Bug 1490561 - Make sure we remove our RefreshDriver observers in CompleteAsyncScroll. r=emilio

layout/generic/nsGfxScrollFrame.cpp

@@ -2270,8 +2270,7 @@ void
 ScrollFrameHelper::CompleteAsyncScroll(const nsRect &aRange, nsAtom* aOrigin)
 {
   // Apply desired destination range since this is the last step of scrolling.
-  mAsyncSmoothMSDScroll = nullptr;
-  mAsyncScroll = nullptr;
+  RemoveObservers();
   AutoWeakFrame weakFrame(mOuter);
   ScrollToImpl(mDestination, aRange, aOrigin);
   if (!weakFrame.IsAlive()) {
@@ -4934,14 +4933,23 @@ ScrollFrameHelper::Destroy(PostDestroyData& aPostDestroyData)
     mScrollActivityTimer->Cancel();
     mScrollActivityTimer = nullptr;
   }
+  RemoveObservers();
+}
+
+void
+ScrollFrameHelper::RemoveObservers()
+{
   if (mAsyncScroll) {
     mAsyncScroll->RemoveObserver();
+    mAsyncScroll = nullptr;
   }
   if (mAsyncSmoothMSDScroll) {
     mAsyncSmoothMSDScroll->RemoveObserver();
+    mAsyncSmoothMSDScroll = nullptr;
   }
 }
 
+
 /**
  * Called when we want to update the scrollbar position, either because scrolling happened
  * or the user moved the scrollbar position and we need to undo that (e.g., when the user

layout/generic/nsGfxScrollFrame.h

@@ -690,6 +690,9 @@ protected:
 
   bool IsForTextControlWithNoScrollbars() const;
 
+  // Removes any RefreshDriver observers we might have registered.
+  void RemoveObservers();
+
   static void EnsureFrameVisPrefsCached();
   static bool sFrameVisPrefsCached;
   // The number of scrollports wide/high to expand when tracking frame visibility.