diff --git a/gfx/layers/Layers.cpp b/gfx/layers/Layers.cpp
index ef1ea7ca9dd0..2f1d855016cf 100644
--- a/gfx/layers/Layers.cpp
+++ b/gfx/layers/Layers.cpp
@@ -47,7 +47,12 @@
 #include "gfxPlatform.h"
 
 using namespace mozilla::layers;
- 
+
+typedef FrameMetrics::ViewID ViewID;
+const ViewID FrameMetrics::NULL_SCROLL_ID = 0;
+const ViewID FrameMetrics::ROOT_SCROLL_ID = 1;
+const ViewID FrameMetrics::START_SCROLL_ID = 2;
+
 #ifdef MOZ_LAYERS_HAVE_LOG
 FILE*
 FILEOrDefault(FILE* aFile)
@@ -79,6 +84,15 @@ AppendToString(nsACString& s, const gfxPattern::GraphicsFilter& f,
   return s += sfx;
 }
 
+nsACString&
+AppendToString(nsACString& s, ViewID n,
+               const char* pfx="", const char* sfx="")
+{
+  s += pfx;
+  s.AppendInt(n);
+  return s += sfx;
+}
+
 nsACString&
 AppendToString(nsACString& s, const gfxRGBA& c,
                const char* pfx="", const char* sfx="")
@@ -164,9 +178,10 @@ AppendToString(nsACString& s, const FrameMetrics& m,
                const char* pfx="", const char* sfx="")
 {
   s += pfx;
-  AppendToString(s, m.mViewportSize, "{ viewport=");
+  AppendToString(s, m.mViewport, "{ viewport=");
   AppendToString(s, m.mViewportScrollOffset, " viewportScroll=");
-  AppendToString(s, m.mDisplayPort, " displayport=", " }");
+  AppendToString(s, m.mDisplayPort, " displayport=");
+  AppendToString(s, m.mScrollId, " scrollId=", " }");
   return s += sfx;
 }
 
diff --git a/gfx/layers/Layers.h b/gfx/layers/Layers.h
index 176107d2e7c5..d85ddf7c40c2 100644
--- a/gfx/layers/Layers.h
+++ b/gfx/layers/Layers.h
@@ -87,18 +87,28 @@ class SpecificLayerAttributes;
  * atomically with new pixels.
  */
 struct FrameMetrics {
+public:
+  // We use IDs to identify frames across processes.
+  typedef PRUint64 ViewID;
+  static const ViewID NULL_SCROLL_ID;   // This container layer does not scroll.
+  static const ViewID ROOT_SCROLL_ID;   // This is the root scroll frame.
+  static const ViewID START_SCROLL_ID;  // This is the ID that scrolling subframes
+                                        // will begin at.
+
   FrameMetrics()
-    : mViewportSize(0, 0)
+    : mViewport(0, 0, 0, 0)
     , mViewportScrollOffset(0, 0)
+    , mScrollId(NULL_SCROLL_ID)
   {}
 
   // Default copy ctor and operator= are fine
 
   PRBool operator==(const FrameMetrics& aOther) const
   {
-    return (mViewportSize == aOther.mViewportSize &&
+    return (mViewport == aOther.mViewport &&
             mViewportScrollOffset == aOther.mViewportScrollOffset &&
-            mDisplayPort == aOther.mDisplayPort);
+            mDisplayPort == aOther.mDisplayPort &&
+            mScrollId == aOther.mScrollId);
   }
 
   PRBool IsDefault() const
@@ -106,9 +116,20 @@ struct FrameMetrics {
     return (FrameMetrics() == *this);
   }
 
-  nsIntSize mViewportSize;
+  PRBool IsRootScrollable() const
+  {
+    return mScrollId == ROOT_SCROLL_ID;
+  }
+
+  PRBool IsScrollable() const
+  {
+    return mScrollId != NULL_SCROLL_ID;
+  }
+
+  nsIntRect mViewport;
   nsIntPoint mViewportScrollOffset;
   nsIntRect mDisplayPort;
+  ViewID mScrollId;
 };
 
 #define MOZ_LAYER_DECL_NAME(n, e)                           \
diff --git a/gfx/layers/ipc/ShadowLayerUtils.h b/gfx/layers/ipc/ShadowLayerUtils.h
index 099d8e594a63..4fff40cfe2ba 100644
--- a/gfx/layers/ipc/ShadowLayerUtils.h
+++ b/gfx/layers/ipc/ShadowLayerUtils.h
@@ -63,16 +63,18 @@ struct ParamTraits<mozilla::layers::FrameMetrics>
 
   static void Write(Message* aMsg, const paramType& aParam)
   {
-    WriteParam(aMsg, aParam.mViewportSize);
+    WriteParam(aMsg, aParam.mViewport);
     WriteParam(aMsg, aParam.mViewportScrollOffset);
     WriteParam(aMsg, aParam.mDisplayPort);
+    WriteParam(aMsg, aParam.mScrollId);
   }
 
   static bool Read(const Message* aMsg, void** aIter, paramType* aResult)
   {
-    return (ReadParam(aMsg, aIter, &aResult->mViewportSize) &&
+    return (ReadParam(aMsg, aIter, &aResult->mViewport) &&
             ReadParam(aMsg, aIter, &aResult->mViewportScrollOffset) &&
-            ReadParam(aMsg, aIter, &aResult->mDisplayPort));
+            ReadParam(aMsg, aIter, &aResult->mDisplayPort) &&
+            ReadParam(aMsg, aIter, &aResult->mScrollId));
   }
 };
 
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index e8ad5cb4f823..2fbc9e1c7e3a 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -67,6 +67,7 @@
 
 using namespace mozilla;
 using namespace mozilla::layers;
+typedef FrameMetrics::ViewID ViewID;
 
 nsDisplayListBuilder::nsDisplayListBuilder(nsIFrame* aReferenceFrame,
     Mode aMode, PRBool aBuildCaret)
@@ -147,6 +148,38 @@ static void UnmarkFrameForDisplay(nsIFrame* aFrame) {
   }
 }
 
+static void RecordFrameMetrics(nsIFrame* aForFrame,
+                               ContainerLayer* aRoot,
+                               nsRect aVisibleRect,
+                               nsRect aViewport,
+                               ViewID aScrollId) {
+  nsPresContext* presContext = aForFrame->PresContext();
+  nsIPresShell* presShell = presContext->GetPresShell();
+
+  nsIntRect visible = aVisibleRect.ToNearestPixels(presContext->AppUnitsPerDevPixel());
+  aRoot->SetVisibleRegion(nsIntRegion(visible));
+
+  FrameMetrics metrics;
+
+  PRInt32 auPerDevPixel = presContext->AppUnitsPerDevPixel();
+  metrics.mViewport = aViewport.ToNearestPixels(auPerDevPixel);
+  if (presShell->UsingDisplayPort()) {
+    metrics.mDisplayPort =
+      presShell->GetDisplayPort().ToNearestPixels(auPerDevPixel);
+  }
+
+  nsIScrollableFrame* rootScrollableFrame =
+    presShell->GetRootScrollFrameAsScrollable();
+  if (rootScrollableFrame) {
+    metrics.mViewportScrollOffset =
+      rootScrollableFrame->GetScrollPosition().ToNearestPixels(auPerDevPixel);
+    
+    metrics.mScrollId = aScrollId;
+  }
+
+  aRoot->SetFrameMetrics(metrics);
+}
+
 nsDisplayListBuilder::~nsDisplayListBuilder() {
   NS_ASSERTION(mFramesMarkedForDisplay.Length() == 0,
                "All frames should have been unmarked");
@@ -467,28 +500,10 @@ void nsDisplayList::PaintForFrame(nsDisplayListBuilder* aBuilder,
   nsPresContext* presContext = aForFrame->PresContext();
   nsIPresShell* presShell = presContext->GetPresShell();
 
-  nsIntRect visible = mVisibleRect.ToNearestPixels(presContext->AppUnitsPerDevPixel());
-  root->SetVisibleRegion(nsIntRegion(visible));
+  ViewID id = presContext->IsRootContentDocument() ? FrameMetrics::ROOT_SCROLL_ID
+                                                   : FrameMetrics::NULL_SCROLL_ID;
 
-  // Collect frame metrics with which to stamp the root layer.
-  FrameMetrics metrics;
-
-  PRInt32 auPerCSSPixel = nsPresContext::AppUnitsPerCSSPixel();
-  metrics.mViewportSize =
-    presContext->GetVisibleArea().ToNearestPixels(auPerCSSPixel).Size();
-  if (presShell->UsingDisplayPort()) {
-    metrics.mDisplayPort =
-      presShell->GetDisplayPort().ToNearestPixels(auPerCSSPixel);
-  }
-
-  nsIScrollableFrame* rootScrollableFrame =
-    presShell->GetRootScrollFrameAsScrollable();
-  if (rootScrollableFrame) {
-    metrics.mViewportScrollOffset =
-      rootScrollableFrame->GetScrollPosition().ToNearestPixels(auPerCSSPixel);
-  }
-
-  root->SetFrameMetrics(metrics);
+  RecordFrameMetrics(aForFrame, root, mVisibleRect, mVisibleRect, id);
 
   // If the layer manager supports resolution scaling, set that up
   if (LayerManager::LAYERS_BASIC == layerManager->GetBackendType()) {