diff --git a/widget/src/cocoa/nsChildView.h b/widget/src/cocoa/nsChildView.h
index 11a1449bf52a..276632ee3e59 100644
--- a/widget/src/cocoa/nsChildView.h
+++ b/widget/src/cocoa/nsChildView.h
@@ -357,6 +357,7 @@ class ChildViewMouseTracker {
 public:
 
   static void MouseMoved(NSEvent* aEvent);
+  static void MouseScrolled(NSEvent* aEvent);
   static void OnDestroyView(ChildView* aView);
   static void OnDestroyWindow(NSWindow* aWindow);
   static BOOL WindowAcceptsEvent(NSWindow* aWindow, NSEvent* aEvent,
@@ -370,6 +371,7 @@ public:
   static ChildView* sLastMouseEventView;
   static NSEvent* sLastMouseMoveEvent;
   static NSWindow* sWindowUnderMouse;
+  static NSPoint sLastScrollEventScreenLocation;
 };
 
 //-------------------------------------------------------------------------
diff --git a/widget/src/cocoa/nsChildView.mm b/widget/src/cocoa/nsChildView.mm
index 8b50f1236933..4baf54cfc3b2 100644
--- a/widget/src/cocoa/nsChildView.mm
+++ b/widget/src/cocoa/nsChildView.mm
@@ -128,6 +128,7 @@ extern nsISupportsArray *gDraggedTransferables;
 ChildView* ChildViewMouseTracker::sLastMouseEventView = nil;
 NSEvent* ChildViewMouseTracker::sLastMouseMoveEvent = nil;
 NSWindow* ChildViewMouseTracker::sWindowUnderMouse = nil;
+NSPoint ChildViewMouseTracker::sLastScrollEventScreenLocation = NSZeroPoint;
 
 #ifdef INVALIDATE_DEBUGGING
 static void blinkRect(Rect* r);
@@ -3691,8 +3692,7 @@ NSEvent* gLastDragMouseDownEvent = nil;
     // No sense in firing off a Gecko event.
      return;
 
-  BOOL isMomentumScroll = [theEvent respondsToSelector:@selector(_scrollPhase)] &&
-                          [theEvent _scrollPhase] != 0;
+  BOOL isMomentumScroll = nsCocoaUtils::IsMomentumScrollEvent(theEvent);
 
   if (scrollDelta != 0) {
     // Send the line scroll event.
@@ -3814,14 +3814,11 @@ NSEvent* gLastDragMouseDownEvent = nil;
 
 -(void)scrollWheel:(NSEvent*)theEvent
 {
-  NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
-
   nsAutoRetainCocoaObject kungFuDeathGrip(self);
 
-  if ([self maybeRollup:theEvent])
-    return;
+  ChildViewMouseTracker::MouseScrolled(theEvent);
 
-  if (!mGeckoChild)
+  if ([self maybeRollup:theEvent])
     return;
 
   // It's possible for a single NSScrollWheel event to carry both useful
@@ -3829,11 +3826,7 @@ NSEvent* gLastDragMouseDownEvent = nil;
   // NSMouseScrollEvent can only carry one axis at a time, so the system
   // event will be split into two Gecko events if necessary.
   [self scrollWheel:theEvent forAxis:nsMouseScrollEvent::kIsVertical];
-  if (!mGeckoChild)
-    return;
   [self scrollWheel:theEvent forAxis:nsMouseScrollEvent::kIsHorizontal];
-
-  NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
 -(NSMenu*)menuForEvent:(NSEvent*)theEvent
@@ -4967,6 +4960,15 @@ ChildViewMouseTracker::MouseMoved(NSEvent* aEvent)
   }
 }
 
+void
+ChildViewMouseTracker::MouseScrolled(NSEvent* aEvent)
+{
+  if (!nsCocoaUtils::IsMomentumScrollEvent(aEvent)) {
+    // Store the position so we can pin future momentum scroll events.
+    sLastScrollEventScreenLocation = nsCocoaUtils::ScreenLocationForEvent(aEvent);
+  }
+}
+
 ChildView*
 ChildViewMouseTracker::ViewForEvent(NSEvent* aEvent)
 {
diff --git a/widget/src/cocoa/nsCocoaUtils.h b/widget/src/cocoa/nsCocoaUtils.h
index 2460ce13a3a2..713e66b3025c 100644
--- a/widget/src/cocoa/nsCocoaUtils.h
+++ b/widget/src/cocoa/nsCocoaUtils.h
@@ -141,6 +141,8 @@ class nsCocoaUtils
   // the event was originally targeted at is still alive!
   static NSPoint EventLocationForWindow(NSEvent* anEvent, NSWindow* aWindow);
 
+  static BOOL IsMomentumScrollEvent(NSEvent* aEvent);
+
   // Hides the Menu bar and the Dock. Multiple hide/show requests can be nested.
   static void HideOSChromeOnScreen(PRBool aShouldHide, NSScreen* aScreen);
 
diff --git a/widget/src/cocoa/nsCocoaUtils.mm b/widget/src/cocoa/nsCocoaUtils.mm
index 95a89c2a7feb..2d9df4b9c71d 100644
--- a/widget/src/cocoa/nsCocoaUtils.mm
+++ b/widget/src/cocoa/nsCocoaUtils.mm
@@ -40,6 +40,7 @@
 
 #include "gfxImageSurface.h"
 #include "nsCocoaUtils.h"
+#include "nsChildView.h"
 #include "nsMenuBarX.h"
 #include "nsCocoaWindow.h"
 #include "nsCOMPtr.h"
@@ -105,6 +106,11 @@ NSPoint nsCocoaUtils::ScreenLocationForEvent(NSEvent* anEvent)
   if (!anEvent || [anEvent type] == NSMouseMoved)
     return [NSEvent mouseLocation];
 
+  // Pin momentum scroll events to the location of the last user-controlled
+  // scroll event.
+  if (IsMomentumScrollEvent(anEvent))
+    return ChildViewMouseTracker::sLastScrollEventScreenLocation;
+
   return [[anEvent window] convertBaseToScreen:[anEvent locationInWindow]];
 
   NS_OBJC_END_TRY_ABORT_BLOCK_RETURN(NSMakePoint(0.0, 0.0));
@@ -128,6 +134,13 @@ NSPoint nsCocoaUtils::EventLocationForWindow(NSEvent* anEvent, NSWindow* aWindow
   NS_OBJC_END_TRY_ABORT_BLOCK_RETURN(NSMakePoint(0.0, 0.0));
 }
 
+BOOL nsCocoaUtils::IsMomentumScrollEvent(NSEvent* aEvent)
+{
+  return [aEvent type] == NSScrollWheel &&
+         [aEvent respondsToSelector:@selector(_scrollPhase)] &&
+         [aEvent _scrollPhase] != 0;
+}
+
 void nsCocoaUtils::HideOSChromeOnScreen(PRBool aShouldHide, NSScreen* aScreen)
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;