diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index 83d9320f6cc7..d0dd7495069b 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -1272,6 +1272,26 @@ IsSmoothScrollingEnabled()
   return PR_FALSE;
 }
 
+class ScrollFrameActivityTracker : public nsExpirationTracker<nsGfxScrollFrameInner,4> {
+public:
+  // Wait for 75-100ms between scrolls before we switch the appearance back to
+  // subpixel AA. That's 4 generations of 25ms each.
+  enum { TIMEOUT_MS = 25 };
+  ScrollFrameActivityTracker()
+    : nsExpirationTracker<nsGfxScrollFrameInner,4>(TIMEOUT_MS) {}
+  ~ScrollFrameActivityTracker() {
+    AgeAllGenerations();
+  }
+
+  virtual void NotifyExpired(nsGfxScrollFrameInner *aObject) {
+    RemoveObject(aObject);
+    aObject->mScrollingActive = PR_FALSE;
+    aObject->mOuter->InvalidateOverflowRect();
+  }
+};
+
+static ScrollFrameActivityTracker *gScrollFrameActivityTracker = nsnull;
+
 nsGfxScrollFrameInner::nsGfxScrollFrameInner(nsContainerFrame* aOuter,
                                              PRBool aIsRoot,
                                              PRBool aIsXUL)
@@ -1307,6 +1327,14 @@ nsGfxScrollFrameInner::nsGfxScrollFrameInner(nsContainerFrame* aOuter,
 
 nsGfxScrollFrameInner::~nsGfxScrollFrameInner()
 {
+  if (mActivityExpirationState.IsTracked()) {
+    gScrollFrameActivityTracker->RemoveObject(this);
+  }
+  if (gScrollFrameActivityTracker &&
+      gScrollFrameActivityTracker->IsEmpty()) {
+    delete gScrollFrameActivityTracker;
+    gScrollFrameActivityTracker = nsnull;
+  }
   delete mAsyncScroll;
 }
 
@@ -1562,6 +1590,41 @@ InvalidateFixedBackgroundFrames(nsIFrame* aRootFrame,
   InvalidateFixedBackgroundFramesFromList(&builder, list);
 }
 
+PRBool nsGfxScrollFrameInner::IsAlwaysActive() const
+{
+  // The root scrollframe for a non-chrome document which is the direct
+  // child of a chrome document is always treated as "active".
+  if (!mIsRoot)
+    return PR_FALSE;
+  nsPresContext* presContext = mOuter->PresContext();
+  if (presContext->IsChrome())
+    return PR_FALSE;
+  nsIFrame* rootFrame = mOuter->PresContext()->PresShell()->GetRootFrame();
+  nsIFrame* rootParent = nsLayoutUtils::GetCrossDocParentFrame(rootFrame);
+  return !rootParent || rootParent->PresContext()->IsChrome(); 
+}
+
+PRBool nsGfxScrollFrameInner::IsScrollingActive() const
+{
+  return mScrollingActive || IsAlwaysActive();
+}
+
+void nsGfxScrollFrameInner::MarkActive()
+{
+  if (IsAlwaysActive())
+    return;
+
+  mScrollingActive = PR_TRUE;
+  if (mActivityExpirationState.IsTracked()) {
+    gScrollFrameActivityTracker->MarkUsed(this);
+  } else {
+    if (!gScrollFrameActivityTracker) {
+      gScrollFrameActivityTracker = new ScrollFrameActivityTracker();
+    }
+    gScrollFrameActivityTracker->AddObject(this);
+  }
+}
+
 void nsGfxScrollFrameInner::ScrollVisual(nsIntPoint aPixDelta)
 {
   nsRootPresContext* rootPresContext =
@@ -1595,10 +1658,10 @@ void nsGfxScrollFrameInner::ScrollVisual(nsIntPoint aPixDelta)
   // to be consistent with the view and frame hierarchy.
   PRUint32 flags = nsIFrame::INVALIDATE_REASON_SCROLL_REPAINT;
   nsIFrame* displayRoot = nsLayoutUtils::GetDisplayRootFrame(mOuter);
-  if (CanScrollWithBlitting(mOuter, displayRoot) && mScrollingActive) {
+  if (IsScrollingActive() && CanScrollWithBlitting(mOuter, displayRoot)) {
     flags |= nsIFrame::INVALIDATE_NO_THEBES_LAYERS;
   }
-  mScrollingActive = PR_TRUE;
+  MarkActive();
   mOuter->InvalidateWithFlags(mScrollPort, flags);
 
   if (flags & nsIFrame::INVALIDATE_NO_THEBES_LAYERS) {
diff --git a/layout/generic/nsGfxScrollFrame.h b/layout/generic/nsGfxScrollFrame.h
index 0482a2204965..f00560621014 100644
--- a/layout/generic/nsGfxScrollFrame.h
+++ b/layout/generic/nsGfxScrollFrame.h
@@ -54,6 +54,7 @@
 #ifdef MOZ_SVG
 #include "nsSVGIntegrationUtils.h"
 #endif
+#include "nsExpirationTracker.h"
 
 class nsPresContext;
 class nsIPresShell;
@@ -213,7 +214,7 @@ public:
   nsMargin GetDesiredScrollbarSizes(nsBoxLayoutState* aState);
   PRBool IsLTR() const;
   PRBool IsScrollbarOnRight() const;
-  PRBool IsScrollingActive() { return mScrollingActive; }
+  PRBool IsScrollingActive() const;
   // adjust the scrollbar rectangle aRect to account for any visible resizer.
   // aHasResizer specifies if there is a content resizer, however this method
   // will also check if a widget resizer is present as well.
@@ -227,6 +228,10 @@ public:
                         const nsRect& aContentArea,
                         const nsRect& aOldScrollArea);
 
+  PRBool IsAlwaysActive() const;
+  void MarkActive();
+  nsExpirationState* GetExpirationState() { return &mActivityExpirationState; }
+
   // owning references to the nsIAnonymousContentCreator-built content
   nsCOMPtr<nsIContent> mHScrollbarContent;
   nsCOMPtr<nsIContent> mVScrollbarContent;
@@ -253,6 +258,8 @@ public:
   nsPoint mRestorePos;
   nsPoint mLastPos;
 
+  nsExpirationState mActivityExpirationState;
+
   PRPackedBool mNeverHasVerticalScrollbar:1;
   PRPackedBool mNeverHasHorizontalScrollbar:1;
   PRPackedBool mHasVerticalScrollbar:1;
diff --git a/xpcom/ds/nsExpirationTracker.h b/xpcom/ds/nsExpirationTracker.h
index 1dcca72d7eac..8a93589a9775 100644
--- a/xpcom/ds/nsExpirationTracker.h
+++ b/xpcom/ds/nsExpirationTracker.h
@@ -259,12 +259,20 @@ template <class T, PRUint32 K> class nsExpirationTracker {
     
     friend class Iterator;
 
+    PRBool IsEmpty() {
+      for (PRUint32 i = 0; i < K; ++i) {
+        if (!mGenerations[i].IsEmpty())
+          return PR_FALSE;
+      }
+      return PR_TRUE;
+    }
+
   protected:
     /**
      * This must be overridden to catch notifications. It is called whenever
      * we detect that an object has not been used for at least (K-1)*mTimerPeriod
-     * seconds. If timer events are not delayed, it will be called within
-     * roughly K*mTimerPeriod seconds after the last use. (Unless AgeOneGeneration
+     * milliseconds. If timer events are not delayed, it will be called within
+     * roughly K*mTimerPeriod milliseconds after the last use. (Unless AgeOneGeneration
      * or AgeAllGenerations have been called to accelerate the aging process.)
      * 
      * NOTE: These bounds ignore delays in timer firings due to actual work being
@@ -298,13 +306,10 @@ template <class T, PRUint32 K> class nsExpirationTracker {
       nsExpirationTracker* tracker = static_cast<nsExpirationTracker*>(aThis);
       tracker->AgeOneGeneration();
       // Cancel the timer if we have no objects to track
-      PRUint32 i;
-      for (i = 0; i < K; ++i) {
-        if (!tracker->mGenerations[i].IsEmpty())
-          return;
+      if (tracker->IsEmpty()) {
+        tracker->mTimer->Cancel();
+        tracker->mTimer = nsnull;
       }
-      tracker->mTimer->Cancel();
-      tracker->mTimer = nsnull;
     }
 
     nsresult CheckStartTimer() {