Only generate a synthetic fling cancel at the start of a touchpad scroll

As part of issue 855840, we will need to hit test touchpad gesture fling
cancel events. However, we currently generate a synthetic fling cancel
before every wheel event in a scroll. This is excessive, since we have
wheel scroll latching, and would become expensive, since we would need
to hit test them all.

We now only generate a synthetic fling cancel at the beginning of a
touchpad scroll.

Bug: 855840
Change-Id: I108393814065c90cea80ba5326772a1764526ad7
Reviewed-on: https://chromium-review.googlesource.com/1138637
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576486}

content/browser/renderer_host/render_widget_host_view_aura_unittest.cc

@@ -384,6 +384,20 @@ class MockRenderWidgetHostImpl : public RenderWidgetHostImpl {
     lastWheelOrTouchEventLatencyInfo = ui::LatencyInfo(ui_latency);
   }
 
+  void ForwardGestureEventWithLatencyInfo(
+      const blink::WebGestureEvent& gesture_event,
+      const ui::LatencyInfo& ui_latency) override {
+    RenderWidgetHostImpl::ForwardGestureEventWithLatencyInfo(gesture_event,
+                                                             ui_latency);
+    last_forwarded_gesture_event_ = gesture_event;
+  }
+
+  base::Optional<WebGestureEvent> GetAndResetLastForwardedGestureEvent() {
+    base::Optional<WebGestureEvent> ret;
+    last_forwarded_gesture_event_.swap(ret);
+    return ret;
+  }
+
   static MockRenderWidgetHostImpl* Create(RenderWidgetHostDelegate* delegate,
                                           RenderProcessHost* process,
                                           int32_t routing_id) {
@@ -430,6 +444,7 @@ class MockRenderWidgetHostImpl : public RenderWidgetHostImpl {
 
   bool new_content_rendering_timeout_fired_ = false;
   std::unique_ptr<MockWidgetImpl> widget_impl_;
+  base::Optional<WebGestureEvent> last_forwarded_gesture_event_;
 };
 
 class TestScopedKeyboardHook : public aura::ScopedKeyboardHook {
@@ -2440,6 +2455,43 @@ TEST_F(RenderWidgetHostViewAuraTest,
   EXPECT_EQ(blink::kWebGestureDeviceTouchscreen, gesture_event->SourceDevice());
 }
 
+TEST_F(RenderWidgetHostViewAuraTest,
+       SyntheticFlingCancelAtTouchpadScrollBegin) {
+  ui::ScrollEvent scroll_event(ui::ET_SCROLL, gfx::Point(2, 2),
+                               ui::EventTimeForNow(), 0, 0, 5, 0, 5, 2);
+
+  // Send the beginning scroll event. This should generate a synthetic fling
+  // cancel to cancel any ongoing flings before the start of this scroll.
+  view_->OnScrollEvent(&scroll_event);
+  base::RunLoop().RunUntilIdle();
+  base::Optional<WebGestureEvent> last_gesture =
+      widget_host_->GetAndResetLastForwardedGestureEvent();
+  ASSERT_TRUE(last_gesture);
+  EXPECT_EQ(WebInputEvent::kGestureFlingCancel, last_gesture->GetType());
+
+  // Consume the wheel to prevent gesture scrolls from interfering with the
+  // rest of the test.
+  MockWidgetInputHandler::MessageVector dispatched_events =
+      GetAndResetDispatchedMessages();
+  EXPECT_EQ("MouseWheel", GetMessageNames(dispatched_events));
+  dispatched_events[0]->ToEvent()->CallCallback(INPUT_EVENT_ACK_STATE_CONSUMED);
+  dispatched_events = GetAndResetDispatchedMessages();
+  EXPECT_EQ(0U, dispatched_events.size());
+
+  // Send a scroll update. A synthetic fling cancel has already been sent for
+  // this sequence, so we should not generate another.
+  view_->OnScrollEvent(&scroll_event);
+  base::RunLoop().RunUntilIdle();
+  last_gesture = widget_host_->GetAndResetLastForwardedGestureEvent();
+  EXPECT_FALSE(last_gesture);
+
+  dispatched_events = GetAndResetDispatchedMessages();
+  EXPECT_EQ("MouseWheel", GetMessageNames(dispatched_events));
+  dispatched_events[0]->ToEvent()->CallCallback(INPUT_EVENT_ACK_STATE_CONSUMED);
+  dispatched_events = GetAndResetDispatchedMessages();
+  EXPECT_EQ(0U, dispatched_events.size());
+}
+
 // Checks that touch-event state is maintained correctly for multiple touch
 // points.
 TEST_F(RenderWidgetHostViewAuraTest, MultiTouchPointsStates) {

content/browser/renderer_host/render_widget_host_view_event_handler.cc

@@ -433,23 +433,29 @@ void RenderWidgetHostViewEventHandler::OnScrollEvent(ui::ScrollEvent* event) {
     if (event->finger_count() != 2)
       return;
 #endif
-    blink::WebGestureEvent gesture_event = ui::MakeWebGestureEventFlingCancel();
-    // Coordinates need to be transferred to the fling cancel gesture only
-    // for Surface-targeting to ensure that it is targeted to the correct
-    // RenderWidgetHost.
-    gesture_event.SetPositionInWidget(event->location_f());
     blink::WebMouseWheelEvent mouse_wheel_event = ui::MakeWebMouseWheelEvent(
         *event, base::Bind(&GetScreenLocationFromEvent));
     mouse_wheel_phase_handler_.AddPhaseIfNeededAndScheduleEndEvent(
         mouse_wheel_event, should_route_event);
+
+    base::Optional<blink::WebGestureEvent> maybe_synthetic_fling_cancel;
+    if (mouse_wheel_event.phase == blink::WebMouseWheelEvent::kPhaseBegan) {
+      maybe_synthetic_fling_cancel =
+          ui::MakeWebGestureEventFlingCancel(mouse_wheel_event);
+    }
+
     if (should_route_event) {
-      host_->delegate()->GetInputEventRouter()->RouteGestureEvent(
-          host_view_, &gesture_event,
-          ui::LatencyInfo(ui::SourceEventType::WHEEL));
+      if (maybe_synthetic_fling_cancel) {
+        host_->delegate()->GetInputEventRouter()->RouteGestureEvent(
+            host_view_, &*maybe_synthetic_fling_cancel,
+            ui::LatencyInfo(ui::SourceEventType::WHEEL));
+      }
       host_->delegate()->GetInputEventRouter()->RouteMouseWheelEvent(
           host_view_, &mouse_wheel_event, *event->latency());
     } else {
-      host_->ForwardGestureEvent(gesture_event);
+      if (maybe_synthetic_fling_cancel) {
+        host_->ForwardGestureEvent(*maybe_synthetic_fling_cancel);
+      }
       host_->ForwardWheelEventWithLatencyInfo(mouse_wheel_event,
                                               *event->latency());
     }

ui/events/blink/web_input_event.cc

@@ -392,11 +392,17 @@ blink::WebGestureEvent MakeWebGestureEvent(
   return gesture_event;
 }
 
-blink::WebGestureEvent MakeWebGestureEventFlingCancel() {
+blink::WebGestureEvent MakeWebGestureEventFlingCancel(
+    const blink::WebMouseWheelEvent& wheel_event) {
   blink::WebGestureEvent gesture_event(
       blink::WebInputEvent::kGestureFlingCancel,
-      blink::WebInputEvent::kNoModifiers, EventTimeForNow(),
+      blink::WebInputEvent::kNoModifiers, wheel_event.TimeStamp(),
       blink::kWebGestureDeviceTouchpad);
+  // Coordinates need to be transferred to the fling cancel gesture only
+  // for Surface-targeting to ensure that it is targeted to the correct
+  // RenderWidgetHost.
+  gesture_event.SetPositionInWidget(wheel_event.PositionInWidget());
+  gesture_event.SetPositionInScreen(wheel_event.PositionInScreen());
   // All other fields are ignored on a GestureFlingCancel event.
   return gesture_event;
 }

ui/events/blink/web_input_event.h

@@ -51,7 +51,8 @@ blink::WebGestureEvent MakeWebGestureEvent(
     const ScrollEvent& event,
     const base::Callback<gfx::PointF(const ui::LocatedEvent& event)>&
         screen_location_callback);
-blink::WebGestureEvent MakeWebGestureEventFlingCancel();
+blink::WebGestureEvent MakeWebGestureEventFlingCancel(
+    const blink::WebMouseWheelEvent& wheel_event);
 
 }  // namespace ui