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/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,
-    StructuredSerializedData, WindowSizeType,
+    LoadData, NavigationHistoryBehavior, ScriptToConstellationChan, StructuredSerializedData,
+    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;
+use webrender_api::units::{DevicePixel, LayoutVector2D};
+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;
+use webrender_api::units::{DeviceIntSize, LayoutVector2D};
+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 |