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/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 {
-                scroll_id,
-                scroll_offset: Vector2D::new(-x, -y),
-            }),
+            ReflowGoal::UpdateScrollNode(scroll_id, 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() {
-                    let scroll_offset = scroll_state.scroll_offset;
-                    if scroll_state.scroll_id.is_root() {
+                for (scroll_id, scroll_offset) in scroll_states.into_iter() {
+                    if scroll_id.is_root() {
                         window.update_viewport_for_scroll(-scroll_offset.x, -scroll_offset.y);
-                    } else if let Some(node_id) =
-                        node_id_from_scroll_id(scroll_state.scroll_id.0 as usize)
-                    {
+                    } else if let Some(node_id) = node_id_from_scroll_id(scroll_id.0 as usize) {
                         scroll_offsets.insert(OpaqueNode(node_id), -scroll_offset);
                     }
                 }