Touchpad scroll latching enabled for Mac behind flag.

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index 1d4a119420bbfc7103437d70a83da022085c87af..377f6bddcbf73ce8b50cd054d5c9dc40524408dc 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -132,6 +132,12 @@ cc::ScrollState CreateScrollStateForGesture(const WebGestureEvent& event) {
       scroll_state_data.position_x = event.x;
       scroll_state_data.position_y = event.y;
       scroll_state_data.is_beginning = true;
+      // On Mac, a GestureScrollBegin in the inertial phase indicates a fling
+      // start.
+      if (event.data.scrollBegin.inertialPhase ==
+          WebGestureEvent::MomentumPhase) {
+        scroll_state_data.is_in_inertial_phase = true;
+      }
       break;
     case WebInputEvent::GestureFlingStart:
       scroll_state_data.velocity_x = event.data.flingStart.velocityX;
@@ -490,27 +496,8 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::ScrollByMouseWheel(
         blink::WebGestureDeviceTouchpad,
         cc::MainThreadScrollingReason::kPageBasedScrolling);
 
-  } else if (ShouldAnimate(wheel_event.hasPreciseScrollingDeltas)) {
-    cc::InputHandler::ScrollStatus scroll_status =
-        input_handler_->ScrollAnimated(gfx::Point(wheel_event.x, wheel_event.y),
-                                       scroll_delta);
-
-    RecordMainThreadScrollingReasons(
-        blink::WebGestureDeviceTouchpad,
-        scroll_status.main_thread_scrolling_reasons);
-
-    switch (scroll_status.thread) {
-      case cc::InputHandler::SCROLL_ON_IMPL_THREAD:
-        result = DID_HANDLE;
-        break;
-      case cc::InputHandler::SCROLL_IGNORED:
-        result = DROP_EVENT;
-        break;
-      default:
-        result = DID_NOT_HANDLE;
-        break;
-    }
   } else {
+    DCHECK(!ShouldAnimate(wheel_event.hasPreciseScrollingDeltas));
     cc::ScrollStateData scroll_state_begin_data;
     scroll_state_begin_data.position_x = wheel_event.x;
     scroll_state_begin_data.position_y = wheel_event.y;
@@ -639,6 +626,7 @@ InputHandlerProxy::HandleGestureScrollUpdate(
 
   if (ShouldAnimate(gesture_event.data.scrollUpdate.deltaUnits !=
                     blink::WebGestureEvent::ScrollUnits::Pixels)) {
+    DCHECK(!scroll_state.is_in_inertial_phase());
     switch (input_handler_->ScrollAnimated(scroll_point, scroll_delta).thread) {
       case cc::InputHandler::SCROLL_ON_IMPL_THREAD:
         return DID_HANDLE;