Auto merge of #29490 - mrobinson:move-hit-testing-out, r=mukilan

Move hit testing information out of WebRender

Store hit testing information in a data structure that sits alongside the display list in the compositor. This will allow the compositor to store more information per-node. The data structure also takes care of de-duplicating information between successive display list entries. In the future, the data structure can be even more aggressive in producing smaller side hit testing lists, if necessary.

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---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they should not change behavior.

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components/compositing/compositor.rs

@@ -28,16 +28,16 @@ use msg::constellation_msg::{
 };
 use net_traits::image::base::Image;
 use net_traits::image_cache::CorsStatus;
-use num_traits::FromPrimitive;
 #[cfg(feature = "gl")]
 use pixels::PixelFormat;
 use profile_traits::time::{self as profile_time, profile, ProfilerCategory};
+use script_traits::compositor::HitTestInfo;
 use script_traits::CompositorEvent::{MouseButtonEvent, MouseMoveEvent, TouchEvent, WheelEvent};
-use script_traits::{AnimationState, AnimationTickType, LayoutControlMsg};
 use script_traits::{
-    MouseButton, MouseEventType, ScrollState, TouchEventType, TouchId, WheelDelta,
+    AnimationState, AnimationTickType, CompositorHitTestResult, LayoutControlMsg, MouseButton,
+    MouseEventType, ScrollState, TouchEventType, TouchId, UntrustedNodeAddress, WheelDelta,
+    WindowSizeData, WindowSizeType,
 };
-use script_traits::{UntrustedNodeAddress, WindowSizeData, WindowSizeType};
 use servo_geometry::{DeviceIndependentPixel, FramebufferUintLength};
 use std::collections::HashMap;
 use std::env;
@@ -48,8 +48,10 @@ use std::rc::Rc;
 use style_traits::viewport::ViewportConstraints;
 use style_traits::{CSSPixel, DevicePixel, PinchZoomFactor};
 use time::{now, precise_time_ns, precise_time_s};
-use webrender_api::units::{DeviceIntPoint, DeviceIntSize, DevicePoint, LayoutVector2D};
-use webrender_api::{self, HitTestFlags, HitTestResult, ScrollLocation};
+use webrender_api::units::{
+    DeviceIntPoint, DeviceIntSize, DevicePoint, LayoutVector2D, WorldPoint,
+};
+use webrender_api::{self, HitTestFlags, ScrollLocation};
 use webrender_surfman::WebrenderSurfman;
 
 #[derive(Debug, PartialEq)]
@@ -263,6 +265,10 @@ struct PipelineDetails {
 
     /// Whether this pipeline is visible
     visible: bool,
+
+    /// Hit test items for this pipeline. This is used to map WebRender hit test
+    /// information to the full information necessary for Servo.
+    hit_test_items: Vec<HitTestInfo>,
 }
 
 impl PipelineDetails {
@@ -272,6 +278,7 @@ impl PipelineDetails {
             animations_running: false,
             animation_callbacks_running: false,
             visible: true,
+            hit_test_items: Vec::new(),
         }
     }
 }
@@ -381,17 +388,16 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         self.webrender.deinit();
     }
 
-    pub fn update_cursor(&mut self, hit_test_results: HitTestResult) {
-        if let Some(item) = hit_test_results.items.first() {
-            if let Some(cursor) = Cursor::from_u8(item.tag.1 as _) {
-                if cursor != self.cursor {
-                    self.cursor = cursor;
-                    let msg = ConstellationMsg::SetCursor(cursor);
-                    if let Err(e) = self.constellation_chan.send(msg) {
-                        warn!("Sending event to constellation failed ({:?}).", e);
-                    }
-                }
-            }
+    fn update_cursor(&mut self, result: CompositorHitTestResult) {
+        let cursor = match result.cursor {
+            Some(cursor) if cursor != self.cursor => cursor,
+            _ => return,
+        };
+
+        self.cursor = cursor;
+        let msg = ConstellationMsg::SetCursor(cursor);
+        if let Err(e) = self.constellation_chan.send(msg) {
+            warn!("Sending event to constellation failed ({:?}).", e);
         }
     }
 
@@ -514,7 +520,9 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
 
             (Msg::NewScrollFrameReady(recomposite_needed), ShutdownState::NotShuttingDown) => {
                 self.waiting_for_results_of_scroll = false;
-                self.update_cursor(self.hit_test_at_point(self.cursor_pos));
+                if let Some(result) = self.hit_test_at_device_point(self.cursor_pos) {
+                    self.update_cursor(result);
+                }
                 if recomposite_needed {
                     self.composition_request = CompositionRequest::CompositeNow(
                         CompositingReason::NewWebRenderScrollFrame,
@@ -628,9 +636,14 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
                 size2,
                 receiver,
                 descriptor,
+                compositor_display_list_info,
             )) => match receiver.recv() {
                 Ok(data) => {
                     self.waiting_on_pending_frame = true;
+
+                    let details = self.pipeline_details(PipelineId::from_webrender(pipeline));
+                    details.hit_test_items = compositor_display_list_info.hit_test_info;
+
                     let mut txn = webrender_api::Transaction::new();
                     txn.set_display_list(
                         epoch,
@@ -656,9 +669,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
                 flags,
                 sender,
             )) => {
-                let result =
-                    self.webrender_api
-                        .hit_test(self.webrender_document, pipeline, point, flags);
+                let result = self.hit_test_at_point_with_flags_and_pipeline(point, flags, pipeline);
                 let _ = sender.send(result);
             },
 
@@ -904,8 +915,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             MouseWindowEvent::MouseUp(_, p) => p,
         };
 
-        let results = self.hit_test_at_point(point);
-        let result = match results.items.first() {
+        let result = match self.hit_test_at_device_point(point) {
             Some(result) => result,
             None => return,
         };
@@ -920,29 +930,68 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             event_type,
             button,
             result.point_in_viewport.to_untyped(),
-            Some(UntrustedNodeAddress(result.tag.0 as *const c_void)),
-            Some(result.point_relative_to_item.to_untyped()),
+            Some(result.node),
+            Some(result.point_relative_to_item),
             button as u16,
         );
 
-        let pipeline_id = PipelineId::from_webrender(result.pipeline);
-        let msg = ConstellationMsg::ForwardEvent(pipeline_id, event_to_send);
+        let msg = ConstellationMsg::ForwardEvent(result.pipeline_id, event_to_send);
         if let Err(e) = self.constellation_chan.send(msg) {
             warn!("Sending event to constellation failed ({:?}).", e);
         }
     }
 
-    fn hit_test_at_point(&self, point: DevicePoint) -> HitTestResult {
+    fn hit_test_at_device_point(&self, point: DevicePoint) -> Option<CompositorHitTestResult> {
         let dppx = self.page_zoom * self.hidpi_factor();
         let scaled_point = (point / dppx).to_untyped();
+        let world_point = WorldPoint::from_untyped(scaled_point);
+        return self.hit_test_at_point(world_point);
+    }
 
-        let world_cursor = webrender_api::units::WorldPoint::from_untyped(scaled_point);
-        self.webrender_api.hit_test(
-            self.webrender_document,
-            None,
-            world_cursor,
-            HitTestFlags::empty(),
-        )
+    fn hit_test_at_point(&self, point: WorldPoint) -> Option<CompositorHitTestResult> {
+        return self
+            .hit_test_at_point_with_flags_and_pipeline(point, HitTestFlags::empty(), None)
+            .first()
+            .cloned();
+    }
+
+    fn hit_test_at_point_with_flags_and_pipeline(
+        &self,
+        point: WorldPoint,
+        flags: HitTestFlags,
+        pipeline_id: Option<webrender_api::PipelineId>,
+    ) -> Vec<CompositorHitTestResult> {
+        let root_pipeline_id = match self.root_pipeline.id {
+            Some(root_pipeline_id) => root_pipeline_id,
+            None => return vec![],
+        };
+        if self.pipeline(root_pipeline_id).is_none() {
+            return vec![];
+        }
+        let results =
+            self.webrender_api
+                .hit_test(self.webrender_document, pipeline_id, point, flags);
+
+        results
+            .items
+            .iter()
+            .filter_map(|item| {
+                let pipeline_id = PipelineId::from_webrender(item.pipeline);
+                let details = match self.pipeline_details.get(&pipeline_id) {
+                    Some(details) => details,
+                    None => return None,
+                };
+
+                let info = &details.hit_test_items[item.tag.0 as usize];
+                Some(CompositorHitTestResult {
+                    pipeline_id,
+                    point_in_viewport: item.point_in_viewport.to_untyped(),
+                    point_relative_to_item: item.point_relative_to_item.to_untyped(),
+                    node: UntrustedNodeAddress(info.node as *const c_void),
+                    cursor: info.cursor,
+                })
+            })
+            .collect()
     }
 
     pub fn on_mouse_window_move_event_class(&mut self, cursor: DevicePoint) {
@@ -955,25 +1004,17 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
     }
 
     fn dispatch_mouse_window_move_event_class(&mut self, cursor: DevicePoint) {
-        let root_pipeline_id = match self.root_pipeline.id {
-            Some(root_pipeline_id) => root_pipeline_id,
+        let result = match self.hit_test_at_device_point(cursor) {
+            Some(result) => result,
             None => return,
         };
-        if self.pipeline(root_pipeline_id).is_none() {
-            return;
-        }
 
-        let results = self.hit_test_at_point(cursor);
-        if let Some(item) = results.items.first() {
-            let node_address = Some(UntrustedNodeAddress(item.tag.0 as *const c_void));
-            let event = MouseMoveEvent(item.point_in_viewport.to_untyped(), node_address, 0);
-            let pipeline_id = PipelineId::from_webrender(item.pipeline);
-            let msg = ConstellationMsg::ForwardEvent(pipeline_id, event);
-            if let Err(e) = self.constellation_chan.send(msg) {
-                warn!("Sending event to constellation failed ({:?}).", e);
-            }
-            self.update_cursor(results);
+        let event = MouseMoveEvent(result.point_in_viewport, Some(result.node), 0);
+        let msg = ConstellationMsg::ForwardEvent(result.pipeline_id, event);
+        if let Err(e) = self.constellation_chan.send(msg) {
+            warn!("Sending event to constellation failed ({:?}).", e);
         }
+        self.update_cursor(result);
     }
 
     fn send_touch_event(
@@ -982,16 +1023,14 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         identifier: TouchId,
         point: DevicePoint,
     ) {
-        let results = self.hit_test_at_point(point);
-        if let Some(item) = results.items.first() {
+        if let Some(result) = self.hit_test_at_device_point(point) {
             let event = TouchEvent(
                 event_type,
                 identifier,
-                item.point_in_viewport.to_untyped(),
-                Some(UntrustedNodeAddress(item.tag.0 as *const c_void)),
+                result.point_in_viewport,
+                Some(result.node),
             );
-            let pipeline_id = PipelineId::from_webrender(item.pipeline);
-            let msg = ConstellationMsg::ForwardEvent(pipeline_id, event);
+            let msg = ConstellationMsg::ForwardEvent(result.pipeline_id, event);
             if let Err(e) = self.constellation_chan.send(msg) {
                 warn!("Sending event to constellation failed ({:?}).", e);
             }
@@ -999,15 +1038,9 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
     }
 
     pub fn send_wheel_event(&mut self, delta: WheelDelta, point: DevicePoint) {
-        let results = self.hit_test_at_point(point);
-        if let Some(item) = results.items.first() {
-            let event = WheelEvent(
-                delta,
-                item.point_in_viewport.to_untyped(),
-                Some(UntrustedNodeAddress(item.tag.0 as *const c_void)),
-            );
-            let pipeline_id = PipelineId::from_webrender(item.pipeline);
-            let msg = ConstellationMsg::ForwardEvent(pipeline_id, event);
+        if let Some(result) = self.hit_test_at_device_point(point) {
+            let event = WheelEvent(delta, result.point_in_viewport, Some(result.node));
+            let msg = ConstellationMsg::ForwardEvent(result.pipeline_id, event);
             if let Err(e) = self.constellation_chan.send(msg) {
                 warn!("Sending event to constellation failed ({:?}).", e);
             }
@@ -1169,7 +1202,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
                 sl @ ScrollLocation::Start | sl @ ScrollLocation::End => sl,
             };
             let cursor = (combined_event.cursor.to_f32() / self.scale).to_untyped();
-            let cursor = webrender_api::units::WorldPoint::from_untyped(cursor);
+            let cursor = WorldPoint::from_untyped(cursor);
             let mut txn = webrender_api::Transaction::new();
             txn.scroll(scroll_location, cursor);
             if combined_event.magnification != 1.0 {

components/embedder_traits/lib.rs

@@ -26,7 +26,7 @@ pub use webxr_api::MainThreadWaker as EventLoopWaker;
 /// A cursor for the window. This is different from a CSS cursor (see
 /// `CursorKind`) in that it has no `Auto` value.
 #[repr(u8)]
-#[derive(Clone, Copy, Deserialize, Eq, FromPrimitive, PartialEq, Serialize)]
+#[derive(Clone, Copy, Debug, Deserialize, Eq, FromPrimitive, PartialEq, Serialize)]
 pub enum Cursor {
     None,
     Default,

components/layout/display_list/builder.rs

@@ -413,11 +413,7 @@ impl<'a> DisplayListBuildState<'a> {
         clipping_and_scrolling: ClippingAndScrolling,
     ) -> BaseDisplayItem {
         BaseDisplayItem::new(
-            DisplayItemMetadata {
-                node,
-                // Store cursor id in display list.
-                pointing: cursor.map(|x| x as u16),
-            },
+            DisplayItemMetadata { node, cursor },
             clip_rect.to_layout(),
             section,
             self.current_stacking_context_id,

components/layout/display_list/items.rs

@@ -12,6 +12,7 @@
 //! They are therefore not exactly analogous to constructs like Skia pictures, which consist of
 //! low-level drawing primitives.
 
+use embedder_traits::Cursor;
 use euclid::{SideOffsets2D, Vector2D};
 use gfx_traits::print_tree::PrintTree;
 use gfx_traits::{self, StackingContextId};
@@ -465,7 +466,7 @@ impl BaseDisplayItem {
         BaseDisplayItem {
             metadata: DisplayItemMetadata {
                 node: OpaqueNode(0),
-                pointing: None,
+                cursor: None,
             },
             // Create a rectangle of maximal size.
             clip_rect: LayoutRect::max_rect(),
@@ -542,7 +543,7 @@ pub struct DisplayItemMetadata {
     pub node: OpaqueNode,
     /// The value of the `cursor` property when the mouse hovers over this display item. If `None`,
     /// this display item is ineligible for pointer events (`pointer-events: none`).
-    pub pointing: Option<u16>,
+    pub cursor: Option<Cursor>,
 }
 
 #[derive(Clone, Eq, PartialEq, Serialize)]

components/layout/display_list/webrender_helpers.rs

@@ -10,6 +10,7 @@
 use crate::display_list::items::{BaseDisplayItem, ClipScrollNode, ClipScrollNodeType, ClipType};
 use crate::display_list::items::{DisplayItem, DisplayList, StackingContextType};
 use msg::constellation_msg::PipelineId;
+use script_traits::compositor::CompositorDisplayListInfo;
 use webrender_api::units::{LayoutPoint, LayoutVector2D};
 use webrender_api::{
     self, ClipId, CommonItemProperties, DisplayItem as WrDisplayItem, DisplayListBuilder,
@@ -22,6 +23,7 @@ struct ClipScrollState {
     spatial_ids: Vec<Option<SpatialId>>,
     active_clip_id: ClipId,
     active_spatial_id: SpatialId,
+    compositor_info: CompositorDisplayListInfo,
 }
 
 impl ClipScrollState {
@@ -34,6 +36,7 @@ impl ClipScrollState {
             spatial_ids: vec![None; size],
             active_clip_id: root_clip_id,
             active_spatial_id: root_scroll_node_id,
+            compositor_info: CompositorDisplayListInfo::default(),
         };
 
         // We need to register the WebRender root reference frame and root scroll node ids
@@ -82,7 +85,7 @@ impl DisplayList {
     pub fn convert_to_webrender(
         &mut self,
         pipeline_id: PipelineId,
-    ) -> (DisplayListBuilder, IsContentful) {
+    ) -> (DisplayListBuilder, CompositorDisplayListInfo, IsContentful) {
         let webrender_pipeline = pipeline_id.to_webrender();
         let mut state = ClipScrollState::new(self.clip_scroll_nodes.len(), webrender_pipeline);
 
@@ -99,7 +102,7 @@ impl DisplayList {
                 .0;
         }
 
-        (builder, is_contentful)
+        (builder, state.compositor_info, is_contentful)
     }
 }
 
@@ -132,9 +135,14 @@ impl DisplayItem {
             state.active_clip_id = cur_clip_id;
         }
 
-        let build_common_item_properties = |base: &BaseDisplayItem| {
-            let tag = match base.metadata.pointing {
-                Some(cursor) => Some((base.metadata.node.0 as u64, cursor)),
+        let mut build_common_item_properties = |base: &BaseDisplayItem| {
+            let tag = match base.metadata.cursor {
+                Some(cursor) => {
+                    let hit_test_index = state
+                        .compositor_info
+                        .add_hit_test_info(base.metadata.node.0 as u64, Some(cursor));
+                    Some((hit_test_index as u64, 0u16))
+                },
                 None => None,
             };
             CommonItemProperties {

components/layout_2020/display_list/mod.rs

@@ -13,6 +13,7 @@ use euclid::{Point2D, SideOffsets2D, Size2D};
 use gfx::text::glyph::GlyphStore;
 use mitochondria::OnceCell;
 use net_traits::image_cache::UsePlaceholder;
+use script_traits::compositor::CompositorDisplayListInfo;
 use std::sync::Arc;
 use style::computed_values::text_decoration_style::T as ComputedTextDecorationStyle;
 use style::dom::OpaqueNode;
@@ -46,6 +47,7 @@ pub struct DisplayListBuilder<'a> {
     element_for_canvas_background: OpaqueNode,
     pub context: &'a LayoutContext<'a>,
     pub wr: wr::DisplayListBuilder,
+    pub compositor_info: CompositorDisplayListInfo,
 
     /// Contentful paint, for the purpose of
     /// https://w3c.github.io/paint-timing/#first-contentful-paint
@@ -66,6 +68,7 @@ impl<'a> DisplayListBuilder<'a> {
             is_contentful: false,
             context,
             wr: wr::DisplayListBuilder::new(pipeline_id, fragment_tree.scrollable_overflow()),
+            compositor_info: CompositorDisplayListInfo::default(),
         }
     }
 
@@ -85,6 +88,21 @@ impl<'a> DisplayListBuilder<'a> {
             flags: style.get_webrender_primitive_flags(),
         }
     }
+
+    fn hit_info(&mut self, style: &ComputedValues, tag: Tag, auto_cursor: Cursor) -> HitInfo {
+        use style::computed_values::pointer_events::T as PointerEvents;
+
+        let inherited_ui = style.get_inherited_ui();
+        if inherited_ui.pointer_events == PointerEvents::None {
+            None
+        } else {
+            let hit_test_index = self.compositor_info.add_hit_test_info(
+                tag.node().0 as u64,
+                Some(cursor(inherited_ui.cursor.keyword, auto_cursor)),
+            );
+            Some((hit_test_index as u64, 0u16))
+        }
+    }
 }
 
 impl Fragment {
@@ -157,7 +175,7 @@ impl Fragment {
         }
 
         let mut common = builder.common_properties(rect.to_webrender(), &fragment.parent_style);
-        common.hit_info = hit_info(&fragment.parent_style, fragment.tag, Cursor::Text);
+        common.hit_info = builder.hit_info(&fragment.parent_style, fragment.tag, Cursor::Text);
 
         let color = fragment.parent_style.clone_color();
         let font_metrics = &fragment.font_metrics;
@@ -351,7 +369,7 @@ impl<'a> BuilderForBoxFragment<'a> {
     }
 
     fn build_hit_test(&self, builder: &mut DisplayListBuilder) {
-        let hit_info = hit_info(&self.fragment.style, self.fragment.tag, Cursor::Default);
+        let hit_info = builder.hit_info(&self.fragment.style, self.fragment.tag, Cursor::Default);
         if hit_info.is_some() {
             let mut common = builder.common_properties(self.border_rect, &self.fragment.style);
             common.hit_info = hit_info;
@@ -559,18 +577,6 @@ fn glyphs(
     glyphs
 }
 
-fn hit_info(style: &ComputedValues, tag: Tag, auto_cursor: Cursor) -> HitInfo {
-    use style::computed_values::pointer_events::T as PointerEvents;
-
-    let inherited_ui = style.get_inherited_ui();
-    if inherited_ui.pointer_events == PointerEvents::None {
-        None
-    } else {
-        let cursor = cursor(inherited_ui.cursor.keyword, auto_cursor);
-        Some((tag.node().0 as u64, cursor as u16))
-    }
-}
-
 fn cursor(kind: CursorKind, auto_cursor: Cursor) -> Cursor {
     match kind {
         CursorKind::Auto => auto_cursor,

components/layout_thread/lib.rs

@@ -1146,7 +1146,8 @@ impl LayoutThread {
                 debug!("Layout done!");
 
                 // TODO: Avoid the temporary conversion and build webrender sc/dl directly!
-                let (builder, is_contentful) = display_list.convert_to_webrender(self.id);
+                let (builder, compositor_info, is_contentful) =
+                    display_list.convert_to_webrender(self.id);
 
                 let viewport_size = Size2D::new(
                     self.viewport_size.width.to_f32_px(),
@@ -1165,8 +1166,12 @@ impl LayoutThread {
                 self.paint_time_metrics
                     .maybe_observe_paint_time(self, epoch, is_contentful.0);
 
-                self.webrender_api
-                    .send_display_list(epoch, viewport_size, builder.finalize());
+                self.webrender_api.send_display_list(
+                    epoch,
+                    viewport_size,
+                    compositor_info,
+                    builder.finalize(),
+                );
             },
         );
     }
@@ -1594,11 +1599,8 @@ impl LayoutThread {
                         flags,
                     );
 
-                    rw_data.nodes_from_point_response = results
-                        .items
-                        .iter()
-                        .map(|item| UntrustedNodeAddress(item.tag.0 as *const c_void))
-                        .collect()
+                    rw_data.nodes_from_point_response =
+                        results.iter().map(|result| result.node).collect()
                 },
                 &QueryMsg::ElementInnerTextQuery(node) => {
                     let node = unsafe { ServoLayoutNode::new(&node) };

components/layout_thread_2020/lib.rs

@@ -1345,8 +1345,12 @@ impl LayoutThread {
             self.viewport_size.width.to_f32_px(),
             self.viewport_size.height.to_f32_px(),
         ));
-        self.webrender_api
-            .send_display_list(epoch, viewport_size, display_list.wr.finalize());
+        self.webrender_api.send_display_list(
+            epoch,
+            viewport_size,
+            display_list.compositor_info,
+            display_list.wr.finalize(),
+        );
 
         if self.trace_layout {
             layout_debug::end_trace(self.generation.get());

components/script_traits/compositor.rs

@@ -0,0 +1,44 @@
+/* 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/. */
+
+//! Defines data structures which are consumed by the Compositor.
+
+use embedder_traits::Cursor;
+
+/// Information that Servo keeps alongside WebRender display items
+/// in order to add more context to hit test results.
+#[derive(Debug, Deserialize, Serialize)]
+pub struct HitTestInfo {
+    /// The id of the node of this hit test item.
+    pub node: u64,
+
+    /// The cursor of this node's hit test item.
+    pub cursor: Option<Cursor>,
+}
+
+/// A data structure which stores compositor-side information about
+/// display lists sent to the compositor.
+/// by a WebRender display list.
+#[derive(Debug, Default, Deserialize, Serialize)]
+pub struct CompositorDisplayListInfo {
+    /// An array of `HitTestInfo` which is used to store information
+    /// to assist the compositor to take various actions (set the cursor,
+    /// scroll without layout) using a WebRender hit test result.
+    pub hit_test_info: Vec<HitTestInfo>,
+}
+
+impl CompositorDisplayListInfo {
+    /// Add or re-use a duplicate HitTestInfo entry in this `CompositorHitTestInfo`
+    /// and return the index.
+    pub fn add_hit_test_info(&mut self, node: u64, cursor: Option<Cursor>) -> usize {
+        if let Some(last) = self.hit_test_info.last() {
+            if node == last.node && cursor == last.cursor {
+                return self.hit_test_info.len() - 1;
+            }
+        }
+
+        self.hit_test_info.push(HitTestInfo { node, cursor });
+        self.hit_test_info.len() - 1
+    }
+}

components/script_traits/lib.rs

@@ -18,11 +18,13 @@ extern crate malloc_size_of_derive;
 #[macro_use]
 extern crate serde;
 
+pub mod compositor;
 mod script_msg;
 pub mod serializable;
 pub mod transferable;
 pub mod webdriver_msg;
 
+use crate::compositor::CompositorDisplayListInfo;
 use crate::serializable::{BlobData, BlobImpl};
 use crate::transferable::MessagePortImpl;
 use crate::webdriver_msg::{LoadStatus, WebDriverScriptCommand};
@@ -30,7 +32,7 @@ use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLPipeline;
 use crossbeam_channel::{Receiver, RecvTimeoutError, Sender};
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, WorkerId};
-use embedder_traits::EventLoopWaker;
+use embedder_traits::{Cursor, EventLoopWaker};
 use euclid::{default::Point2D, Length, Rect, Scale, Size2D, UnknownUnit, Vector2D};
 use gfx_traits::Epoch;
 use http::HeaderMap;
@@ -74,7 +76,7 @@ use webrender_api::{
     BuiltDisplayList, DocumentId, ExternalImageData, ExternalScrollId, ImageData, ImageDescriptor,
     ImageKey, ScrollClamping,
 };
-use webrender_api::{BuiltDisplayListDescriptor, HitTestFlags, HitTestResult};
+use webrender_api::{BuiltDisplayListDescriptor, HitTestFlags};
 
 pub use crate::script_msg::{
     DOMMessage, HistoryEntryReplacement, Job, JobError, JobResult, JobResultValue, JobType,
@@ -1110,6 +1112,25 @@ impl From<i32> for MediaSessionActionType {
     }
 }
 
+/// The result of a hit test in the compositor.
+#[derive(Clone, Debug, Deserialize, Serialize)]
+pub struct CompositorHitTestResult {
+    /// The pipeline id of the resulting item.
+    pub pipeline_id: PipelineId,
+
+    /// The hit test point in the item's viewport.
+    pub point_in_viewport: euclid::default::Point2D<f32>,
+
+    /// The hit test point relative to the item itself.
+    pub point_relative_to_item: euclid::default::Point2D<f32>,
+
+    /// The node address of the hit test result.
+    pub node: UntrustedNodeAddress,
+
+    /// The cursor that should be used when hovering the item hit by the hit test.
+    pub cursor: Option<Cursor>,
+}
+
 /// The set of WebRender operations that can be initiated by the content process.
 #[derive(Deserialize, Serialize)]
 pub enum WebrenderMsg {
@@ -1125,6 +1146,7 @@ pub enum WebrenderMsg {
         LayoutSize,
         ipc::IpcBytesReceiver,
         BuiltDisplayListDescriptor,
+        CompositorDisplayListInfo,
     ),
     /// Perform a hit test operation. The result will be returned via
     /// the provided channel sender.
@@ -1132,7 +1154,7 @@ pub enum WebrenderMsg {
         Option<webrender_api::PipelineId>,
         WorldPoint,
         HitTestFlags,
-        IpcSender<HitTestResult>,
+        IpcSender<Vec<CompositorHitTestResult>>,
     ),
     /// Create a new image key. The result will be returned via the
     /// provided channel sender.
@@ -1178,6 +1200,7 @@ impl WebrenderIpcSender {
         &self,
         epoch: Epoch,
         size: LayoutSize,
+        display_list_info: CompositorDisplayListInfo,
         (pipeline, size2, list): (webrender_api::PipelineId, LayoutSize, BuiltDisplayList),
     ) {
         let (data, descriptor) = list.into_data();
@@ -1189,6 +1212,7 @@ impl WebrenderIpcSender {
             size2,
             receiver,
             descriptor,
+            display_list_info,
         )) {
             warn!("Error sending display list: {}", e);
         }
@@ -1205,7 +1229,7 @@ impl WebrenderIpcSender {
         pipeline: Option<webrender_api::PipelineId>,
         point: WorldPoint,
         flags: HitTestFlags,
-    ) -> HitTestResult {
+    ) -> Vec<CompositorHitTestResult> {
         let (sender, receiver) = ipc::channel().unwrap();
         self.0
             .send(WebrenderMsg::HitTest(pipeline, point, flags, sender))