors of the
+        // scroll root selected for the picture cache).
+        if real_scroll_root == ROOT_SPATIAL_NODE_INDEX {
+            outermost_scroll_root
+        } else {
+            real_scroll_root
+        }
+    }
+
+    fn print_node<T: PrintTreePrinter>(
+        &self,
+        index: SpatialNodeIndex,
+        pt: &mut T,
+    ) {
+        let node = &self.spatial_nodes[index.0 as usize];
+        match node.node_type {
+            SpatialNodeType::StickyFrame(ref sticky_frame_info) => {
+                pt.new_level(format!("StickyFrame"));
+                pt.add_item(format!("sticky info: {:?}", sticky_frame_info));
+            }
+            SpatialNodeType::ScrollFrame(scrolling_info) => {
+                pt.new_level(format!("ScrollFrame"));
+                pt.add_item(format!("viewport: {:?}", scrolling_info.viewport_rect));
+                pt.add_item(format!("scrollable_size: {:?}", scrolling_info.scrollable_size));
+                pt.add_item(format!("scroll offset: {:?}", scrolling_info.offset));
+                pt.add_item(format!("external_scroll_offset: {:?}", scrolling_info.external_scroll_offset));
+                pt.add_item(format!("kind: {:?}", scrolling_info.frame_kind));
+            }
+            SpatialNodeType::ReferenceFrame(ref info) => {
+                pt.new_level(format!("ReferenceFrame"));
+                pt.add_item(format!("kind: {:?}", info.kind));
+                pt.add_item(format!("transform_style: {:?}", info.transform_style));
+                pt.add_item(format!("source_transform: {:?}", info.source_transform));
+                pt.add_item(format!("origin_in_parent_reference_frame: {:?}", info.origin_in_parent_reference_frame));
+            }
+        }
+
+        pt.add_item(format!("index: {:?}", index));
+        pt.add_item(format!("content_transform: {:?}", node.content_transform));
+        pt.add_item(format!("viewport_transform: {:?}", node.viewport_transform));
+        pt.add_item(format!("snapping_transform: {:?}", node.snapping_transform));
+        pt.add_item(format!("coordinate_system_id: {:?}", node.coordinate_system_id));
+
+        for child_index in &node.children {
+            self.print_node(*child_index, pt);
+        }
+
+        pt.end_level();
+    }
+
+    /// Get the visible face of the transfrom from the specified node to its parent.
+    pub fn get_local_visible_face(&self, node_index: SpatialNodeIndex) -> VisibleFace {
+        let node = &self.spatial_nodes[node_index.0 as usize];
+        let mut face = VisibleFace::Front;
+        if let Some(parent_index) = node.parent {
+            self.get_relative_transform_with_face(node_index, parent_index, Some(&mut face));
+        }
+        face
+    }
+
+    #[allow(dead_code)]
+    pub fn print(&self) {
+        if !self.spatial_nodes.is_empty() {
+            let mut buf = Vec::<u8>::new();
+            {
+                let mut pt = PrintTree::new_with_sink("spatial tree", &mut buf);
+                self.print_with(&mut pt);
+            }
+            // If running in Gecko, set RUST_LOG=webrender::spatial_tree=debug
+            // to get this logging to be emitted to stderr/logcat.
+            debug!("{}", std::str::from_utf8(&buf).unwrap_or("(Tree printer emitted non-utf8)"));
+        }
+    }
+}
+
+impl PrintableTree for SpatialTree {
+    fn print_with<T: PrintTreePrinter>(&self, pt: &mut T) {
+        if !self.spatial_nodes.is_empty() {
+            self.print_node(self.root_reference_frame_index(), pt);
+        }
+    }
+}
+
+#[cfg(test)]
+fn add_reference_frame(
+    cst: &mut SpatialTree,
+    parent: Option<SpatialNodeIndex>,
+    transform: LayoutTransform,
+    origin_in_parent_reference_frame: LayoutVector2D,
+) -> SpatialNodeIndex {
+    cst.add_reference_frame(
+        parent,
+        TransformStyle::Preserve3D,
+        PropertyBinding::Value(transform),
+        ReferenceFrameKind::Transform,
+        origin_in_parent_reference_frame,
+        PipelineId::dummy(),
+    )
+}
+
+#[cfg(test)]
+fn test_pt(
+    px: f32,
+    py: f32,
+    cst: &SpatialTree,
+    child: SpatialNodeIndex,
+    parent: SpatialNodeIndex,
+    expected_x: f32,
+    expected_y: f32,
+) {
+    use euclid::approxeq::ApproxEq;
+    const EPSILON: f32 = 0.0001;
+
+    let p = LayoutPoint::new(px, py);
+    let m = cst.get_relative_transform(child, parent).into_transform();
+    let pt = m.transform_point2d(p).unwrap();
+    assert!(pt.x.approx_eq_eps(&expected_x, &EPSILON) &&
+            pt.y.approx_eq_eps(&expected_y, &EPSILON),
+            "p: {:?} -> {:?}\nm={:?}",
+            p, pt, m,
+            );
+}
+
+#[test]
+fn test_cst_simple_translation() {
+    // Basic translations only
+
+    let mut cst = SpatialTree::new();
+
+    let root = add_reference_frame(
+        &mut cst,
+        None,
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child1 = add_reference_frame(
+        &mut cst,
+        Some(root),
+        LayoutTransform::translation(100.0, 0.0, 0.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child2 = add_reference_frame(
+        &mut cst,
+        Some(child1),
+        LayoutTransform::translation(0.0, 50.0, 0.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child3 = add_reference_frame(
+        &mut cst,
+        Some(child2),
+        LayoutTransform::translation(200.0, 200.0, 0.0),
+        LayoutVector2D::zero(),
+    );
+
+    cst.update_tree(WorldPoint::zero(), DevicePixelScale::new(1.0), &SceneProperties::new());
+
+    test_pt(100.0, 100.0, &cst, child1, root, 200.0, 100.0);
+    test_pt(100.0, 100.0, &cst, child2, root, 200.0, 150.0);
+    test_pt(100.0, 100.0, &cst, child2, child1, 100.0, 150.0);
+    test_pt(100.0, 100.0, &cst, child3, root, 400.0, 350.0);
+}
+
+#[test]
+fn test_cst_simple_scale() {
+    // Basic scale only
+
+    let mut cst = SpatialTree::new();
+
+    let root = add_reference_frame(
+        &mut cst,
+        None,
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child1 = add_reference_frame(
+        &mut cst,
+        Some(root),
+        LayoutTransform::scale(4.0, 1.0, 1.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child2 = add_reference_frame(
+        &mut cst,
+        Some(child1),
+        LayoutTransform::scale(1.0, 2.0, 1.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child3 = add_reference_frame(
+        &mut cst,
+        Some(child2),
+        LayoutTransform::scale(2.0, 2.0, 1.0),
+        LayoutVector2D::zero(),
+    );
+
+    cst.update_tree(WorldPoint::zero(), DevicePixelScale::new(1.0), &SceneProperties::new());
+
+    test_pt(100.0, 100.0, &cst, child1, root, 400.0, 100.0);
+    test_pt(100.0, 100.0, &cst, child2, root, 400.0, 200.0);
+    test_pt(100.0, 100.0, &cst, child3, root, 800.0, 400.0);
+    test_pt(100.0, 100.0, &cst, child2, child1, 100.0, 200.0);
+    test_pt(100.0, 100.0, &cst, child3, child1, 200.0, 400.0);
+}
+
+#[test]
+fn test_cst_scale_translation() {
+    // Scale + translation
+
+    let mut cst = SpatialTree::new();
+
+    let root = add_reference_frame(
+        &mut cst,
+        None,
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child1 = add_reference_frame(
+        &mut cst,
+        Some(root),
+        LayoutTransform::translation(100.0, 50.0, 0.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child2 = add_reference_frame(
+        &mut cst,
+        Some(child1),
+        LayoutTransform::scale(2.0, 4.0, 1.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child3 = add_reference_frame(
+        &mut cst,
+        Some(child2),
+        LayoutTransform::translation(200.0, -100.0, 0.0),
+        LayoutVector2D::zero(),
+    );
+
+    let child4 = add_reference_frame(
+        &mut cst,
+        Some(child3),
+        LayoutTransform::scale(3.0, 2.0, 1.0),
+        LayoutVector2D::zero(),
+    );
+
+    cst.update_tree(WorldPoint::zero(), DevicePixelScale::new(1.0), &SceneProperties::new());
+
+    test_pt(100.0, 100.0, &cst, child1, root, 200.0, 150.0);
+    test_pt(100.0, 100.0, &cst, child2, root, 300.0, 450.0);
+    test_pt(100.0, 100.0, &cst, child4, root, 1100.0, 450.0);
+
+    test_pt(0.0, 0.0, &cst, child4, child1, 400.0, -400.0);
+    test_pt(100.0, 100.0, &cst, child4, child1, 1000.0, 400.0);
+    test_pt(100.0, 100.0, &cst, child2, child1, 200.0, 400.0);
+
+    test_pt(100.0, 100.0, &cst, child3, child1, 600.0, 0.0);
+}
+
+#[test]
+fn test_cst_translation_rotate() {
+    // Rotation + translation
+    use euclid::Angle;
+
+    let mut cst = SpatialTree::new();
+
+    let root = add_reference_frame(
+        &mut cst,
+        None,
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child1 = add_reference_frame(
+        &mut cst,
+        Some(root),
+        LayoutTransform::rotation(0.0, 0.0, 1.0, Angle::degrees(-90.0)),
+        LayoutVector2D::zero(),
+    );
+
+    cst.update_tree(WorldPoint::zero(), DevicePixelScale::new(1.0), &SceneProperties::new());
+
+    test_pt(100.0, 0.0, &cst, child1, root, 0.0, -100.0);
+}
+
+#[test]
+fn test_is_ancestor1() {
+    let mut st = SpatialTree::new();
+
+    let root = add_reference_frame(
+        &mut st,
+        None,
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child1_0 = add_reference_frame(
+        &mut st,
+        Some(root),
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child1_1 = add_reference_frame(
+        &mut st,
+        Some(child1_0),
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    let child2 = add_reference_frame(
+        &mut st,
+        Some(root),
+        LayoutTransform::identity(),
+        LayoutVector2D::zero(),
+    );
+
+    st.update_tree(
+        WorldPoint::zero(),
+        DevicePixelScale::new(1.0),
+        &SceneProperties::new(),
+    );
+
+    assert!(!st.is_ancestor(root, root));
+    assert!(!st.is_ancestor(child1_0, child1_0));
+    assert!(!st.is_ancestor(child1_1, child1_1));
+    assert!(!st.is_ancestor(child2, child2));
+
+    assert!(st.is_ancestor(root, child1_0));
+    assert!(st.is_ancestor(root, child1_1));
+    assert!(st.is_ancestor(child1_0, child1_1));
+
+    assert!(!st.is_ancestor(child1_0, root));
+    assert!(!st.is_ancestor(child1_1, root));
+    assert!(!st.is_ancestor(child1_1, child1_0));
+
+    assert!(st.is_ancestor(root, child2));
+    assert!(!st.is_ancestor(child2, root));
+
+    assert!(!st.is_ancestor(child1_0, child2));
+    assert!(!st.is_ancestor(child1_1, child2));
+    assert!(!st.is_ancestor(child2, child1_0));
+    assert!(!st.is_ancestor(child2, child1_1));
+}