layout: Store scroll offsets in the ScrollTree (#37428)

There are currently five places that scroll offsets are stored:

 - DOM: A set of scroll offsets used for script.
 - Layout: An array of scroll offsets that is used for tracking
   layout-side scroll offsets.
 - Layout: The scroll offsets stored in the `ScrollTree`. These are
   currently unset and unused.
 - Compositor: The scroll offsets stored in the `ScrollTree` mirrored
   from layout.
 - WebRender: The scrolled offsets stored in the WebRender spatial tree.

This change is the first step in combining the first three into the
layout `ScrollTree`. It eliminates the extra array of scroll offsets
stored in layout in favor of the storing them in the `ScrollTree`. A
followup change will eliminate the ones stored in the DOM.

- In addition the `ScrollState` data structure is eliminated as these
are
now stored in a `HashMap` everywhere when passing them via IPC.
- The offsests stored in layout can now never scroll past the boundaries
of the scrolled content.

Testing: This should not change behavior and is thus covered by existing
WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: stevennovaryo <steven.novaryo@gmail.com>

components/compositing/compositor.rs

@@ -30,7 +30,6 @@ use embedder_traits::{
     TouchEventType, UntrustedNodeAddress, ViewportDetails, WheelDelta, WheelEvent, WheelMode,
 };
 use euclid::{Point2D, Rect, Scale, Size2D, Transform3D, Vector2D};
-use fnv::FnvHashMap;
 use ipc_channel::ipc::{self, IpcSharedMemory};
 use libc::c_void;
 use log::{debug, info, trace, warn};
@@ -48,10 +47,10 @@ use webrender_api::units::{
 };
 use webrender_api::{
     self, BuiltDisplayList, DirtyRect, DisplayListPayload, DocumentId, Epoch as WebRenderEpoch,
-    ExternalScrollId, FontInstanceFlags, FontInstanceKey, FontInstanceOptions, FontKey,
+    FontInstanceFlags, FontInstanceKey, FontInstanceOptions, FontKey, HitTestFlags,
-    HitTestFlags, PipelineId as WebRenderPipelineId, PropertyBinding, ReferenceFrameKind,
+    PipelineId as WebRenderPipelineId, PropertyBinding, ReferenceFrameKind, RenderReasons,
-    RenderReasons, SampledScrollOffset, ScrollLocation, SpaceAndClipInfo, SpatialId,
+    SampledScrollOffset, ScrollLocation, SpaceAndClipInfo, SpatialId, SpatialTreeItemKey,
-    SpatialTreeItemKey, TransformStyle,
+    TransformStyle,
 };
 
 use crate::InitialCompositorState;
@@ -260,26 +259,9 @@ impl PipelineDetails {
     }
 
     fn install_new_scroll_tree(&mut self, new_scroll_tree: ScrollTree) {
-        let old_scroll_offsets: FnvHashMap<ExternalScrollId, LayoutVector2D> = self
+        let old_scroll_offsets = self.scroll_tree.scroll_offsets();
-            .scroll_tree
-            .nodes
-            .drain(..)
-            .filter_map(|node| match (node.external_id(), node.offset()) {
-                (Some(external_id), Some(offset)) => Some((external_id, offset)),
-                _ => None,
-            })
-            .collect();
-
         self.scroll_tree = new_scroll_tree;
-        for node in self.scroll_tree.nodes.iter_mut() {
+        self.scroll_tree.set_all_scroll_offsets(&old_scroll_offsets);
-            match node.external_id() {
-                Some(external_id) => match old_scroll_offsets.get(&external_id) {
-                    Some(new_offset) => node.set_offset(*new_offset),
-                    None => continue,
-                },
-                _ => continue,
-            };
-        }
     }
 }
 
@@ -728,7 +710,7 @@ impl IOCompositor {
                 self.global.borrow_mut().send_transaction(txn);
             },
 
-            CompositorMsg::SendScrollNode(webview_id, pipeline_id, point, external_scroll_id) => {
+            CompositorMsg::SendScrollNode(webview_id, pipeline_id, offset, external_scroll_id) => {
                 let Some(webview_renderer) = self.webview_renderers.get_mut(webview_id) else {
                     return;
                 };
@@ -739,15 +721,17 @@ impl IOCompositor {
                     return;
                 };
 
-                let offset = LayoutVector2D::new(point.x, point.y);
                 let Some(offset) = pipeline_details
                     .scroll_tree
-                    .set_scroll_offsets_for_node_with_external_scroll_id(
+                    .set_scroll_offset_for_node_with_external_scroll_id(
                         external_scroll_id,
-                        -offset,
+                        offset,
                         ScrollType::Script,
                     )
                 else {
+                    // The renderer should be fully up-to-date with script at this point and script
+                    // should never try to scroll to an invalid location.
+                    warn!("Could not scroll node with id: {external_scroll_id:?}");
                     return;
                 };
 

components/compositing/webview_renderer.rs

@@ -13,7 +13,7 @@ use compositing_traits::viewport_description::{
     DEFAULT_ZOOM, MAX_ZOOM, MIN_ZOOM, ViewportDescription,
 };
 use compositing_traits::{SendableFrameTree, WebViewTrait};
-use constellation_traits::{EmbedderToConstellationMessage, ScrollState, WindowSizeType};
+use constellation_traits::{EmbedderToConstellationMessage, WindowSizeType};
 use embedder_traits::{
     AnimationState, CompositorHitTestResult, InputEvent, MouseButton, MouseButtonAction,
     MouseButtonEvent, MouseMoveEvent, ShutdownState, TouchEvent, TouchEventResult, TouchEventType,
@@ -204,18 +204,18 @@ impl WebViewRenderer {
             return;
         };
 
-        let mut scroll_states = Vec::new();
+        let scroll_offsets = details.scroll_tree.scroll_offsets();
-        details.scroll_tree.nodes.iter().for_each(|node| {
+
-            if let (Some(scroll_id), Some(scroll_offset)) = (node.external_id(), node.offset()) {
+        // This might be true if we have not received a display list from the layout
-                scroll_states.push(ScrollState {
+        // associated with this pipeline yet. In that case, the layout is not ready to
-                    scroll_id,
+        // receive scroll offsets anyway, so just save time and prevent other issues by
-                    scroll_offset,
+        // not sending them.
-                });
+        if scroll_offsets.is_empty() {
-            }
+            return;
-        });
+        }
 
         let _ = self.global.borrow().constellation_sender.send(
-            EmbedderToConstellationMessage::SetScrollStates(pipeline_id, scroll_states),
+            EmbedderToConstellationMessage::SetScrollStates(pipeline_id, scroll_offsets),
         );
     }
 

components/constellation/constellation.rs

@@ -116,9 +116,8 @@ use constellation_traits::{
     EmbedderToConstellationMessage, IFrameLoadInfo, IFrameLoadInfoWithData, IFrameSandboxState,
     IFrameSizeMsg, Job, LoadData, LoadOrigin, LogEntry, MessagePortMsg, NavigationHistoryBehavior,
     PaintMetricEvent, PortMessageTask, PortTransferInfo, SWManagerMsg, SWManagerSenders,
-    ScriptToConstellationChan, ScriptToConstellationMessage, ScrollState,
+    ScriptToConstellationChan, ScriptToConstellationMessage, ServiceWorkerManagerFactory,
-    ServiceWorkerManagerFactory, ServiceWorkerMsg, StructuredSerializedData, TraversalDirection,
+    ServiceWorkerMsg, StructuredSerializedData, TraversalDirection, WindowSizeType,
-    WindowSizeType,
 };
 use crossbeam_channel::{Receiver, Select, Sender, unbounded};
 use devtools_traits::{
@@ -167,7 +166,8 @@ use webgpu_traits::{WebGPU, WebGPURequest};
 #[cfg(feature = "webgpu")]
 use webrender::RenderApi;
 use webrender::RenderApiSender;
-use webrender_api::{DocumentId, ImageKey};
+use webrender_api::units::LayoutVector2D;
+use webrender_api::{DocumentId, ExternalScrollId, ImageKey};
 
 use crate::browsingcontext::{
     AllBrowsingContextsIterator, BrowsingContext, FullyActiveBrowsingContextsIterator,
@@ -6051,7 +6051,11 @@ where
         feature = "tracing",
         tracing::instrument(skip_all, fields(servo_profiling = true), level = "trace")
     )]
-    fn handle_set_scroll_states(&self, pipeline_id: PipelineId, scroll_states: Vec<ScrollState>) {
+    fn handle_set_scroll_states(
+        &self,
+        pipeline_id: PipelineId,
+        scroll_states: HashMap<ExternalScrollId, LayoutVector2D>,
+    ) {
         let Some(pipeline) = self.pipelines.get(&pipeline_id) else {
             warn!("Discarding scroll offset update for unknown pipeline");
             return;

components/layout/layout_impl.rs

@@ -15,16 +15,16 @@ use app_units::Au;
 use base::Epoch;
 use base::id::{PipelineId, WebViewId};
 use compositing_traits::CrossProcessCompositorApi;
-use constellation_traits::ScrollState;
+use compositing_traits::display_list::ScrollType;
 use embedder_traits::{Theme, UntrustedNodeAddress, ViewportDetails};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect};
-use euclid::{Point2D, Scale, Size2D, Vector2D};
+use euclid::{Point2D, Scale, Size2D};
 use fnv::FnvHashMap;
 use fonts::{FontContext, FontContextWebFontMethods};
 use fonts_traits::StylesheetWebFontLoadFinishedCallback;
 use fxhash::FxHashMap;
 use ipc_channel::ipc::IpcSender;
-use log::{debug, error};
+use log::{debug, error, warn};
 use malloc_size_of::{MallocConditionalSizeOf, MallocSizeOf, MallocSizeOfOps};
 use net_traits::image_cache::{ImageCache, UsePlaceholder};
 use parking_lot::{Mutex, RwLock};
@@ -74,7 +74,7 @@ use style::{Zero, driver};
 use style_traits::{CSSPixel, SpeculativePainter};
 use stylo_atoms::Atom;
 use url::Url;
-use webrender_api::units::{DevicePixel, DevicePoint, LayoutPixel, LayoutPoint, LayoutSize};
+use webrender_api::units::{DevicePixel, DevicePoint, LayoutSize, LayoutVector2D};
 use webrender_api::{ExternalScrollId, HitTestFlags};
 
 use crate::context::{CachedImageOrError, LayoutContext};
@@ -149,6 +149,12 @@ pub struct LayoutThread {
     /// layout trees remain the same.
     need_new_display_list: Cell<bool>,
 
+    /// Whether or not the existing stacking context tree is dirty and needs to be
+    /// rebuilt. This happens after a relayout or overflow update. The reason that we
+    /// don't simply clear the stacking context tree when it becomes dirty is that we need
+    /// to preserve scroll offsets from the old tree to the new one.
+    need_new_stacking_context_tree: Cell<bool>,
+
     /// The box tree.
     box_tree: RefCell<Option<Arc<BoxTree>>>,
 
@@ -161,9 +167,6 @@ pub struct LayoutThread {
     /// A counter for epoch messages
     epoch: Cell<Epoch>,
 
-    /// Scroll offsets of nodes that scroll.
-    scroll_offsets: RefCell<HashMap<ExternalScrollId, Vector2D<f32, LayoutPixel>>>,
-
     // A cache that maps image resources specified in CSS (e.g as the `url()` value
     // for `background-image` or `content` properties) to either the final resolved
     // image data, or an error if the image cache failed to load/decode the image.
@@ -485,11 +488,20 @@ impl Layout for LayoutThread {
     ) {
     }
 
-    fn set_scroll_offsets(&mut self, scroll_states: &[ScrollState]) {
+    fn set_scroll_offsets_from_renderer(
-        *self.scroll_offsets.borrow_mut() = scroll_states
+        &mut self,
-            .iter()
+        scroll_states: &HashMap<ExternalScrollId, LayoutVector2D>,
-            .map(|scroll_state| (scroll_state.scroll_id, scroll_state.scroll_offset))
+    ) {
-            .collect();
+        let mut stacking_context_tree = self.stacking_context_tree.borrow_mut();
+        let Some(stacking_context_tree) = stacking_context_tree.as_mut() else {
+            warn!("Received scroll offsets before finishing layout.");
+            return;
+        };
+
+        stacking_context_tree
+            .compositor_info
+            .scroll_tree
+            .set_all_scroll_offsets(scroll_states);
     }
 }
 
@@ -533,13 +545,13 @@ impl LayoutThread {
             have_added_user_agent_stylesheets: false,
             have_ever_generated_display_list: Cell::new(false),
             need_new_display_list: Cell::new(false),
+            need_new_stacking_context_tree: Cell::new(false),
             box_tree: Default::default(),
             fragment_tree: Default::default(),
             stacking_context_tree: Default::default(),
             // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
             epoch: Cell::new(Epoch(1)),
             compositor_api: config.compositor_api,
-            scroll_offsets: Default::default(),
             stylist: Stylist::new(device, QuirksMode::NoQuirks),
             resolved_images_cache: Default::default(),
             debug: opts::get().debug.clone(),
@@ -674,8 +686,9 @@ impl LayoutThread {
         let built_display_list =
             self.build_display_list(&reflow_request, damage, &mut layout_context);
 
-        if let ReflowGoal::UpdateScrollNode(scroll_state) = reflow_request.reflow_goal {
+        if let ReflowGoal::UpdateScrollNode(external_scroll_id, offset) = reflow_request.reflow_goal
-            self.update_scroll_node_state(&scroll_state);
+        {
+            self.set_scroll_offset_from_script(external_scroll_id, offset);
         }
 
         let pending_images = std::mem::take(&mut *layout_context.pending_images.lock());
@@ -828,12 +841,10 @@ impl LayoutThread {
 
         *self.fragment_tree.borrow_mut() = Some(fragment_tree);
 
-        // The FragmentTree has been updated, so any existing StackingContext tree that layout
+        // Changes to layout require us to generate a new stacking context tree and display
-        // had is now out of date and should be rebuilt.
+        // list the next time one is requested.
-        *self.stacking_context_tree.borrow_mut() = None;
-
-        // Force display list generation as layout has changed.
         self.need_new_display_list.set(true);
+        self.need_new_stacking_context_tree.set(true);
 
         if self.debug.dump_style_tree {
             println!(
@@ -866,6 +877,11 @@ impl LayoutThread {
                 fragment_tree.print();
             }
         }
+
+        // Changes to overflow require us to generate a new stacking context tree and
+        // display list the next time one is requested.
+        self.need_new_display_list.set(true);
+        self.need_new_stacking_context_tree.set(true);
     }
 
     fn build_stacking_context_tree(&self, reflow_request: &ReflowRequest, damage: RestyleDamage) {
@@ -878,7 +894,7 @@ impl LayoutThread {
             return;
         };
         if !damage.contains(RestyleDamage::REBUILD_STACKING_CONTEXT) &&
-            self.stacking_context_tree.borrow().is_some()
+            !self.need_new_stacking_context_tree.get()
         {
             return;
         }
@@ -889,19 +905,39 @@ impl LayoutThread {
             viewport_size.height.to_f32_px(),
         );
 
+        let mut stacking_context_tree = self.stacking_context_tree.borrow_mut();
+        let old_scroll_offsets = stacking_context_tree
+            .as_ref()
+            .map(|tree| tree.compositor_info.scroll_tree.scroll_offsets());
+
         // Build the StackingContextTree. This turns the `FragmentTree` into a
         // tree of fragments in CSS painting order and also creates all
         // applicable spatial and clip nodes.
-        *self.stacking_context_tree.borrow_mut() = Some(StackingContextTree::new(
+        let mut new_stacking_context_tree = StackingContextTree::new(
             fragment_tree,
             viewport_size,
             self.id.into(),
             !self.have_ever_generated_display_list.get(),
             &self.debug,
-        ));
+        );
+
+        // When a new StackingContextTree is built, it contains a freshly built
+        // ScrollTree. We want to preserve any existing scroll offsets in that tree,
+        // adjusted by any new scroll constraints.
+        if let Some(old_scroll_offsets) = old_scroll_offsets {
+            new_stacking_context_tree
+                .compositor_info
+                .scroll_tree
+                .set_all_scroll_offsets(&old_scroll_offsets);
+        }
+
+        *stacking_context_tree = Some(new_stacking_context_tree);
 
         // Force display list generation as layout has changed.
         self.need_new_display_list.set(true);
+
+        // The stacking context tree is up-to-date again.
+        self.need_new_stacking_context_tree.set(false);
     }
 
     /// Build the display list for the current layout and send it to the renderer. If no display
@@ -959,17 +995,32 @@ impl LayoutThread {
         true
     }
 
-    fn update_scroll_node_state(&self, state: &ScrollState) {
+    fn set_scroll_offset_from_script(
-        self.scroll_offsets
+        &self,
-            .borrow_mut()
+        external_scroll_id: ExternalScrollId,
-            .insert(state.scroll_id, state.scroll_offset);
+        offset: LayoutVector2D,
-        let point = Point2D::new(-state.scroll_offset.x, -state.scroll_offset.y);
+    ) {
-        self.compositor_api.send_scroll_node(
+        let mut stacking_context_tree = self.stacking_context_tree.borrow_mut();
-            self.webview_id,
+        let Some(stacking_context_tree) = stacking_context_tree.as_mut() else {
-            self.id.into(),
+            return;
-            LayoutPoint::from_untyped(point),
+        };
-            state.scroll_id,
+
-        );
+        if let Some(offset) = stacking_context_tree
+            .compositor_info
+            .scroll_tree
+            .set_scroll_offset_for_node_with_external_scroll_id(
+                external_scroll_id,
+                offset,
+                ScrollType::Script,
+            )
+        {
+            self.compositor_api.send_scroll_node(
+                self.webview_id,
+                self.id.into(),
+                offset,
+                external_scroll_id,
+            );
+        }
     }
 
     /// Returns profiling information which is passed to the time profiler.

components/script/dom/window.rs

@@ -24,7 +24,7 @@ use canvas_traits::webgl::WebGLChan;
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::{
     DocumentState, LoadData, LoadOrigin, NavigationHistoryBehavior, ScriptToConstellationChan,
-    ScriptToConstellationMessage, ScrollState, StructuredSerializedData, WindowSizeType,
+    ScriptToConstellationMessage, StructuredSerializedData, WindowSizeType,
 };
 use crossbeam_channel::{Sender, unbounded};
 use cssparser::SourceLocation;
@@ -2097,10 +2097,7 @@ impl Window {
         // TODO(mrobinson, #18709): Add smooth scrolling support to WebRender so that we can
         // properly process ScrollBehavior here.
         self.reflow(
-            ReflowGoal::UpdateScrollNode(ScrollState {
+            ReflowGoal::UpdateScrollNode(scroll_id, Vector2D::new(-x, -y)),
-                scroll_id,
-                scroll_offset: Vector2D::new(-x, -y),
-            }),
             can_gc,
         );
     }
@@ -3234,7 +3231,7 @@ fn should_move_clip_rect(clip_rect: UntypedRect<Au>, new_viewport: UntypedRect<f
 fn debug_reflow_events(id: PipelineId, reflow_goal: &ReflowGoal) {
     let goal_string = match *reflow_goal {
         ReflowGoal::UpdateTheRendering => "\tFull",
-        ReflowGoal::UpdateScrollNode(_) => "\tUpdateScrollNode",
+        ReflowGoal::UpdateScrollNode(..) => "\tUpdateScrollNode",
         ReflowGoal::LayoutQuery(ref query_msg) => match *query_msg {
             QueryMsg::ContentBox => "\tContentBoxQuery",
             QueryMsg::ContentBoxes => "\tContentBoxesQuery",

components/script/script_thread.rs

@@ -39,7 +39,7 @@ use chrono::{DateTime, Local};
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::{
     JsEvalResult, LoadData, LoadOrigin, NavigationHistoryBehavior, ScriptToConstellationChan,
-    ScriptToConstellationMessage, ScrollState, StructuredSerializedData, WindowSizeType,
+    ScriptToConstellationMessage, StructuredSerializedData, WindowSizeType,
 };
 use content_security_policy::{self as csp};
 use crossbeam_channel::unbounded;
@@ -99,7 +99,8 @@ use timers::{TimerEventRequest, TimerId, TimerScheduler};
 use url::Position;
 #[cfg(feature = "webgpu")]
 use webgpu_traits::{WebGPUDevice, WebGPUMsg};
-use webrender_api::units::DevicePixel;
+use webrender_api::ExternalScrollId;
+use webrender_api::units::{DevicePixel, LayoutVector2D};
 
 use crate::document_collection::DocumentCollection;
 use crate::document_loader::DocumentLoader;
@@ -2010,7 +2011,11 @@ impl ScriptThread {
         }
     }
 
-    fn handle_set_scroll_states(&self, pipeline_id: PipelineId, scroll_states: Vec<ScrollState>) {
+    fn handle_set_scroll_states(
+        &self,
+        pipeline_id: PipelineId,
+        scroll_states: HashMap<ExternalScrollId, LayoutVector2D>,
+    ) {
         let Some(window) = self.documents.borrow().find_window(pipeline_id) else {
             warn!("Received scroll states for closed pipeline {pipeline_id}");
             return;
@@ -2020,16 +2025,15 @@ impl ScriptThread {
             ScriptThreadEventCategory::SetScrollState,
             Some(pipeline_id),
             || {
-                window.layout_mut().set_scroll_offsets(&scroll_states);
+                window
+                    .layout_mut()
+                    .set_scroll_offsets_from_renderer(&scroll_states);
 
                 let mut scroll_offsets = HashMap::new();
-                for scroll_state in scroll_states.into_iter() {
+                for (scroll_id, scroll_offset) in scroll_states.into_iter() {
-                    let scroll_offset = scroll_state.scroll_offset;
+                    if scroll_id.is_root() {
-                    if scroll_state.scroll_id.is_root() {
                         window.update_viewport_for_scroll(-scroll_offset.x, -scroll_offset.y);
-                    } else if let Some(node_id) =
+                    } else if let Some(node_id) = node_id_from_scroll_id(scroll_id.0 as usize) {
-                        node_id_from_scroll_id(scroll_state.scroll_id.0 as usize)
-                    {
                         scroll_offsets.insert(OpaqueNode(node_id), -scroll_offset);
                     }
                 }

components/shared/compositing/display_list.rs

@@ -4,6 +4,8 @@
 
 //! Defines data structures which are consumed by the Compositor.
 
+use std::collections::HashMap;
+
 use base::id::ScrollTreeNodeId;
 use bitflags::bitflags;
 use embedder_traits::Cursor;
@@ -338,7 +340,7 @@ impl ScrollTree {
 
     /// Given an [`ExternalScrollId`] and an offset, update the scroll offset of the scroll node
     /// with the given id.
-    pub fn set_scroll_offsets_for_node_with_external_scroll_id(
+    pub fn set_scroll_offset_for_node_with_external_scroll_id(
         &mut self,
         external_scroll_id: ExternalScrollId,
         offset: LayoutVector2D,
@@ -353,6 +355,29 @@ impl ScrollTree {
             _ => None,
         })
     }
+
+    /// Given a set of all scroll offsets coming from the Servo renderer, update all of the offsets
+    /// for nodes that actually exist in this tree.
+    pub fn set_all_scroll_offsets(&mut self, offsets: &HashMap<ExternalScrollId, LayoutVector2D>) {
+        for node in self.nodes.iter_mut() {
+            if let SpatialTreeNodeInfo::Scroll(ref mut scroll_info) = node.info {
+                if let Some(offset) = offsets.get(&scroll_info.external_id) {
+                    scroll_info.offset = *offset;
+                }
+            }
+        }
+    }
+
+    /// Collect all of the scroll offsets of the scrolling nodes of this tree into a
+    /// [`HashMap`] which can be applied to another tree.
+    pub fn scroll_offsets(&self) -> HashMap<ExternalScrollId, LayoutVector2D> {
+        HashMap::from_iter(self.nodes.iter().filter_map(|node| match node.info {
+            SpatialTreeNodeInfo::Scroll(ref scroll_info) => {
+                Some((scroll_info.external_id, scroll_info.offset))
+            },
+            _ => None,
+        }))
+    }
 }
 
 /// A data structure which stores compositor-side information about

components/shared/compositing/lib.rs

@@ -35,7 +35,7 @@ use ipc_channel::ipc::{self, IpcSharedMemory};
 use profile_traits::mem::{OpaqueSender, ReportsChan};
 use serde::{Deserialize, Serialize};
 use servo_geometry::{DeviceIndependentIntRect, DeviceIndependentIntSize};
-use webrender_api::units::{DevicePoint, LayoutPoint, TexelRect};
+use webrender_api::units::{DevicePoint, LayoutVector2D, TexelRect};
 use webrender_api::{
     BuiltDisplayList, BuiltDisplayListDescriptor, ExternalImage, ExternalImageData,
     ExternalImageHandler, ExternalImageId, ExternalImageSource, ExternalScrollId,
@@ -124,7 +124,7 @@ pub enum CompositorMsg {
     SendScrollNode(
         WebViewId,
         WebRenderPipelineId,
-        LayoutPoint,
+        LayoutVector2D,
         ExternalScrollId,
     ),
     /// Inform WebRender of a new display list for the given pipeline.
@@ -232,7 +232,7 @@ impl CrossProcessCompositorApi {
         &self,
         webview_id: WebViewId,
         pipeline_id: WebRenderPipelineId,
-        point: LayoutPoint,
+        point: LayoutVector2D,
         scroll_id: ExternalScrollId,
     ) {
         if let Err(e) = self.0.send(CompositorMsg::SendScrollNode(

components/shared/constellation/lib.rs

@@ -22,7 +22,6 @@ use embedder_traits::{
     CompositorHitTestResult, Cursor, InputEvent, JavaScriptEvaluationId, MediaSessionActionType,
     Theme, ViewportDetails, WebDriverCommandMsg,
 };
-use euclid::Vector2D;
 pub use from_script_message::*;
 use ipc_channel::ipc::IpcSender;
 use malloc_size_of_derive::MallocSizeOf;
@@ -32,7 +31,7 @@ use servo_url::{ImmutableOrigin, ServoUrl};
 pub use structured_data::*;
 use strum_macros::IntoStaticStr;
 use webrender_api::ExternalScrollId;
-use webrender_api::units::LayoutPixel;
+use webrender_api::units::LayoutVector2D;
 
 /// Messages to the Constellation from the embedding layer, whether from `ServoRenderer` or
 /// from `libservo` itself.
@@ -90,7 +89,7 @@ pub enum EmbedderToConstellationMessage {
     SetWebViewThrottled(WebViewId, bool),
     /// The Servo renderer scrolled and is updating the scroll states of the nodes in the
     /// given pipeline via the constellation.
-    SetScrollStates(PipelineId, Vec<ScrollState>),
+    SetScrollStates(PipelineId, HashMap<ExternalScrollId, LayoutVector2D>),
     /// Notify the constellation that a particular paint metric event has happened for the given pipeline.
     PaintMetric(PipelineId, PaintMetricEvent),
     /// Evaluate a JavaScript string in the context of a `WebView`. When execution is complete or an
@@ -136,15 +135,6 @@ pub enum WindowSizeType {
     Resize,
 }
 
-/// The scroll state of a stacking context.
-#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
-pub struct ScrollState {
-    /// The ID of the scroll root.
-    pub scroll_id: ExternalScrollId,
-    /// The scrolling offset of this stacking context.
-    pub scroll_offset: Vector2D<f32, LayoutPixel>,
-}
-
 /// The direction of a history traversal
 #[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
 pub enum TraversalDirection {

components/shared/script/lib.rs

@@ -9,6 +9,7 @@
 #![deny(missing_docs)]
 #![deny(unsafe_code)]
 
+use std::collections::HashMap;
 use std::fmt;
 use std::sync::Arc;
 
@@ -20,8 +21,8 @@ use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLPipeline;
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::{
-    LoadData, NavigationHistoryBehavior, ScriptToConstellationChan, ScrollState,
+    LoadData, NavigationHistoryBehavior, ScriptToConstellationChan, StructuredSerializedData,
-    StructuredSerializedData, WindowSizeType,
+    WindowSizeType,
 };
 use crossbeam_channel::{RecvTimeoutError, Sender};
 use devtools_traits::ScriptToDevtoolsControlMsg;
@@ -47,8 +48,8 @@ use style_traits::{CSSPixel, SpeculativePainter};
 use stylo_atoms::Atom;
 #[cfg(feature = "webgpu")]
 use webgpu_traits::WebGPUMsg;
-use webrender_api::ImageKey;
+use webrender_api::units::{DevicePixel, LayoutVector2D};
-use webrender_api::units::DevicePixel;
+use webrender_api::{ExternalScrollId, ImageKey};
 
 /// The initial data required to create a new layout attached to an existing script thread.
 #[derive(Debug, Deserialize, Serialize)]
@@ -246,7 +247,7 @@ pub enum ScriptThreadMessage {
     SetWebGPUPort(IpcReceiver<WebGPUMsg>),
     /// The compositor scrolled and is updating the scroll states of the nodes in the given
     /// pipeline via the Constellation.
-    SetScrollStates(PipelineId, Vec<ScrollState>),
+    SetScrollStates(PipelineId, HashMap<ExternalScrollId, LayoutVector2D>),
     /// Evaluate the given JavaScript and return a result via a corresponding message
     /// to the Constellation.
     EvaluateJavaScript(PipelineId, JavaScriptEvaluationId, String),

components/shared/script_layout/lib.rs

@@ -11,6 +11,7 @@
 pub mod wrapper_traits;
 
 use std::any::Any;
+use std::collections::HashMap;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicIsize, AtomicU64, Ordering};
 
@@ -19,7 +20,7 @@ use atomic_refcell::AtomicRefCell;
 use base::Epoch;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use compositing_traits::CrossProcessCompositorApi;
-use constellation_traits::{LoadData, ScrollState};
+use constellation_traits::LoadData;
 use embedder_traits::{Theme, UntrustedNodeAddress, ViewportDetails};
 use euclid::default::{Point2D, Rect};
 use fnv::FnvHashMap;
@@ -48,8 +49,8 @@ use style::properties::PropertyId;
 use style::properties::style_structs::Font;
 use style::selector_parser::{PseudoElement, RestyleDamage, Snapshot};
 use style::stylesheets::Stylesheet;
-use webrender_api::ImageKey;
+use webrender_api::units::{DeviceIntSize, LayoutVector2D};
-use webrender_api::units::DeviceIntSize;
+use webrender_api::{ExternalScrollId, ImageKey};
 
 pub trait GenericLayoutDataTrait: Any + MallocSizeOfTrait {
     fn as_any(&self) -> &dyn Any;
@@ -245,7 +246,10 @@ pub trait Layout {
     );
 
     /// Set the scroll states of this layout after a compositor scroll.
-    fn set_scroll_offsets(&mut self, scroll_states: &[ScrollState]);
+    fn set_scroll_offsets_from_renderer(
+        &mut self,
+        scroll_states: &HashMap<ExternalScrollId, LayoutVector2D>,
+    );
 
     fn query_content_box(&self, node: TrustedNodeAddress) -> Option<Rect<Au>>;
     fn query_content_boxes(&self, node: TrustedNodeAddress) -> Vec<Rect<Au>>;
@@ -333,7 +337,7 @@ pub enum ReflowGoal {
 
     /// Tells layout about a single new scrolling offset from the script. The rest will
     /// remain untouched and layout won't forward this back to script.
-    UpdateScrollNode(ScrollState),
+    UpdateScrollNode(ExternalScrollId, LayoutVector2D),
 }
 
 impl ReflowGoal {
@@ -341,7 +345,7 @@ impl ReflowGoal {
     /// be present or false if it only needs stacking-relative positions.
     pub fn needs_display_list(&self) -> bool {
         match *self {
-            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(_) => true,
+            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(..) => true,
             ReflowGoal::LayoutQuery(ref querymsg) => match *querymsg {
                 QueryMsg::ElementInnerOuterTextQuery |
                 QueryMsg::InnerWindowDimensionsQuery |
@@ -363,7 +367,7 @@ impl ReflowGoal {
     /// false if a layout_thread display list is sufficient.
     pub fn needs_display(&self) -> bool {
         match *self {
-            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(_) => true,
+            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(..) => true,
             ReflowGoal::LayoutQuery(ref querymsg) => match *querymsg {
                 QueryMsg::NodesFromPointQuery |
                 QueryMsg::TextIndexQuery |