compositing: Keep the event loop awake when scrolling.

components/compositing/compositor.rs

@@ -82,6 +82,8 @@ pub struct IOCompositor<Window: WindowMethods> {
     /// The device pixel ratio for this window.
     hidpi_factor: ScaleFactor<ScreenPx, DevicePixel, f32>,
 
+    channel_to_self: Box<CompositorProxy + Send>,
+
     /// A handle to the scrolling timer.
     scrolling_timer: ScrollingTimerProxy,
 
@@ -206,6 +208,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             }),
             window_size: window_size,
             hidpi_factor: hidpi_factor,
+            channel_to_self: sender.clone_compositor_proxy(),
             scrolling_timer: ScrollingTimerProxy::new(sender),
             composition_request: CompositionRequest::NoCompositingNecessary,
             pending_scroll_events: Vec::new(),
@@ -360,6 +363,11 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 }
             }
 
+            (Msg::RecompositeAfterScroll, ShutdownState::NotShuttingDown) => {
+                self.composition_request =
+                    CompositionRequest::CompositeNow(CompositingReason::ContinueScroll)
+            }
+
             (Msg::KeyEvent(key, state, modified), ShutdownState::NotShuttingDown) => {
                 if state == KeyState::Pressed {
                     self.window.handle_key(key, modified);
@@ -755,11 +763,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                                 layer_id: LayerId,
                                 point: Point2D<f32>) {
         if self.move_layer(pipeline_id, layer_id, Point2D::from_untyped(&point)) {
-            if self.send_buffer_requests_for_all_layers() {
+            self.perform_updates_after_scroll()
-                self.start_scrolling_timer_if_necessary();
-            }
         } else {
-            self.fragment_point = Some(point);
+            self.fragment_point = Some(point)
         }
     }
 
@@ -899,8 +905,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 layer.handle_scroll_event(delta, cursor);
             }
 
-            self.start_scrolling_timer_if_necessary();
+            self.perform_updates_after_scroll();
-            self.send_buffer_requests_for_all_layers();
         }
 
         if had_scroll_events {
@@ -908,6 +913,16 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         }
     }
 
+    /// Performs buffer requests and starts the scrolling timer or schedules a recomposite as
+    /// necessary.
+    fn perform_updates_after_scroll(&mut self) {
+        if self.send_buffer_requests_for_all_layers() {
+            self.start_scrolling_timer_if_necessary();
+        } else {
+            self.channel_to_self.send(Msg::RecompositeAfterScroll);
+        }
+    }
+
     /// If there are any animations running, dispatches appropriate messages to the constellation.
     fn process_animations(&mut self) {
         for (pipeline_id, pipeline_details) in self.pipeline_details.iter() {

components/compositing/compositor_task.rs

@@ -212,6 +212,7 @@ pub enum Msg {
     /// Indicates that the scrolling timeout with the given starting timestamp has happened and a
     /// composite should happen. (See the `scrolling` module.)
     ScrollTimeout(u64),
+    RecompositeAfterScroll,
     /// Sends an unconsumed key event back to the compositor.
     KeyEvent(Key, KeyState, KeyModifiers),
     /// Changes the cursor.
@@ -240,6 +241,7 @@ impl Debug for Msg {
             Msg::SetFrameTree(..) => write!(f, "SetFrameTree"),
             Msg::LoadComplete => write!(f, "LoadComplete"),
             Msg::ScrollTimeout(..) => write!(f, "ScrollTimeout"),
+            Msg::RecompositeAfterScroll => write!(f, "RecompositeAfterScroll"),
             Msg::KeyEvent(..) => write!(f, "KeyEvent"),
             Msg::SetCursor(..) => write!(f, "SetCursor"),
             Msg::PaintTaskExited(..) => write!(f, "PaintTaskExited"),

components/compositing/headless.rs

@@ -103,6 +103,7 @@ impl CompositorEventListener for NullCompositor {
             Msg::LoadComplete |
             Msg::PaintMsgDiscarded(..) |
             Msg::ScrollTimeout(..) |
+            Msg::RecompositeAfterScroll |
             Msg::ChangePageTitle(..) |
             Msg::ChangePageUrl(..) |
             Msg::KeyEvent(..) |