Proxy all WR interactions for layout/font/script/canvas threads to the compositor

thread. There is now a single RenderApi that is used, and all transactions are serialized
through the compositor.

16 files changed, 345 insertions, 262 deletions

diff --git a/Cargo.lock b/Cargo.lock
index a961d3ae81d..e13c7dff6a9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -755,6 +755,7 @@ dependencies = [
 name = "compositing"
 version = "0.0.1"
 dependencies = [
+ "app_units",
  "canvas",
  "crossbeam-channel",
  "embedder_traits",
@@ -1892,10 +1893,12 @@ dependencies = [
 name = "gfx_traits"
 version = "0.0.1"
 dependencies = [
+ "app_units",
  "malloc_size_of",
  "malloc_size_of_derive",
  "range",
  "serde",
+ "webrender_api",
 ]
 
 [[package]]
@@ -3018,6 +3021,7 @@ dependencies = [
 name = "libservo"
 version = "0.0.1"
 dependencies = [
+ "app_units",
  "background_hang_monitor",
  "bluetooth",
  "bluetooth_traits",
@@ -3034,6 +3038,7 @@ dependencies = [
  "euclid",
  "gaol",
  "gfx",
+ "gfx_traits",
  "gleam 0.11.0",
  "gstreamer",
  "ipc-channel",
@@ -3928,7 +3933,7 @@ dependencies = [
 [[package]]
 name = "peek-poke"
 version = "0.2.0"
-source = "git+https://github.com/servo/webrender#01082a9091ab98c392af8934d04271eb1dd546df"
+source = "git+https://github.com/servo/webrender#de3999583ab20aad7c57ea35a3e0394ed45be627"
 dependencies = [
  "euclid",
  "peek-poke-derive 0.2.1 (git+https://github.com/servo/webrender)",
@@ -3946,7 +3951,7 @@ dependencies = [
 [[package]]
 name = "peek-poke-derive"
 version = "0.2.1"
-source = "git+https://github.com/servo/webrender#01082a9091ab98c392af8934d04271eb1dd546df"
+source = "git+https://github.com/servo/webrender#de3999583ab20aad7c57ea35a3e0394ed45be627"
 dependencies = [
  "proc-macro2 1.0.17",
  "quote 1.0.2",
@@ -4413,15 +4418,6 @@ checksum = "cabe4fa914dec5870285fa7f71f602645da47c486e68486d2b4ceb4a343e90ac"
 
 [[package]]
 name = "ron"
-version = "0.1.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "da06feaa07f69125ab9ddc769b11de29090122170b402547f64b86fe16ebc399"
-dependencies = [
- "serde",
-]
-
-[[package]]
-name = "ron"
 version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2ece421e0c4129b90e4a35b6f625e472e96c552136f5093a2f4fa2bbb75a62d5"
@@ -6401,7 +6397,7 @@ dependencies = [
 [[package]]
 name = "webrender"
 version = "0.61.0"
-source = "git+https://github.com/servo/webrender#01082a9091ab98c392af8934d04271eb1dd546df"
+source = "git+https://github.com/servo/webrender#de3999583ab20aad7c57ea35a3e0394ed45be627"
 dependencies = [
  "base64 0.10.1",
  "bincode",
@@ -6427,7 +6423,7 @@ dependencies = [
  "num-traits",
  "plane-split",
  "rayon",
- "ron 0.1.7",
+ "ron",
  "serde",
  "serde_json",
  "smallvec 1.3.0",
@@ -6443,7 +6439,7 @@ dependencies = [
 [[package]]
 name = "webrender_api"
 version = "0.61.0"
-source = "git+https://github.com/servo/webrender#01082a9091ab98c392af8934d04271eb1dd546df"
+source = "git+https://github.com/servo/webrender#de3999583ab20aad7c57ea35a3e0394ed45be627"
 dependencies = [
  "app_units",
  "bitflags",
@@ -6464,7 +6460,7 @@ dependencies = [
 [[package]]
 name = "webrender_build"
 version = "0.0.1"
-source = "git+https://github.com/servo/webrender#01082a9091ab98c392af8934d04271eb1dd546df"
+source = "git+https://github.com/servo/webrender#de3999583ab20aad7c57ea35a3e0394ed45be627"
 dependencies = [
  "bitflags",
  "lazy_static",
@@ -6544,7 +6540,7 @@ dependencies = [
  "naga",
  "parking_lot 0.10.2",
  "peek-poke 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "ron 0.5.1",
+ "ron",
  "serde",
  "smallvec 1.3.0",
  "spirv_headers",
@@ -6644,7 +6640,7 @@ dependencies = [
 [[package]]
 name = "wr_malloc_size_of"
 version = "0.0.1"
-source = "git+https://github.com/servo/webrender#01082a9091ab98c392af8934d04271eb1dd546df"
+source = "git+https://github.com/servo/webrender#de3999583ab20aad7c57ea35a3e0394ed45be627"
 dependencies = [
  "app_units",
  "euclid",
diff --git a/components/canvas/canvas_data.rs b/components/canvas/canvas_data.rs
index 9c3aae1d0be..1a4d1ccfc98 100644
--- a/components/canvas/canvas_data.rs
+++ b/components/canvas/canvas_data.rs
@@ -2,7 +2,7 @@
  * 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/. */
 
-use crate::canvas_paint_thread::AntialiasMode;
+use crate::canvas_paint_thread::{AntialiasMode, ImageUpdate, WebrenderApi};
 use crate::raqote_backend::Repetition;
 use canvas_traits::canvas::*;
 use cssparser::RGBA;
@@ -13,7 +13,6 @@ use num_traits::ToPrimitive;
 use std::marker::PhantomData;
 use std::mem;
 use std::sync::Arc;
-use webrender::api::DirtyRect;
 use webrender_api::units::RectExt as RectExt_;
 
 /// The canvas data stores a state machine for the current status of
@@ -367,8 +366,7 @@ pub struct CanvasData<'a> {
     path_state: Option<PathState>,
     state: CanvasPaintState<'a>,
     saved_states: Vec<CanvasPaintState<'a>>,
-    webrender_api: webrender_api::RenderApi,
-    webrender_doc: webrender_api::DocumentId,
+    webrender_api: Box<dyn WebrenderApi>,
     image_key: Option<webrender_api::ImageKey>,
     /// An old webrender image key that can be deleted when the next epoch ends.
     old_image_key: Option<webrender_api::ImageKey>,
@@ -384,22 +382,19 @@ fn create_backend() -> Box<dyn Backend> {
 impl<'a> CanvasData<'a> {
     pub fn new(
         size: Size2D<u64>,
-        webrender_api_sender: webrender_api::RenderApiSender,
-        webrender_doc: webrender_api::DocumentId,
+        webrender_api: Box<dyn WebrenderApi>,
         antialias: AntialiasMode,
         canvas_id: CanvasId,
     ) -> CanvasData<'a> {
         let backend = create_backend();
         let draw_target = backend.create_drawtarget(size);
-        let webrender_api = webrender_api_sender.create_api();
         CanvasData {
             backend,
             drawtarget: draw_target,
             path_state: None,
             state: CanvasPaintState::new(antialias),
             saved_states: vec![],
-            webrender_api: webrender_api,
-            webrender_doc,
+            webrender_api,
             image_key: None,
             old_image_key: None,
             very_old_image_key: None,
@@ -979,27 +974,28 @@ impl<'a> CanvasData<'a> {
         let data = self.drawtarget.snapshot_data_owned();
         let data = webrender_api::ImageData::Raw(Arc::new(data));
 
-        let mut txn = webrender_api::Transaction::new();
+        let mut updates = vec![];
 
         match self.image_key {
             Some(image_key) => {
                 debug!("Updating image {:?}.", image_key);
-                txn.update_image(image_key, descriptor, data, &DirtyRect::All);
+                updates.push(ImageUpdate::Update(image_key, descriptor, data));
             },
             None => {
-                self.image_key = Some(self.webrender_api.generate_image_key());
+                let key = self.webrender_api.generate_key();
+                updates.push(ImageUpdate::Add(key, descriptor, data));
+                self.image_key = Some(key);
                 debug!("New image {:?}.", self.image_key);
-                txn.add_image(self.image_key.unwrap(), descriptor, data, None);
             },
         }
 
         if let Some(image_key) =
             mem::replace(&mut self.very_old_image_key, self.old_image_key.take())
         {
-            txn.delete_image(image_key);
+            updates.push(ImageUpdate::Delete(image_key));
         }
 
-        self.webrender_api.send_transaction(self.webrender_doc, txn);
+        self.webrender_api.update_images(updates);
 
         let data = CanvasImageData {
             image_key: self.image_key.unwrap(),
@@ -1110,16 +1106,15 @@ impl<'a> CanvasData<'a> {
 
 impl<'a> Drop for CanvasData<'a> {
     fn drop(&mut self) {
-        let mut txn = webrender_api::Transaction::new();
-
+        let mut updates = vec![];
         if let Some(image_key) = self.old_image_key.take() {
-            txn.delete_image(image_key);
+            updates.push(ImageUpdate::Delete(image_key));
         }
         if let Some(image_key) = self.very_old_image_key.take() {
-            txn.delete_image(image_key);
+            updates.push(ImageUpdate::Delete(image_key));
         }
 
-        self.webrender_api.send_transaction(self.webrender_doc, txn);
+        self.webrender_api.update_images(updates);
     }
 }
 
diff --git a/components/canvas/canvas_paint_thread.rs b/components/canvas/canvas_paint_thread.rs
index f73d8f7b9a8..6832a5cddc8 100644
--- a/components/canvas/canvas_paint_thread.rs
+++ b/components/canvas/canvas_paint_thread.rs
@@ -12,35 +12,52 @@ use ipc_channel::router::ROUTER;
 use std::borrow::ToOwned;
 use std::collections::HashMap;
 use std::thread;
+use webrender_api::{ImageData, ImageDescriptor, ImageKey};
 
 pub enum AntialiasMode {
     Default,
     None,
 }
 
+pub enum ImageUpdate {
+    Add(ImageKey, ImageDescriptor, ImageData),
+    Update(ImageKey, ImageDescriptor, ImageData),
+    Delete(ImageKey),
+}
+
+pub trait WebrenderApi {
+    fn generate_key(&self) -> webrender_api::ImageKey;
+    fn update_images(&self, updates: Vec<ImageUpdate>);
+    fn clone(&self) -> Box<dyn WebrenderApi>;
+}
+
 pub struct CanvasPaintThread<'a> {
     canvases: HashMap<CanvasId, CanvasData<'a>>,
     next_canvas_id: CanvasId,
+    webrender_api: Box<dyn WebrenderApi>,
 }
 
 impl<'a> CanvasPaintThread<'a> {
-    fn new() -> CanvasPaintThread<'a> {
+    fn new(webrender_api: Box<dyn WebrenderApi>) -> CanvasPaintThread<'a> {
         CanvasPaintThread {
             canvases: HashMap::new(),
             next_canvas_id: CanvasId(0),
+            webrender_api,
         }
     }
 
     /// Creates a new `CanvasPaintThread` and returns an `IpcSender` to
     /// communicate with it.
-    pub fn start() -> (Sender<ConstellationCanvasMsg>, IpcSender<CanvasMsg>) {
+    pub fn start(
+        webrender_api: Box<dyn WebrenderApi + Send>,
+    ) -> (Sender<ConstellationCanvasMsg>, IpcSender<CanvasMsg>) {
         let (ipc_sender, ipc_receiver) = ipc::channel::<CanvasMsg>().unwrap();
         let msg_receiver = ROUTER.route_ipc_receiver_to_new_crossbeam_receiver(ipc_receiver);
         let (create_sender, create_receiver) = unbounded();
         thread::Builder::new()
             .name("CanvasThread".to_owned())
             .spawn(move || {
-                let mut canvas_paint_thread = CanvasPaintThread::new();
+                let mut canvas_paint_thread = CanvasPaintThread::new(webrender_api);
                 loop {
                     select! {
                         recv(msg_receiver) -> msg => {
@@ -74,16 +91,9 @@ impl<'a> CanvasPaintThread<'a> {
                                 Ok(ConstellationCanvasMsg::Create {
                                     id_sender: creator,
                                     size,
-                                    webrender_sender: webrenderer_api_sender,
-                                    webrender_doc,
                                     antialias
                                 }) => {
-                                    let canvas_id = canvas_paint_thread.create_canvas(
-                                        size,
-                                        webrenderer_api_sender,
-                                        webrender_doc,
-                                        antialias,
-                                    );
+                                    let canvas_id = canvas_paint_thread.create_canvas(size, antialias);
                                     creator.send(canvas_id).unwrap();
                                 },
                                 Ok(ConstellationCanvasMsg::Exit) => break,
@@ -101,13 +111,7 @@ impl<'a> CanvasPaintThread<'a> {
         (create_sender, ipc_sender)
     }
 
-    pub fn create_canvas(
-        &mut self,
-        size: Size2D<u64>,
-        webrender_api_sender: webrender_api::RenderApiSender,
-        webrender_doc: webrender_api::DocumentId,
-        antialias: bool,
-    ) -> CanvasId {
+    pub fn create_canvas(&mut self, size: Size2D<u64>, antialias: bool) -> CanvasId {
         let antialias = if antialias {
             AntialiasMode::Default
         } else {
@@ -119,8 +123,7 @@ impl<'a> CanvasPaintThread<'a> {
 
         let canvas_data = CanvasData::new(
             size,
-            webrender_api_sender,
-            webrender_doc,
+            self.webrender_api.clone(),
             antialias,
             canvas_id.clone(),
         );
diff --git a/components/canvas/webgl_thread.rs b/components/canvas/webgl_thread.rs
index 9b7b4ed4683..0d8bc69cb77 100644
--- a/components/canvas/webgl_thread.rs
+++ b/components/canvas/webgl_thread.rs
@@ -655,7 +655,7 @@ impl WebGLThread {
         );
 
         let image_key = Self::create_wr_external_image(
-            &self.webrender_api,
+            &mut self.webrender_api,
             self.webrender_doc,
             size.to_i32(),
             has_alpha,
@@ -718,7 +718,7 @@ impl WebGLThread {
             .contains(ContextAttributeFlags::ALPHA);
         let texture_target = current_wr_texture_target(&self.device);
         Self::update_wr_external_image(
-            &self.webrender_api,
+            &mut self.webrender_api,
             self.webrender_doc,
             size.to_i32(),
             has_alpha,
@@ -1021,7 +1021,7 @@ impl WebGLThread {
 
     /// Creates a `webrender_api::ImageKey` that uses shared textures.
     fn create_wr_external_image(
-        webrender_api: &webrender_api::RenderApi,
+        webrender_api: &mut webrender_api::RenderApi,
         webrender_doc: webrender_api::DocumentId,
         size: Size2D<i32>,
         alpha: bool,
@@ -1041,7 +1041,7 @@ impl WebGLThread {
 
     /// Updates a `webrender_api::ImageKey` that uses shared textures.
     fn update_wr_external_image(
-        webrender_api: &webrender_api::RenderApi,
+        webrender_api: &mut webrender_api::RenderApi,
         webrender_doc: webrender_api::DocumentId,
         size: Size2D<i32>,
         alpha: bool,
diff --git a/components/canvas_traits/lib.rs b/components/canvas_traits/lib.rs
index ec510e6281d..467eb432e0f 100644
--- a/components/canvas_traits/lib.rs
+++ b/components/canvas_traits/lib.rs
@@ -26,8 +26,6 @@ pub enum ConstellationCanvasMsg {
     Create {
         id_sender: Sender<CanvasId>,
         size: Size2D<u64>,
-        webrender_sender: webrender_api::RenderApiSender,
-        webrender_doc: webrender_api::DocumentId,
         antialias: bool,
     },
     Exit,
diff --git a/components/compositing/Cargo.toml b/components/compositing/Cargo.toml
index 027d16233bf..97419f37752 100644
--- a/components/compositing/Cargo.toml
+++ b/components/compositing/Cargo.toml
@@ -16,6 +16,7 @@ default = []
 gl = ["gleam", "pixels"]
 
 [dependencies]
+app_units = "0.7"
 canvas = { path = "../canvas" }
 crossbeam-channel = "0.4"
 embedder_traits = { path = "../embedder_traits" }
diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index c3c545af4bb..1b0929dda74 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -3,7 +3,9 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use crate::compositor_thread::CompositorReceiver;
-use crate::compositor_thread::{InitialCompositorState, Msg};
+use crate::compositor_thread::{
+    InitialCompositorState, Msg, WebrenderCanvasMsg, WebrenderFontMsg, WebrenderMsg,
+};
 #[cfg(feature = "gl")]
 use crate::gl;
 use crate::touch::{TouchAction, TouchHandler};
@@ -11,10 +13,11 @@ use crate::windowing::{
     self, EmbedderCoordinates, MouseWindowEvent, WebRenderDebugOption, WindowMethods,
 };
 use crate::{CompositionPipeline, ConstellationMsg, SendableFrameTree};
+use canvas::canvas_paint_thread::ImageUpdate;
 use crossbeam_channel::Sender;
 use embedder_traits::Cursor;
 use euclid::{Point2D, Rect, Scale, Vector2D};
-use gfx_traits::Epoch;
+use gfx_traits::{Epoch, FontData};
 #[cfg(feature = "gl")]
 use image::{DynamicImage, ImageFormat};
 use ipc_channel::ipc;
@@ -40,8 +43,6 @@ use std::fs::{create_dir_all, File};
 use std::io::Write;
 use std::num::NonZeroU32;
 use std::rc::Rc;
-use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
 use style_traits::viewport::ViewportConstraints;
 use style_traits::{CSSPixel, DevicePixel, PinchZoomFactor};
 use time::{now, precise_time_ns, precise_time_s};
@@ -218,7 +219,7 @@ pub struct IOCompositor<Window: WindowMethods + ?Sized> {
     /// True if a WR frame render has been requested. Screenshots
     /// taken before the render is complete will not reflect the
     /// most up to date rendering.
-    waiting_on_pending_frame: Arc<AtomicBool>,
+    waiting_on_pending_frame: bool,
 }
 
 #[derive(Clone, Copy)]
@@ -334,7 +335,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             is_running_problem_test,
             exit_after_load,
             convert_mouse_to_touch,
-            waiting_on_pending_frame: state.pending_wr_frame,
+            waiting_on_pending_frame: false,
         }
     }
 
@@ -443,7 +444,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             },
 
             (Msg::Recomposite(reason), ShutdownState::NotShuttingDown) => {
-                self.waiting_on_pending_frame.store(false, Ordering::SeqCst);
+                self.waiting_on_pending_frame = false;
                 self.composition_request = CompositionRequest::CompositeNow(reason)
             },
 
@@ -474,7 +475,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
                     self.ready_to_save_state,
                     ReadyState::WaitingForConstellationReply
                 );
-                if is_ready && !self.waiting_on_pending_frame.load(Ordering::SeqCst) {
+                if is_ready && !self.waiting_on_pending_frame {
                     self.ready_to_save_state = ReadyState::ReadyToSaveImage;
                     if self.is_running_problem_test {
                         println!("ready to save image!");
@@ -569,6 +570,10 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
                 }
             },
 
+            (Msg::Webrender(msg), ShutdownState::NotShuttingDown) => {
+                self.handle_webrender_message(msg);
+            },
+
             // When we are shutting_down, we need to avoid performing operations
             // such as Paint that may crash because we have begun tearing down
             // the rest of our resources.
@@ -578,6 +583,148 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         true
     }
 
+    /// Accept messages from content processes that need to be relayed to the WebRender
+    /// instance in the parent process.
+    fn handle_webrender_message(&mut self, msg: WebrenderMsg) {
+        match msg {
+            WebrenderMsg::Layout(script_traits::WebrenderMsg::SendInitialTransaction(
+                _doc,
+                pipeline,
+            )) => {
+                self.waiting_on_pending_frame = true;
+                let mut txn = webrender_api::Transaction::new();
+                txn.set_display_list(
+                    webrender_api::Epoch(0),
+                    None,
+                    Default::default(),
+                    (pipeline, Default::default(), Default::default()),
+                    false,
+                );
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+            },
+
+            WebrenderMsg::Layout(script_traits::WebrenderMsg::SendScrollNode(
+                _doc,
+                point,
+                scroll_id,
+                clamping,
+            )) => {
+                let mut txn = webrender_api::Transaction::new();
+                txn.scroll_node_with_id(point, scroll_id, clamping);
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+            },
+
+            WebrenderMsg::Layout(script_traits::WebrenderMsg::SendDisplayList(
+                _doc,
+                epoch,
+                size,
+                pipeline,
+                size2,
+                data,
+                descriptor,
+            )) => {
+                self.waiting_on_pending_frame = true;
+                let mut txn = webrender_api::Transaction::new();
+                txn.set_display_list(
+                    epoch,
+                    None,
+                    size,
+                    (
+                        pipeline,
+                        size2,
+                        webrender_api::BuiltDisplayList::from_data(data, descriptor),
+                    ),
+                    true,
+                );
+                txn.generate_frame();
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+            },
+
+            WebrenderMsg::Layout(script_traits::WebrenderMsg::HitTest(
+                _doc,
+                pipeline,
+                point,
+                flags,
+                sender,
+            )) => {
+                let result =
+                    self.webrender_api
+                        .hit_test(self.webrender_document, pipeline, point, flags);
+                let _ = sender.send(result);
+            },
+
+            WebrenderMsg::Layout(script_traits::WebrenderMsg::GenerateImageKey(sender)) |
+            WebrenderMsg::Net(net_traits::WebrenderImageMsg::GenerateImageKey(sender)) => {
+                let _ = sender.send(self.webrender_api.generate_image_key());
+            },
+
+            WebrenderMsg::Layout(script_traits::WebrenderMsg::UpdateImages(updates)) => {
+                let mut txn = webrender_api::Transaction::new();
+                for update in updates {
+                    match update {
+                        script_traits::ImageUpdate::AddImage(key, desc, data) => {
+                            txn.add_image(key, desc, data, None)
+                        },
+                        script_traits::ImageUpdate::DeleteImage(key) => txn.delete_image(key),
+                        script_traits::ImageUpdate::UpdateImage(key, desc, data) => {
+                            txn.update_image(key, desc, data, &webrender_api::DirtyRect::All)
+                        },
+                    }
+                }
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+            },
+
+            WebrenderMsg::Net(net_traits::WebrenderImageMsg::AddImage(key, desc, data)) => {
+                let mut txn = webrender_api::Transaction::new();
+                txn.add_image(key, desc, data, None);
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+            },
+            WebrenderMsg::Font(WebrenderFontMsg::AddFontInstance(font_key, size, sender)) => {
+                let key = self.webrender_api.generate_font_instance_key();
+                let mut txn = webrender_api::Transaction::new();
+                txn.add_font_instance(key, font_key, size, None, None, Vec::new());
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+                let _ = sender.send(key);
+            },
+            WebrenderMsg::Font(WebrenderFontMsg::AddFont(data, sender)) => {
+                let font_key = self.webrender_api.generate_font_key();
+                let mut txn = webrender_api::Transaction::new();
+                match data {
+                    FontData::Raw(bytes) => txn.add_raw_font(font_key, bytes, 0),
+                    FontData::Native(native_font) => txn.add_native_font(font_key, native_font),
+                }
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+                let _ = sender.send(font_key);
+            },
+            WebrenderMsg::Canvas(WebrenderCanvasMsg::GenerateKey(sender)) => {
+                let _ = sender.send(self.webrender_api.generate_image_key());
+            },
+            WebrenderMsg::Canvas(WebrenderCanvasMsg::UpdateImages(updates)) => {
+                let mut txn = webrender_api::Transaction::new();
+                for update in updates {
+                    match update {
+                        ImageUpdate::Add(key, descriptor, data) => {
+                            txn.add_image(key, descriptor, data, None)
+                        },
+                        ImageUpdate::Update(key, descriptor, data) => {
+                            txn.update_image(key, descriptor, data, &webrender_api::DirtyRect::All)
+                        },
+                        ImageUpdate::Delete(key) => txn.delete_image(key),
+                    }
+                }
+                self.webrender_api
+                    .send_transaction(self.webrender_document, txn);
+            },
+        }
+    }
+
     /// Sets or unsets the animations-running flag for the given pipeline, and schedules a
     /// recomposite if necessary.
     fn change_running_animations_state(
diff --git a/components/compositing/compositor_thread.rs b/components/compositing/compositor_thread.rs
index cbd715af0dd..01fb92a544e 100644
--- a/components/compositing/compositor_thread.rs
+++ b/components/compositing/compositor_thread.rs
@@ -6,6 +6,7 @@
 
 use crate::compositor::CompositingReason;
 use crate::{ConstellationMsg, SendableFrameTree};
+use canvas::canvas_paint_thread::ImageUpdate;
 use crossbeam_channel::{Receiver, Sender};
 use embedder_traits::EventLoopWaker;
 use euclid::Rect;
@@ -18,8 +19,6 @@ use profile_traits::time;
 use script_traits::{AnimationState, EventResult, MouseButton, MouseEventType};
 use std::fmt::{Debug, Error, Formatter};
 use std::rc::Rc;
-use std::sync::atomic::AtomicBool;
-use std::sync::Arc;
 use style_traits::viewport::ViewportConstraints;
 use style_traits::CSSPixel;
 use webrender_api;
@@ -122,6 +121,30 @@ pub enum Msg {
     GetScreenSize(IpcSender<DeviceIntSize>),
     /// Get screen available size.
     GetScreenAvailSize(IpcSender<DeviceIntSize>),
+
+    /// Webrender operations requested from non-compositor threads.
+    Webrender(WebrenderMsg),
+}
+
+pub enum WebrenderFontMsg {
+    AddFontInstance(
+        webrender_api::FontKey,
+        app_units::Au,
+        Sender<webrender_api::FontInstanceKey>,
+    ),
+    AddFont(gfx_traits::FontData, Sender<webrender_api::FontKey>),
+}
+
+pub enum WebrenderCanvasMsg {
+    GenerateKey(Sender<webrender_api::ImageKey>),
+    UpdateImages(Vec<ImageUpdate>),
+}
+
+pub enum WebrenderMsg {
+    Layout(script_traits::WebrenderMsg),
+    Net(net_traits::WebrenderImageMsg),
+    Font(WebrenderFontMsg),
+    Canvas(WebrenderCanvasMsg),
 }
 
 impl Debug for Msg {
@@ -148,6 +171,7 @@ impl Debug for Msg {
             Msg::GetClientWindow(..) => write!(f, "GetClientWindow"),
             Msg::GetScreenSize(..) => write!(f, "GetScreenSize"),
             Msg::GetScreenAvailSize(..) => write!(f, "GetScreenAvailSize"),
+            Msg::Webrender(..) => write!(f, "Webrender"),
         }
     }
 }
@@ -171,5 +195,4 @@ pub struct InitialCompositorState {
     pub webrender_surfman: WebrenderSurfman,
     pub webrender_gl: Rc<dyn gleam::gl::Gl>,
     pub webxr_main_thread: webxr::MainThreadRegistry,
-    pub pending_wr_frame: Arc<AtomicBool>,
 }
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index d7408ac549e..1ea79bc3209 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -109,6 +109,7 @@ use canvas_traits::webgl::WebGLThreads;
 use canvas_traits::ConstellationCanvasMsg;
 use compositing::compositor_thread::CompositorProxy;
 use compositing::compositor_thread::Msg as ToCompositorMsg;
+use compositing::compositor_thread::WebrenderMsg;
 use compositing::{ConstellationMsg as FromCompositorMsg, SendableFrameTree};
 use crossbeam_channel::{after, never, unbounded, Receiver, Sender};
 use devtools_traits::{ChromeToDevtoolsControlMsg, DevtoolsControlMsg};
@@ -171,7 +172,6 @@ use std::marker::PhantomData;
 use std::mem::replace;
 use std::process;
 use std::rc::{Rc, Weak};
-use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 use std::thread;
 use style_traits::viewport::ViewportConstraints;
@@ -384,10 +384,6 @@ pub struct Constellation<Message, LTF, STF, SWF> {
     /// A single WebRender document the constellation operates on.
     webrender_document: webrender_api::DocumentId,
 
-    /// A channel for the constellation to send messages to the
-    /// WebRender thread.
-    webrender_api_sender: webrender_api::RenderApiSender,
-
     /// A channel for content processes to send messages that will
     /// be relayed to the WebRender thread.
     webrender_api_ipc_sender: script_traits::WebrenderIpcSender,
@@ -537,9 +533,6 @@ pub struct InitialConstellationState {
     /// Webrender document ID.
     pub webrender_document: webrender_api::DocumentId,
 
-    /// Webrender API.
-    pub webrender_api_sender: webrender_api::RenderApiSender,
-
     /// Entry point to create and get channels to a WebGLThread.
     pub webgl_threads: Option<WebGLThreads>,
 
@@ -554,9 +547,6 @@ pub struct InitialConstellationState {
     /// Mechanism to force the compositor to process events.
     pub event_loop_waker: Option<Box<dyn EventLoopWaker>>,
 
-    /// A flag share with the compositor to indicate that a WR frame is in progress.
-    pub pending_wr_frame: Arc<AtomicBool>,
-
     /// User agent string to report in network requests.
     pub user_agent: Cow<'static, str>,
 }
@@ -726,113 +716,6 @@ where
     mpsc_receiver
 }
 
-enum WebrenderMsg {
-    Layout(script_traits::WebrenderMsg),
-    Net(net_traits::WebrenderImageMsg),
-}
-
-/// Accept messages from content processes that need to be relayed to the WebRender
-/// instance in the parent process.
-fn handle_webrender_message(
-    pending_wr_frame: &AtomicBool,
-    webrender_api: &webrender_api::RenderApi,
-    webrender_doc: webrender_api::DocumentId,
-    msg: WebrenderMsg,
-) {
-    match msg {
-        WebrenderMsg::Layout(script_traits::WebrenderMsg::SendInitialTransaction(
-            doc,
-            pipeline,
-        )) => {
-            pending_wr_frame.store(true, Ordering::SeqCst);
-            let mut txn = webrender_api::Transaction::new();
-            txn.set_display_list(
-                webrender_api::Epoch(0),
-                None,
-                Default::default(),
-                (pipeline, Default::default(), Default::default()),
-                false,
-            );
-            webrender_api.send_transaction(doc, txn);
-        },
-
-        WebrenderMsg::Layout(script_traits::WebrenderMsg::SendScrollNode(
-            doc,
-            point,
-            scroll_id,
-            clamping,
-        )) => {
-            let mut txn = webrender_api::Transaction::new();
-            txn.scroll_node_with_id(point, scroll_id, clamping);
-            webrender_api.send_transaction(doc, txn);
-        },
-
-        WebrenderMsg::Layout(script_traits::WebrenderMsg::SendDisplayList(
-            doc,
-            epoch,
-            size,
-            pipeline,
-            size2,
-            data,
-            descriptor,
-        )) => {
-            pending_wr_frame.store(true, Ordering::SeqCst);
-            let mut txn = webrender_api::Transaction::new();
-            txn.set_display_list(
-                epoch,
-                None,
-                size,
-                (
-                    pipeline,
-                    size2,
-                    webrender_api::BuiltDisplayList::from_data(data, descriptor),
-                ),
-                true,
-            );
-            txn.generate_frame();
-            webrender_api.send_transaction(doc, txn);
-        },
-
-        WebrenderMsg::Layout(script_traits::WebrenderMsg::HitTest(
-            doc,
-            pipeline,
-            point,
-            flags,
-            sender,
-        )) => {
-            let result = webrender_api.hit_test(doc, pipeline, point, flags);
-            let _ = sender.send(result);
-        },
-
-        WebrenderMsg::Layout(script_traits::WebrenderMsg::GenerateImageKey(sender)) |
-        WebrenderMsg::Net(net_traits::WebrenderImageMsg::GenerateImageKey(sender)) => {
-            let _ = sender.send(webrender_api.generate_image_key());
-        },
-
-        WebrenderMsg::Layout(script_traits::WebrenderMsg::UpdateImages(updates)) => {
-            let mut txn = webrender_api::Transaction::new();
-            for update in updates {
-                match update {
-                    script_traits::ImageUpdate::AddImage(key, desc, data) => {
-                        txn.add_image(key, desc, data, None)
-                    },
-                    script_traits::ImageUpdate::DeleteImage(key) => txn.delete_image(key),
-                    script_traits::ImageUpdate::UpdateImage(key, desc, data) => {
-                        txn.update_image(key, desc, data, &webrender_api::DirtyRect::All)
-                    },
-                }
-            }
-            webrender_api.send_transaction(webrender_doc, txn);
-        },
-
-        WebrenderMsg::Net(net_traits::WebrenderImageMsg::AddImage(key, desc, data)) => {
-            let mut txn = webrender_api::Transaction::new();
-            txn.add_image(key, desc, data, None);
-            webrender_api.send_transaction(webrender_doc, txn);
-        },
-    }
-}
-
 impl<Message, LTF, STF, SWF> Constellation<Message, LTF, STF, SWF>
 where
     LTF: LayoutThreadFactory<Message = Message>,
@@ -931,32 +814,23 @@ where
                 let (webrender_image_ipc_sender, webrender_image_ipc_receiver) =
                     ipc::channel().expect("ipc channel failure");
 
-                let webrender_api = state.webrender_api_sender.create_api();
-                let webrender_doc = state.webrender_document;
-                let pending_wr_frame_clone = state.pending_wr_frame.clone();
+                let compositor_proxy = state.compositor_proxy.clone();
                 ROUTER.add_route(
                     webrender_ipc_receiver.to_opaque(),
                     Box::new(move |message| {
-                        handle_webrender_message(
-                            &pending_wr_frame_clone,
-                            &webrender_api,
-                            webrender_doc,
+                        let _ = compositor_proxy.send(ToCompositorMsg::Webrender(
                             WebrenderMsg::Layout(message.to().expect("conversion failure")),
-                        )
+                        ));
                     }),
                 );
 
-                let webrender_api = state.webrender_api_sender.create_api();
-                let pending_wr_frame_clone = state.pending_wr_frame.clone();
+                let compositor_proxy = state.compositor_proxy.clone();
                 ROUTER.add_route(
                     webrender_image_ipc_receiver.to_opaque(),
                     Box::new(move |message| {
-                        handle_webrender_message(
-                            &pending_wr_frame_clone,
-                            &webrender_api,
-                            webrender_doc,
+                        let _ = compositor_proxy.send(ToCompositorMsg::Webrender(
                             WebrenderMsg::Net(message.to().expect("conversion failure")),
-                        )
+                        ));
                     }),
                 );
 
@@ -1009,7 +883,6 @@ where
                     scheduler_receiver,
                     document_states: HashMap::new(),
                     webrender_document: state.webrender_document,
-                    webrender_api_sender: state.webrender_api_sender,
                     webrender_api_ipc_sender: script_traits::WebrenderIpcSender::new(
                         webrender_ipc_sender,
                     ),
@@ -1474,10 +1347,10 @@ where
                 ))
             }
             recv(self.swmanager_receiver) -> msg => {
-                msg.expect("Unexpected panic channel panic in constellation").map(Request::FromSWManager)
+                msg.expect("Unexpected SW channel panic in constellation").map(Request::FromSWManager)
             }
             recv(self.scheduler_receiver) -> msg => {
-                msg.expect("Unexpected panic channel panic in constellation").map(Request::Timer)
+                msg.expect("Unexpected schedule channel panic in constellation").map(Request::Timer)
             }
             recv(scheduler_timeout) -> _ => {
                 // Note: by returning, we go back to the top,
@@ -4386,14 +4259,11 @@ where
         size: UntypedSize2D<u64>,
         response_sender: IpcSender<(IpcSender<CanvasMsg>, CanvasId)>,
     ) {
-        let webrender_api = self.webrender_api_sender.clone();
         let (canvas_id_sender, canvas_id_receiver) = unbounded();
 
         if let Err(e) = self.canvas_chan.send(ConstellationCanvasMsg::Create {
             id_sender: canvas_id_sender,
             size,
-            webrender_sender: webrender_api,
-            webrender_doc: self.webrender_document,
             antialias: self.enable_canvas_antialiasing,
         }) {
             return warn!("Create canvas paint thread failed ({})", e);
diff --git a/components/gfx/font_cache_thread.rs b/components/gfx/font_cache_thread.rs
index 48d4c9e2889..45eead2357d 100644
--- a/components/gfx/font_cache_thread.rs
+++ b/components/gfx/font_cache_thread.rs
@@ -12,6 +12,7 @@ use crate::platform::font_list::system_default_family;
 use crate::platform::font_list::SANS_SERIF_FONT_FAMILY;
 use crate::platform::font_template::FontTemplateData;
 use app_units::Au;
+use gfx_traits::{FontData, WebrenderApi};
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use net_traits::request::{Destination, RequestBuilder};
 use net_traits::{fetch_async, CoreResourceThread, FetchResponseMsg};
@@ -135,9 +136,8 @@ struct FontCache {
     web_families: HashMap<LowercaseString, FontTemplates>,
     font_context: FontContextHandle,
     core_resource_thread: CoreResourceThread,
-    webrender_api: webrender_api::RenderApi,
+    webrender_api: Box<dyn WebrenderApi>,
     webrender_fonts: HashMap<Atom, webrender_api::FontKey>,
-    webrender_doc: webrender_api::DocumentId,
     font_instances: HashMap<(webrender_api::FontKey, Au), webrender_api::FontInstanceKey>,
 }
 
@@ -181,19 +181,11 @@ impl FontCache {
                 },
                 Command::GetFontInstance(font_key, size, result) => {
                     let webrender_api = &self.webrender_api;
-                    let doc = self.webrender_doc;
-
-                    let instance_key =
-                        *self
-                            .font_instances
-                            .entry((font_key, size))
-                            .or_insert_with(|| {
-                                let key = webrender_api.generate_font_instance_key();
-                                let mut txn = webrender_api::Transaction::new();
-                                txn.add_font_instance(key, font_key, size, None, None, Vec::new());
-                                webrender_api.send_transaction(doc, txn);
-                                key
-                            });
+
+                    let instance_key = *self
+                        .font_instances
+                        .entry((font_key, size))
+                        .or_insert_with(|| webrender_api.add_font_instance(font_key, size));
 
                     let _ = result.send(instance_key);
                 },
@@ -391,21 +383,17 @@ impl FontCache {
 
     fn get_font_template_info(&mut self, template: Arc<FontTemplateData>) -> FontTemplateInfo {
         let webrender_api = &self.webrender_api;
-        let doc = self.webrender_doc;
         let webrender_fonts = &mut self.webrender_fonts;
 
         let font_key = *webrender_fonts
             .entry(template.identifier.clone())
             .or_insert_with(|| {
-                let font_key = webrender_api.generate_font_key();
-                let mut txn = webrender_api::Transaction::new();
-                match (template.bytes_if_in_memory(), template.native_font()) {
-                    (Some(bytes), _) => txn.add_raw_font(font_key, bytes, 0),
-                    (None, Some(native_font)) => txn.add_native_font(font_key, native_font),
-                    (None, None) => txn.add_raw_font(font_key, template.bytes(), 0),
-                }
-                webrender_api.send_transaction(doc, txn);
-                font_key
+                let font = match (template.bytes_if_in_memory(), template.native_font()) {
+                    (Some(bytes), _) => FontData::Raw(bytes),
+                    (None, Some(native_font)) => FontData::Native(native_font),
+                    (None, None) => FontData::Raw(template.bytes()),
+                };
+                webrender_api.add_font(font)
             });
 
         FontTemplateInfo {
@@ -444,8 +432,7 @@ pub struct FontCacheThread {
 impl FontCacheThread {
     pub fn new(
         core_resource_thread: CoreResourceThread,
-        webrender_api: webrender_api::RenderApi,
-        webrender_doc: webrender_api::DocumentId,
+        webrender_api: Box<dyn WebrenderApi + Send>,
     ) -> FontCacheThread {
         let (chan, port) = ipc::channel().unwrap();
 
@@ -465,7 +452,6 @@ impl FontCacheThread {
                     font_context: FontContextHandle::new(),
                     core_resource_thread,
                     webrender_api,
-                    webrender_doc,
                     webrender_fonts: HashMap::new(),
                     font_instances: HashMap::new(),
                 };
diff --git a/components/gfx_traits/Cargo.toml b/components/gfx_traits/Cargo.toml
index ff83cc79406..5d7acdd1b61 100644
--- a/components/gfx_traits/Cargo.toml
+++ b/components/gfx_traits/Cargo.toml
@@ -11,7 +11,9 @@ name = "gfx_traits"
 path = "lib.rs"
 
 [dependencies]
+app_units = "0.7"
 malloc_size_of = { path = "../malloc_size_of" }
 malloc_size_of_derive = "0.1"
 range = { path = "../range" }
 serde = "1.0"
+webrender_api = { git = "https://github.com/servo/webrender" }
diff --git a/components/gfx_traits/lib.rs b/components/gfx_traits/lib.rs
index 165059c7996..447be6e5dc2 100644
--- a/components/gfx_traits/lib.rs
+++ b/components/gfx_traits/lib.rs
@@ -103,3 +103,17 @@ pub fn node_id_from_scroll_id(id: usize) -> Option<usize> {
     }
     None
 }
+
+pub enum FontData {
+    Raw(Vec<u8>),
+    Native(webrender_api::NativeFontHandle),
+}
+
+pub trait WebrenderApi {
+    fn add_font_instance(
+        &self,
+        font_key: webrender_api::FontKey,
+        size: app_units::Au,
+    ) -> webrender_api::FontInstanceKey;
+    fn add_font(&self, data: FontData) -> webrender_api::FontKey;
+}
diff --git a/components/layout/display_list/webrender_helpers.rs b/components/layout/display_list/webrender_helpers.rs
index dd187cf3704..9495b2ab166 100644
--- a/components/layout/display_list/webrender_helpers.rs
+++ b/components/layout/display_list/webrender_helpers.rs
@@ -234,6 +234,12 @@ impl DisplayItem {
                     builder.push_iter(&stacking_context.filters);
                 }
 
+                // TODO(jdm): WebRender now requires us to create stacking context items
+                //            with the IS_BLEND_CONTAINER flag enabled if any children
+                //            of the stacking context have a blend mode applied.
+                //            This will require additional tracking during layout
+                //            before we start collecting stacking contexts so that
+                //            information will be available when we reach this point.
                 let wr_item = PushStackingContextDisplayItem {
                     origin: bounds.origin,
                     spatial_id,
@@ -243,9 +249,7 @@ impl DisplayItem {
                         mix_blend_mode: stacking_context.mix_blend_mode,
                         clip_id: None,
                         raster_space: RasterSpace::Screen,
-                        // TODO(pcwalton): Enable picture caching?
-                        cache_tiles: false,
-                        is_backdrop_root: false,
+                        flags: Default::default(),
                     },
                 };
 
diff --git a/components/layout_2020/display_list/stacking_context.rs b/components/layout_2020/display_list/stacking_context.rs
index fce6fb6caec..23357da4984 100644
--- a/components/layout_2020/display_list/stacking_context.rs
+++ b/components/layout_2020/display_list/stacking_context.rs
@@ -234,6 +234,12 @@ impl StackingContext {
             ));
         }
 
+        // TODO(jdm): WebRender now requires us to create stacking context items
+        //            with the IS_BLEND_CONTAINER flag enabled if any children
+        //            of the stacking context have a blend mode applied.
+        //            This will require additional tracking during layout
+        //            before we start collecting stacking contexts so that
+        //            information will be available when we reach this point.
         builder.wr.push_stacking_context(
             LayoutPoint::zero(), // origin
             self.spatial_id,
@@ -245,8 +251,7 @@ impl StackingContext {
             &vec![], // filter_datas
             &vec![], // filter_primitives
             wr::RasterSpace::Screen,
-            false, // cache_tiles,
-            false, // false
+            wr::StackingContextFlags::empty(),
         );
 
         true
diff --git a/components/servo/Cargo.toml b/components/servo/Cargo.toml
index 1cb332e3da9..96b2ad36f68 100644
--- a/components/servo/Cargo.toml
+++ b/components/servo/Cargo.toml
@@ -38,6 +38,7 @@ webrender_debugger = ["webrender/debugger"]
 xr-profile = ["canvas/xr-profile", "canvas_traits/xr-profile", "script/xr-profile", "webxr/profile"]
 
 [dependencies]
+app_units = "0.7"
 background_hang_monitor = { path = "../background_hang_monitor" }
 bluetooth = { path = "../bluetooth" }
 bluetooth_traits = { path = "../bluetooth_traits" }
@@ -53,6 +54,7 @@ embedder_traits = { path = "../embedder_traits" }
 env_logger = "0.7"
 euclid = "0.20"
 gfx = { path = "../gfx" }
+gfx_traits = { path = "../gfx_traits" }
 gleam = "0.11"
 gstreamer = { version = "0.15", optional = true }
 ipc-channel = "0.14"
diff --git a/components/servo/lib.rs b/components/servo/lib.rs
index 9278fb5c0de..25fdd2261b4 100644
--- a/components/servo/lib.rs
+++ b/components/servo/lib.rs
@@ -64,10 +64,12 @@ fn webdriver(_port: u16, _constellation: Sender<ConstellationMsg>) {}
 
 use bluetooth::BluetoothThreadFactory;
 use bluetooth_traits::BluetoothRequest;
+use canvas::canvas_paint_thread::{self, CanvasPaintThread};
 use canvas::{SurfaceProviders, WebGLComm, WebGlExecutor};
 use canvas_traits::webgl::WebGLThreads;
 use compositing::compositor_thread::{
-    CompositorProxy, CompositorReceiver, InitialCompositorState, Msg,
+    CompositorProxy, CompositorReceiver, InitialCompositorState, Msg, WebrenderCanvasMsg,
+    WebrenderFontMsg, WebrenderMsg,
 };
 use compositing::windowing::{EmbedderMethods, WindowEvent, WindowMethods};
 use compositing::{CompositingReason, ConstellationMsg, IOCompositor, ShutdownState};
@@ -115,7 +117,6 @@ use std::borrow::Cow;
 use std::cmp::max;
 use std::path::PathBuf;
 use std::rc::Rc;
-use std::sync::atomic::AtomicBool;
 use std::sync::{Arc, Mutex};
 use surfman::GLApi;
 use webrender::ShaderPrecacheFlags;
@@ -454,7 +455,7 @@ where
             webrender_surfman.clone(),
             webrender_gl.clone(),
             &mut webrender,
-            webrender_api_sender.clone(),
+            webrender_api.create_sender(),
             webrender_document,
             &mut webxr_main_thread,
             &mut external_image_handlers,
@@ -500,8 +501,6 @@ where
             device_pixel_ratio: Scale::new(device_pixel_ratio),
         };
 
-        let pending_wr_frame = Arc::new(AtomicBool::new(false));
-
         // Create the constellation, which maintains the engine
         // pipelines, including the script and layout threads, as well
         // as the navigation context.
@@ -515,14 +514,12 @@ where
             debugger_chan,
             devtools_chan,
             webrender_document,
-            webrender_api_sender,
             webxr_main_thread.registry(),
             player_context,
             webgl_threads,
             glplayer_threads,
             event_loop_waker,
             window_size,
-            pending_wr_frame.clone(),
         );
 
         if cfg!(feature = "webdriver") {
@@ -547,7 +544,6 @@ where
                 webrender_surfman,
                 webrender_gl,
                 webxr_main_thread,
-                pending_wr_frame,
             },
             opts.output_file.clone(),
             opts.is_running_problem_test,
@@ -855,14 +851,12 @@ fn create_constellation(
     debugger_chan: Option<debugger::Sender>,
     devtools_chan: Option<Sender<devtools_traits::DevtoolsControlMsg>>,
     webrender_document: webrender_api::DocumentId,
-    webrender_api_sender: webrender_api::RenderApiSender,
     webxr_registry: webxr_api::Registry,
     player_context: WindowGLContext,
     webgl_threads: Option<WebGLThreads>,
     glplayer_threads: Option<GLPlayerThreads>,
     event_loop_waker: Option<Box<dyn EventLoopWaker>>,
     initial_window_size: WindowSizeData,
-    pending_wr_frame: Arc<AtomicBool>,
 ) -> Sender<ConstellationMsg> {
     // Global configuration options, parsed from the command line.
     let opts = opts::get();
@@ -879,14 +873,14 @@ fn create_constellation(
         config_dir,
         opts.certificate_path.clone(),
     );
+
     let font_cache_thread = FontCacheThread::new(
         public_resource_threads.sender(),
-        webrender_api_sender.create_api(),
-        webrender_document,
+        Box::new(FontCacheWR(compositor_proxy.clone())),
     );
 
     let initial_state = InitialConstellationState {
-        compositor_proxy,
+        compositor_proxy: compositor_proxy.clone(),
         embedder_proxy,
         debugger_chan,
         devtools_chan,
@@ -897,17 +891,16 @@ fn create_constellation(
         time_profiler_chan,
         mem_profiler_chan,
         webrender_document,
-        webrender_api_sender,
         webxr_registry,
         webgl_threads,
         glplayer_threads,
         player_context,
         event_loop_waker,
-        pending_wr_frame,
         user_agent,
     };
 
-    let (canvas_chan, ipc_canvas_chan) = canvas::canvas_paint_thread::CanvasPaintThread::start();
+    let (canvas_chan, ipc_canvas_chan) =
+        CanvasPaintThread::start(Box::new(CanvasWebrenderApi(compositor_proxy)));
 
     let constellation_chan = Constellation::<
         script_layout_interface::message::Msg,
@@ -929,6 +922,50 @@ fn create_constellation(
     constellation_chan
 }
 
+struct FontCacheWR(CompositorProxy);
+
+impl gfx_traits::WebrenderApi for FontCacheWR {
+    fn add_font_instance(
+        &self,
+        font_key: webrender_api::FontKey,
+        size: app_units::Au,
+    ) -> webrender_api::FontInstanceKey {
+        let (sender, receiver) = unbounded();
+        let _ = self.0.send(Msg::Webrender(WebrenderMsg::Font(
+            WebrenderFontMsg::AddFontInstance(font_key, size, sender),
+        )));
+        receiver.recv().unwrap()
+    }
+    fn add_font(&self, data: gfx_traits::FontData) -> webrender_api::FontKey {
+        let (sender, receiver) = unbounded();
+        let _ = self.0.send(Msg::Webrender(WebrenderMsg::Font(
+            WebrenderFontMsg::AddFont(data, sender),
+        )));
+        receiver.recv().unwrap()
+    }
+}
+
+#[derive(Clone)]
+struct CanvasWebrenderApi(CompositorProxy);
+
+impl canvas_paint_thread::WebrenderApi for CanvasWebrenderApi {
+    fn generate_key(&self) -> webrender_api::ImageKey {
+        let (sender, receiver) = unbounded();
+        let _ = self.0.send(Msg::Webrender(WebrenderMsg::Canvas(
+            WebrenderCanvasMsg::GenerateKey(sender),
+        )));
+        receiver.recv().unwrap()
+    }
+    fn update_images(&self, updates: Vec<canvas_paint_thread::ImageUpdate>) {
+        let _ = self.0.send(Msg::Webrender(WebrenderMsg::Canvas(
+            WebrenderCanvasMsg::UpdateImages(updates),
+        )));
+    }
+    fn clone(&self) -> Box<dyn canvas_paint_thread::WebrenderApi> {
+        Box::new(<Self as Clone>::clone(self))
+    }
+}
+
 // A logger that logs to two downstream loggers.
 // This should probably be in the log crate.
 struct BothLogger<Log1, Log2>(Log1, Log2);