If APZ is enabled, do not handle wheel-event scrolling in layout. (bug 1126090 part 1, r=smaug)

dom/events/EventStateManager.cpp

@@ -87,6 +87,7 @@
 #include "mozilla/LookAndFeel.h"
 #include "GeckoProfiler.h"
 #include "Units.h"
+#include "mozilla/layers/APZCTreeManager.h"
 
 #ifdef XP_MACOSX
 #import <ApplicationServices/ApplicationServices.h>
@@ -3007,7 +3008,18 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
       }
 
       WidgetWheelEvent* wheelEvent = aEvent->AsWheelEvent();
-      switch (WheelPrefs::GetInstance()->ComputeActionFor(wheelEvent)) {
+
+      // When APZ is enabled, the actual scroll animation might be handled by
+      // the compositor.
+      WheelPrefs::Action action;
+      if (gfxPrefs::AsyncPanZoomEnabled() &&
+          layers::APZCTreeManager::WillHandleWheelEvent(wheelEvent))
+      {
+        action = WheelPrefs::ACTION_NONE;
+      } else {
+        action = WheelPrefs::GetInstance()->ComputeActionFor(wheelEvent);
+      }
+      switch (action) {
         case WheelPrefs::ACTION_SCROLL: {
           // For scrolling of default action, we should honor the mouse wheel
           // transaction.

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -854,6 +854,13 @@ APZCTreeManager::ProcessWheelEvent(WidgetWheelEvent& aEvent,
   return status;
 }
 
+bool
+APZCTreeManager::WillHandleWheelEvent(WidgetWheelEvent* aEvent)
+{
+  return EventStateManager::WheelEventIsScrollAction(aEvent) &&
+         aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_LINE;
+}
+
 nsEventStatus
 APZCTreeManager::ReceiveInputEvent(WidgetInputEvent& aEvent,
                                    ScrollableLayerGuid* aOutTargetGuid,
@@ -891,9 +898,7 @@ APZCTreeManager::ReceiveInputEvent(WidgetInputEvent& aEvent,
     }
     case eWheelEventClass: {
       WidgetWheelEvent& wheelEvent = *aEvent.AsWheelEvent();
-      if (wheelEvent.deltaMode != nsIDOMWheelEvent::DOM_DELTA_LINE ||
-          !EventStateManager::WheelEventIsScrollAction(&wheelEvent))
-      {
+      if (!WillHandleWheelEvent(&wheelEvent)) {
         // Don't send through APZ if we're not scrolling or if the delta mode
         // is not line-based.
         return ProcessEvent(aEvent, aOutTargetGuid, aOutInputBlockId);

gfx/layers/apz/src/APZCTreeManager.h

@@ -382,6 +382,15 @@ public:
    */
   nsRefPtr<const OverscrollHandoffChain> BuildOverscrollHandoffChain(const nsRefPtr<AsyncPanZoomController>& aInitialTarget);
 
+public:
+  // Returns whether or not a wheel event action will be (or was) performed by
+  // APZ. If this returns true, the event must not perform a synchronous
+  // scroll.
+  //
+  // Even if this returns false, all wheel events in APZ-aware widgets must
+  // be sent through APZ so they are transformed correctly for TabParent.
+  static bool WillHandleWheelEvent(WidgetWheelEvent* aEvent);
+
 protected:
   // Protected destructor, to discourage deletion outside of Release():
   virtual ~APZCTreeManager();