Don't route wheel events through APZ if they require custom deltas. (bug 1146674, r=kats)

dom/events/EventStateManager.cpp

@@ -5514,12 +5514,28 @@ EventStateManager::WheelPrefs::NeedToComputeLineOrPageDelta(
          (mMultiplierY[index] != 1.0 && mMultiplierY[index] != -1.0);
 }
 
+bool
+EventStateManager::WheelPrefs::HasUserPrefsForDelta(WidgetWheelEvent* aEvent)
+{
+  Index index = GetIndexFor(aEvent);
+  Init(index);
+
+  return mMultiplierX[index] != 1.0 ||
+         mMultiplierY[index] != 1.0;
+}
+
 bool
 EventStateManager::WheelEventIsScrollAction(WidgetWheelEvent* aEvent)
 {
   return WheelPrefs::GetInstance()->ComputeActionFor(aEvent) == WheelPrefs::ACTION_SCROLL;
 }
 
+bool
+EventStateManager::WheelEventNeedsDeltaMultipliers(WidgetWheelEvent* aEvent)
+{
+  return WheelPrefs::GetInstance()->HasUserPrefsForDelta(aEvent);
+}
+
 bool
 EventStateManager::WheelPrefs::IsOverOnePageScrollAllowedX(
                                  WidgetWheelEvent* aEvent)

dom/events/EventStateManager.h

@@ -225,6 +225,10 @@ public:
   // Returns true if the given WidgetWheelEvent will resolve to a scroll action.
   static bool WheelEventIsScrollAction(WidgetWheelEvent* aEvent);
 
+  // Returns true if user prefs for wheel deltas apply to the given
+  // WidgetWheelEvent.
+  static bool WheelEventNeedsDeltaMultipliers(WidgetWheelEvent* aEvent);
+
   // Returns whether or not a frame can be vertically scrolled with a mouse
   // wheel (as opposed to, say, a selection or touch scroll).
   static bool CanVerticallyScrollFrameWithWheel(nsIFrame* aFrame);
@@ -423,6 +427,12 @@ protected:
      */
     void ApplyUserPrefsToDelta(WidgetWheelEvent* aEvent);
 
+    /**
+     * Returns whether or not ApplyUserPrefsToDelta() would change the delta
+     * values of an event.
+     */
+    bool HasUserPrefsForDelta(WidgetWheelEvent* aEvent);
+
     /**
      * If ApplyUserPrefsToDelta() changed the delta values with customized
      * prefs, the overflowDelta values would be inflated.

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -903,7 +903,8 @@ APZCTreeManager::WillHandleWheelEvent(WidgetWheelEvent* aEvent)
 {
   return EventStateManager::WheelEventIsScrollAction(aEvent) &&
          aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_LINE &&
-         !gfxPrefs::MouseWheelHasScrollDeltaOverride();
+         !gfxPrefs::MouseWheelHasScrollDeltaOverride() &&
+         !EventStateManager::WheelEventNeedsDeltaMultipliers(aEvent);
 }
 
 nsEventStatus