Touchpad scroll latching enabled for Mac behind flag.

content/browser/renderer_host/input/mouse_wheel_event_queue.cc

Index: content/browser/renderer_host/input/mouse_wheel_event_queue.cc
diff --git a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
index 72d9e95ca706eda95fbe9e2905da993b41e3dd24..901a00f2caad500fb261282def62dfe0470575e5 100644
--- a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
+++ b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
@@ -149,6 +149,8 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
     }
 
     bool current_phase_ended = false;
+    bool scroll_phase_ended = false;
+    bool momentum_phase_ended = false;
     bool has_phase_info = false;
 
     if (event_sent_for_gesture_ack_->event.phase !=
@@ -156,14 +158,15 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
         event_sent_for_gesture_ack_->event.momentumPhase !=
             blink::WebMouseWheelEvent::PhaseNone) {
       has_phase_info = true;
-      current_phase_ended = event_sent_for_gesture_ack_->event.phase ==
-                                blink::WebMouseWheelEvent::PhaseEnded ||
-                            event_sent_for_gesture_ack_->event.phase ==
-                                blink::WebMouseWheelEvent::PhaseCancelled ||
-                            event_sent_for_gesture_ack_->event.momentumPhase ==
-                                blink::WebMouseWheelEvent::PhaseEnded ||
-                            event_sent_for_gesture_ack_->event.momentumPhase ==
-                                blink::WebMouseWheelEvent::PhaseCancelled;
+      scroll_phase_ended = event_sent_for_gesture_ack_->event.phase ==
+                               blink::WebMouseWheelEvent::PhaseEnded ||
+                           event_sent_for_gesture_ack_->event.phase ==
+                               blink::WebMouseWheelEvent::PhaseCancelled;
+      momentum_phase_ended = event_sent_for_gesture_ack_->event.momentumPhase ==
+                                 blink::WebMouseWheelEvent::PhaseEnded ||
+                             event_sent_for_gesture_ack_->event.momentumPhase ==
+                                 blink::WebMouseWheelEvent::PhaseCancelled;
+      current_phase_ended = scroll_phase_ended || momentum_phase_ended;
     }
 
     bool needs_update = scroll_update.data.scrollUpdate.deltaX != 0 ||
@@ -177,11 +180,21 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
     bool empty_sequence =
         !needs_update && needs_scroll_begin_ && current_phase_ended;
 
+    if (enable_scroll_latching_ &&
+        event_sent_for_gesture_ack_->event.momentumPhase ==
+            blink::WebMouseWheelEvent::PhaseBegan) {
+      // Send the pending GSE with |precedeFling = true|.
+      if (scroll_end_timer_.IsRunning()) {
+        scroll_end_timer_.Stop();
+        SendScrollEnd(last_scroll_update_, false, true);
+      }
+    }
+
     if (needs_update || !empty_sequence) {
       if (needs_scroll_begin_) {
         // If no GSB has been sent, it will be a non-synthetic GSB.
         SendScrollBegin(scroll_update, false);
-      } else if (has_phase_info) {
+      } else if (has_phase_info && !enable_scroll_latching_) {
         // If a GSB has been sent, generate a synthetic GSB if we have phase
         // information. This should be removed once crbug.com/526463 is fully
         // implemented.
@@ -196,23 +209,29 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
         client_->ForwardGestureEventWithLatencyInfo(scroll_update, latency);
       }
 
-      if (current_phase_ended) {
-        // Non-synthetic GSEs are sent when the current phase is canceled or
-        // ended.
-        SendScrollEnd(scroll_update, false);
-      } else if (has_phase_info) {
+      if ((enable_scroll_latching_ && momentum_phase_ended) ||
+          (!enable_scroll_latching_ && current_phase_ended)) {
+        // Send GSE with |precedeFling = false| if scroll latching is enabled
+        // and no fling is going to happen next, or if scroll latching is
+        // disabled and current phase ended.
+        SendScrollEnd(scroll_update, false, false);
+      } else if (has_phase_info && !enable_scroll_latching_) {
         // Generate a synthetic GSE for every update to force hit testing so
         // that the non-latching behavior is preserved. Remove once
         // crbug.com/526463 is fully implemented.
-        SendScrollEnd(scroll_update, true);
-      } else {
+        SendScrollEnd(scroll_update, true, false);
+      } else if ((enable_scroll_latching_ && scroll_phase_ended) ||
+                 !has_phase_info) {
+        // If scroll latching is enabled and a fling might happen next, or no
+        // phase info exists, start the scroll_end_timer_.
         scroll_end_timer_.Start(
             FROM_HERE,
             base::TimeDelta::FromMilliseconds(scroll_transaction_ms_),
             base::Bind(&MouseWheelEventQueue::SendScrollEnd,
-                       base::Unretained(this), scroll_update, false));
+                       base::Unretained(this), scroll_update, false, false));
       }
     }
+    last_scroll_update_ = scroll_update;
   }
 
   event_sent_for_gesture_ack_.reset();
@@ -255,7 +274,8 @@ void MouseWheelEventQueue::TryForwardNextEventToRenderer() {
 }
 
 void MouseWheelEventQueue::SendScrollEnd(WebGestureEvent update_event,
-                                         bool synthetic) {
+                                         bool synthetic,
+                                         bool precede_fling) {
   DCHECK((synthetic && !needs_scroll_end_) || needs_scroll_end_);
 
   WebGestureEvent scroll_end(update_event);
@@ -264,6 +284,7 @@ void MouseWheelEventQueue::SendScrollEnd(WebGestureEvent update_event,
   scroll_end.type = WebInputEvent::GestureScrollEnd;
   scroll_end.resendingPluginId = -1;
   scroll_end.data.scrollEnd.synthetic = synthetic;
+  scroll_end.data.scrollEnd.precedeFling = precede_fling;
   scroll_end.data.scrollEnd.inertialPhase =
       update_event.data.scrollUpdate.inertialPhase;
   scroll_end.data.scrollEnd.deltaUnits =