MouseWheelEventQueue should respect ScrollBegin/End state from RenderWidgetHost.

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 1728da15397f81f3146534e1220b523969983213..a602748afc470d3d38230c9d558bd30f4db103fd 100644
--- a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
+++ b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
@@ -37,8 +37,6 @@ class QueuedWebMouseWheelEvent : public MouseWheelEventWithLatencyInfo {
 MouseWheelEventQueue::MouseWheelEventQueue(MouseWheelEventQueueClient* client,
                                            bool enable_scroll_latching)
     : client_(client),
-      needs_scroll_begin_(true),
-      needs_scroll_end_(false),
       enable_scroll_latching_(enable_scroll_latching),
       scrolling_device_(blink::kWebGestureDeviceUninitialized) {
   DCHECK(client);
@@ -180,8 +178,8 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
     // because the events generated will be a GSB (non-synthetic) and GSE
     // (non-synthetic). This situation arises when OSX generates double
     // phase end information.
-    bool empty_sequence =
-        !needs_update && needs_scroll_begin_ && current_phase_ended;
+    bool empty_sequence = !needs_update && !client_->is_in_gesture_scroll() &&
+                          current_phase_ended;
 
     if (enable_scroll_latching_) {
       if (event_sent_for_gesture_ack_->event.momentum_phase ==
@@ -192,7 +190,7 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
       }
 
       if (needs_update || !empty_sequence) {
-        if (needs_scroll_begin_) {
+        if (!client_->is_in_gesture_scroll()) {
           SendScrollBegin(scroll_update, false);
         }
 
@@ -221,7 +219,7 @@ void MouseWheelEventQueue::ProcessMouseWheelAck(
 
     } else {  // !enable_scroll_latching_
       if (needs_update || !empty_sequence) {
-        if (needs_scroll_begin_) {
+        if (!client_->is_in_gesture_scroll()) {
           // If no GSB has been sent, it will be a non-synthetic GSB.
           SendScrollBegin(scroll_update, false);

[dtapuska, 2017/04/12 19:24:46]

It there any potential dangerous scenarios here? We generally assume that the
gesture scroll begin has the delta hints and such from the first gesture scroll
update. (and phase information). If it doesn't then could it be subtly broken
somewhere?

[wjmaclean, 2017/04/12 19:47:35]

I suppose, but (1) it passes the tests, and (2) appears to work fine in manual
testing. If something else does break, then we can fix it when we discover it.

[wjmaclean, 2017/04/12 19:48:42]

Also, it does fix something we know is already broken.

         } else if (has_phase_info) {
@@ -284,8 +282,6 @@ void MouseWheelEventQueue::OnGestureScrollEvent(
         // Don't send the pending ScrollEnd if a fling is happening.
         // The next wheel event will still need a ScrollBegin.
         scroll_end_timer_.Stop();
-        needs_scroll_begin_ = true;
-        needs_scroll_end_ = false;
       } else {
         scroll_end_timer_.Reset();
       }
@@ -307,7 +303,8 @@ void MouseWheelEventQueue::TryForwardNextEventToRenderer() {
 
 void MouseWheelEventQueue::SendScrollEnd(WebGestureEvent update_event,
                                          bool synthetic) {
-  DCHECK((synthetic && !needs_scroll_end_) || needs_scroll_end_);
+  DCHECK((synthetic && !client_->is_in_gesture_scroll()) ||
+         client_->is_in_gesture_scroll());
 
   WebGestureEvent scroll_end(update_event);
   scroll_end.SetTimeStampSeconds(
@@ -320,13 +317,9 @@ void MouseWheelEventQueue::SendScrollEnd(WebGestureEvent update_event,
   scroll_end.data.scroll_end.delta_units =
       update_event.data.scroll_update.delta_units;
 
-  if (!synthetic) {
-    needs_scroll_begin_ = true;
-    needs_scroll_end_ = false;
+  if (!synthetic && scroll_end_timer_.IsRunning())
+    scroll_end_timer_.Reset();
 
-    if (scroll_end_timer_.IsRunning())
-      scroll_end_timer_.Reset();
-  }
   client_->ForwardGestureEventWithLatencyInfo(
       scroll_end, ui::LatencyInfo(ui::SourceEventType::WHEEL));
 }
@@ -334,7 +327,8 @@ void MouseWheelEventQueue::SendScrollEnd(WebGestureEvent update_event,
 void MouseWheelEventQueue::SendScrollBegin(
     const WebGestureEvent& gesture_update,
     bool synthetic) {
-  DCHECK((synthetic && !needs_scroll_begin_) || needs_scroll_begin_);
+  DCHECK((synthetic && client_->is_in_gesture_scroll()) ||
+         !client_->is_in_gesture_scroll());
 
   WebGestureEvent scroll_begin(gesture_update);
   scroll_begin.SetType(WebInputEvent::kGestureScrollBegin);
@@ -349,8 +343,6 @@ void MouseWheelEventQueue::SendScrollBegin(
   scroll_begin.data.scroll_begin.delta_hint_units =
       gesture_update.data.scroll_update.delta_units;
 
-  needs_scroll_begin_ = false;
-  needs_scroll_end_ = true;
   client_->ForwardGestureEventWithLatencyInfo(
       scroll_begin, ui::LatencyInfo(ui::SourceEventType::WHEEL));
 }