Fix APZ using incorrect scroll wheel deltas. (bug 1141728, r=kats)

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -843,8 +843,8 @@ APZCTreeManager::ProcessWheelEvent(WidgetWheelEvent& aEvent,
                          scrollMode,
                          ScrollWheelInput::SCROLLDELTA_LINE,
                          origin,
-                         aEvent.lineOrPageDeltaX,
-                         aEvent.lineOrPageDeltaY);
+                         aEvent.deltaX,
+                         aEvent.deltaY);
 
   nsEventStatus status = ReceiveInputEvent(input, aOutTargetGuid, aOutInputBlockId);
   aEvent.refPoint.x = input.mOrigin.x;
@@ -856,7 +856,8 @@ bool
 APZCTreeManager::WillHandleWheelEvent(WidgetWheelEvent* aEvent)
 {
   return EventStateManager::WheelEventIsScrollAction(aEvent) &&
-         aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_LINE;
+         aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_LINE &&
+         !gfxPrefs::MouseWheelHasScrollDeltaOverride();
 }
 
 nsEventStatus

gfx/thebes/gfxPrefs.h

@@ -342,6 +342,10 @@ private:
 
   // This and code dependent on it should be removed once containerless scrolling looks stable.
   DECL_GFX_PREF(Once, "layout.scroll.root-frame-containers",   LayoutUseContainersForRootFrames, bool, true);
+
+  // This affects whether events will be routed through APZ or not.
+  DECL_GFX_PREF(Once, "mousewheel.system_scroll_override_on_root_content.enabled",
+                                                               MouseWheelHasScrollDeltaOverride, bool, false);
 public:
   // Manage the singleton:
   static gfxPrefs& GetSingleton()