phase based wheel scroll latching for mac

content/browser/renderer_host/render_widget_host_view_mac.mm

Index: content/browser/renderer_host/render_widget_host_view_mac.mm
diff --git a/content/browser/renderer_host/render_widget_host_view_mac.mm b/content/browser/renderer_host/render_widget_host_view_mac.mm
index efe95f0a6e7492a295e168eaa7c62e2d5582fbc3..c6e189dfa5139b2cbf010fe8b192ff3ed55292dc 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -1756,6 +1756,50 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
   UpdateScreenInfo(cocoa_view_);
 }
 
+void RenderWidgetHostViewMac::ScheduleMouseWheelEndDispatching(
+    blink::WebMouseWheelEvent wheel_event,
+    bool should_route_event) {
+  mouse_wheel_end_dispatch_timer_.Start(
+      FROM_HERE,
+      base::TimeDelta::FromMilliseconds(
+          kDefaultMouseWheelLatchingTransactionMs),
+      base::Bind(
+          &RenderWidgetHostViewMac::SendSyntheticWheelEventWithPhaseEnded,
+          base::Unretained(this), wheel_event, should_route_event));
+}
+
+void RenderWidgetHostViewMac::DispatchPendingWheelEndEvent() {
+  if (mouse_wheel_end_dispatch_timer_.IsRunning()) {
+    base::Closure task = mouse_wheel_end_dispatch_timer_.user_task();
+    mouse_wheel_end_dispatch_timer_.Stop();
+    task.Run();
+  }
+}
+
+void RenderWidgetHostViewMac::IgnorePendingWheelEndEvent() {
+  mouse_wheel_end_dispatch_timer_.Stop();
+}
+
+bool RenderWidgetHostViewMac::HasPendingWheelEndEvent() {
+  return mouse_wheel_end_dispatch_timer_.IsRunning();
+}
+
+void RenderWidgetHostViewMac::SendSyntheticWheelEventWithPhaseEnded(
+    blink::WebMouseWheelEvent wheel_event,
+    bool should_route_event) {
+  wheel_event.dispatch_type =
+      blink::WebInputEvent::DispatchType::kEventNonBlocking;
+  if (should_route_event) {
+    render_widget_host_->delegate()
+        ->GetInputEventRouter()
+        ->RouteMouseWheelEvent(this, &wheel_event,
+                               ui::LatencyInfo(ui::SourceEventType::WHEEL));
+  } else {
+    ProcessMouseWheelEvent(wheel_event,
+                           ui::LatencyInfo(ui::SourceEventType::WHEEL));
+  }
+}
+
 }  // namespace content
 
 // RenderWidgetHostViewCocoa ---------------------------------------------------
@@ -2356,10 +2400,14 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
     WebMouseWheelEvent webEvent = WebMouseWheelEventBuilder::Build(
         event, self);
     webEvent.rails_mode = mouseWheelFilter_.UpdateRailsMode(webEvent);
-    ui::LatencyInfo latency_info(ui::SourceEventType::WHEEL);
-    latency_info.AddLatencyNumber(ui::INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
-    renderWidgetHostView_->render_widget_host_->
-        ForwardWheelEventWithLatencyInfo(webEvent, latency_info);
+    if (renderWidgetHostView_->wheel_scroll_latching_enabled()) {
+      renderWidgetHostView_->ScheduleMouseWheelEndDispatching(webEvent, false);
+    } else {
+      ui::LatencyInfo latency_info(ui::SourceEventType::WHEEL);
+      latency_info.AddLatencyNumber(ui::INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
+      renderWidgetHostView_->render_widget_host_
+          ->ForwardWheelEventWithLatencyInfo(webEvent, latency_info);
+    }
   }
 
   if (endWheelMonitor_) {
@@ -2565,6 +2613,28 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
     webEvent.rails_mode = mouseWheelFilter_.UpdateRailsMode(webEvent);
     ui::LatencyInfo latency_info(ui::SourceEventType::WHEEL);
     latency_info.AddLatencyNumber(ui::INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
+    if (renderWidgetHostView_->wheel_scroll_latching_enabled()) {
+      if (webEvent.phase == blink::WebMouseWheelEvent::kPhaseEnded) {
+        // Don't send the wheel end event immediately, start a timer instead to
+        // see whether momentum phase of the scrolling starts or not.
+        renderWidgetHostView_->ScheduleMouseWheelEndDispatching(
+            webEvent, renderWidgetHostView_->ShouldRouteEvent(webEvent));
+        return;
+      }
+      if (webEvent.phase == blink::WebMouseWheelEvent::kPhaseBegan) {
+        // A new scrolling sequence has started, send the pending wheel end
+        // event to end the previous scrolling sequence.
+        renderWidgetHostView_->DispatchPendingWheelEndEvent();
+
+      } else if (webEvent.momentum_phase ==
+                 blink::WebMouseWheelEvent::kPhaseBegan) {
+        // Momentum phase has started, drop the pending wheel end event to make
+        // sure that no wheel end event will be sent during the momentum phase
+        // of scrolling.
+        renderWidgetHostView_->IgnorePendingWheelEndEvent();
+      }
+    }
+
     if (renderWidgetHostView_->ShouldRouteEvent(webEvent)) {
       renderWidgetHostView_->render_widget_host_->delegate()
           ->GetInputEventRouter()