layout: Send accessibility updates directly to embedder from layout. (#44208)

This introduces a `ScriptToEmbedderChan` property on `LayoutThread`,
which is used to send accessibility updates directly to the embedder.

Follow-up to https://github.com/servo/servo/pull/42338.

Testing: Covered by existing tests.

Signed-off-by: Alice Boxhall <alice@igalia.com>

components/layout/layout_impl.rs

@@ -13,7 +13,7 @@ use std::sync::{Arc, LazyLock};
 
 use app_units::Au;
 use bitflags::bitflags;
-use embedder_traits::{Theme, ViewportDetails};
+use embedder_traits::{EmbedderMsg, ScriptToEmbedderChan, Theme, ViewportDetails};
 use euclid::{Point2D, Rect, Scale, Size2D};
 use fonts::{FontContext, FontContextWebFontMethods, WebFontDocumentContext};
 use fonts_traits::StylesheetWebFontLoadFinishedCallback;
@@ -137,6 +137,9 @@ pub struct LayoutThread {
     /// The channel on which messages can be sent to the time profiler.
     time_profiler_chan: profile_time::ProfilerChan,
 
+    /// The channel to send messages to the Embedder.
+    embedder_chan: ScriptToEmbedderChan,
+
     /// Reference to the script thread image cache.
     image_cache: Arc<dyn ImageCache>,
 
@@ -730,6 +733,7 @@ impl LayoutThread {
             is_iframe: config.is_iframe,
             script_chan: config.script_chan.clone(),
             time_profiler_chan: config.time_profiler_chan,
+            embedder_chan: config.embedder_chan.clone(),
             registered_painters: RegisteredPaintersImpl(Default::default()),
             image_cache: config.image_cache,
             font_context: config.font_context,
@@ -890,8 +894,8 @@ impl LayoutThread {
             // finalise after sending, removing accessibility damage? On fail, retain damage
             // for next reflow, as well as retaining document.needs_accessibility_update.
             let _ = self
-                .script_chan
-                .send(ScriptThreadMessage::AccessibilityTreeUpdate(
+                .embedder_chan
+                .send(EmbedderMsg::AccessibilityTreeUpdate(
                     self.webview_id,
                     tree_update,
                     accessibility_tree.epoch(),

components/script/messaging.rs

@@ -106,7 +106,6 @@ impl MixedMessage {
                 ScriptThreadMessage::EmbedderControlResponse(id, _) => Some(id.pipeline_id),
                 ScriptThreadMessage::SetUserContents(..) => None,
                 ScriptThreadMessage::DestroyUserContentManager(..) => None,
-                ScriptThreadMessage::AccessibilityTreeUpdate(..) => None,
                 ScriptThreadMessage::UpdatePinchZoomInfos(id, _) => Some(*id),
                 ScriptThreadMessage::SetAccessibilityActive(..) => None,
                 ScriptThreadMessage::TriggerGarbageCollection => None,

components/script/script_thread.rs

@@ -1969,11 +1969,6 @@ impl ScriptThread {
                     .borrow_mut()
                     .remove(&user_content_manager_id);
             },
-            ScriptThreadMessage::AccessibilityTreeUpdate(webview_id, tree_update, epoch) => {
-                let _ = self.senders.pipeline_to_embedder_sender.send(
-                    EmbedderMsg::AccessibilityTreeUpdate(webview_id, tree_update, epoch),
-                );
-            },
             ScriptThreadMessage::UpdatePinchZoomInfos(id, pinch_zoom_infos) => {
                 self.handle_update_pinch_zoom_infos(id, pinch_zoom_infos, CanGc::from_cx(cx));
             },
@@ -3397,6 +3392,7 @@ impl ScriptThread {
             viewport_details: incomplete.viewport_details,
             user_stylesheets,
             theme: incomplete.theme,
+            embedder_chan: self.senders.pipeline_to_embedder_sender.clone(),
         };
 
         // Create the window and document objects.

components/shared/layout/lib.rs

@@ -25,7 +25,7 @@ use app_units::Au;
 use atomic_refcell::AtomicRefCell;
 use background_hang_monitor_api::BackgroundHangMonitorRegister;
 use bitflags::bitflags;
-use embedder_traits::{Cursor, Theme, UntrustedNodeAddress, ViewportDetails};
+use embedder_traits::{Cursor, ScriptToEmbedderChan, Theme, UntrustedNodeAddress, ViewportDetails};
 use euclid::{Point2D, Rect};
 use fonts::{FontContext, TextByteRange, WebFontDocumentContext};
 pub use layout_damage::LayoutDamage;
@@ -252,6 +252,7 @@ pub struct LayoutConfig {
     pub viewport_details: ViewportDetails,
     pub user_stylesheets: Rc<Vec<DocumentStyleSheet>>,
     pub theme: Theme,
+    pub embedder_chan: ScriptToEmbedderChan,
 }
 
 pub trait LayoutFactory: Send + Sync {

components/shared/script/lib.rs

@@ -312,8 +312,6 @@ pub enum ScriptThreadMessage {
     /// Release all data for the given `UserContentManagerId` from the `ScriptThread`'s
     /// `user_contents_for_manager_id` map.
     DestroyUserContentManager(UserContentManagerId),
-    /// Send the embedder an accessibility tree update.
-    AccessibilityTreeUpdate(WebViewId, accesskit::TreeUpdate, Epoch),
     /// Update the pinch zoom details of a pipeline. Each `Window` stores a `VisualViewport` DOM
     /// instance that gets updated according to the changes from the `Compositor``.
     UpdatePinchZoomInfos(PipelineId, PinchZoomInfos),