Bug 800041 - Set the correct reference frame for nsDisplayScrollLayer when the frame is transformed. r=roc

layout/base/nsDisplayList.cpp

@@ -2780,6 +2780,11 @@ nsDisplayScrollLayer::nsDisplayScrollLayer(nsDisplayListBuilder* aBuilder,
 
   NS_ASSERTION(mScrolledFrame && mScrolledFrame->GetContent(),
                "Need a child frame with content");
+  nsIFrame *parent = nsLayoutUtils::GetCrossDocParentFrame(aForFrame);
+  NS_ASSERTION(parent, "Must have a parent!");
+  mReferenceFrame = 
+    aBuilder->FindReferenceFrameFor(parent);
+  mToReferenceFrame = aForFrame->GetOffsetToCrossDoc(mReferenceFrame);
 }
 
 nsDisplayScrollLayer::nsDisplayScrollLayer(nsDisplayListBuilder* aBuilder,
@@ -2797,6 +2802,11 @@ nsDisplayScrollLayer::nsDisplayScrollLayer(nsDisplayListBuilder* aBuilder,
 
   NS_ASSERTION(mScrolledFrame && mScrolledFrame->GetContent(),
                "Need a child frame with content");
+  nsIFrame *parent = nsLayoutUtils::GetCrossDocParentFrame(aForFrame);
+  NS_ASSERTION(parent, "Must have a parent!");
+  mReferenceFrame = 
+    aBuilder->FindReferenceFrameFor(parent);
+  mToReferenceFrame = aForFrame->GetOffsetToCrossDoc(mReferenceFrame);
 }
 
 nsDisplayScrollLayer::nsDisplayScrollLayer(nsDisplayListBuilder* aBuilder,
@@ -2813,6 +2823,11 @@ nsDisplayScrollLayer::nsDisplayScrollLayer(nsDisplayListBuilder* aBuilder,
 
   NS_ASSERTION(mScrolledFrame && mScrolledFrame->GetContent(),
                "Need a child frame with content");
+  nsIFrame *parent = nsLayoutUtils::GetCrossDocParentFrame(aForFrame);
+  NS_ASSERTION(parent, "Must have a parent!");
+  mReferenceFrame = 
+    aBuilder->FindReferenceFrameFor(parent);
+  mToReferenceFrame = aForFrame->GetOffsetToCrossDoc(mReferenceFrame);
 }
 
 #ifdef NS_BUILD_REFCNT_LOGGING
@@ -2902,6 +2917,9 @@ nsDisplayScrollLayer::TryMerge(nsDisplayListBuilder* aBuilder,
     return false;
   }
 
+  NS_ASSERTION(other->mReferenceFrame == mReferenceFrame,
+               "Must have the same reference frame!");
+
   FrameProperties props = mScrolledFrame->Properties();
   props.Set(nsIFrame::ScrollLayerCount(),
     reinterpret_cast<void*>(GetScrollLayerCount() - 1));