diff --git a/mozilla/layout/xul/base/src/nsNativeScrollbarFrame.cpp b/mozilla/layout/xul/base/src/nsNativeScrollbarFrame.cpp
index 7f5dfca78fd..db6366768c6 100644
--- a/mozilla/layout/xul/base/src/nsNativeScrollbarFrame.cpp
+++ b/mozilla/layout/xul/base/src/nsNativeScrollbarFrame.cpp
@@ -325,8 +325,10 @@ nsNativeScrollbarFrame::Hookup()
   // We can't just pass 'mediator' to the widget, because 'mediator' might go away.
   // So pass a pointer to us. When we go away, we can tell the widget.
   nsIContent* scrollbarContent = parts.mScrollbarFrame->GetContent();
+  // Always configure ourselves as the mediator even if parts.mMediator is null;
+  // the mediator might be changed.
   scrollbar->SetContent(scrollbarContent,
-                        parts.mIScrollbarFrame, parts.mMediator ? this : nsnull);
+                        parts.mIScrollbarFrame, this);
   mScrollbarNeedsContent = PR_FALSE;
 
   if (!scrollbarContent)
diff --git a/mozilla/widget/src/mac/nsNativeScrollbar.cpp b/mozilla/widget/src/mac/nsNativeScrollbar.cpp
index 43e503321e4..481012e9dbd 100644
--- a/mozilla/widget/src/mac/nsNativeScrollbar.cpp
+++ b/mozilla/widget/src/mac/nsNativeScrollbar.cpp
@@ -238,13 +238,11 @@ nsNativeScrollbar::DoScrollAction(ControlPartCode part)
     // lines. Outliner ignores the indexes in ScrollbarButtonPressed() except
     // to check if one is greater than the other to indicate direction.
     //
+    UpdateContentPosition(newPos);
     if (mMediator) {
       BoundsCheck(0, newPos, mMaxValue);
       mMediator->ScrollbarButtonPressed(mScrollbar, oldPos, newPos);
     }
-    else {
-      UpdateContentPosition(newPos);
-    }
   }
   else {
     //