diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 1f714b0b34d..e78dc10f274 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -2033,6 +2033,11 @@ nsCSSFrameConstructor::ConstructRootFrame(nsIPresContext* aPresContext,
                                              nsnull, PR_FALSE,
                                              getter_AddRefs(viewportPseudoStyle));
 
+  { // ensure that the viewport thinks it is a block frame, layout goes pootsy if it doesn't
+    nsStyleDisplay* display = (nsStyleDisplay*)viewportPseudoStyle->GetMutableStyleData(eStyleStruct_Display);
+    display->mDisplay = NS_STYLE_DISPLAY_BLOCK;
+  }
+
   // Initialize the viewport frame. It has a NULL content object
   viewportFrame->Init(*aPresContext, nsnull, nsnull, viewportPseudoStyle, nsnull);
 
diff --git a/layout/html/style/src/nsCSSFrameConstructor.cpp b/layout/html/style/src/nsCSSFrameConstructor.cpp
index 1f714b0b34d..e78dc10f274 100644
--- a/layout/html/style/src/nsCSSFrameConstructor.cpp
+++ b/layout/html/style/src/nsCSSFrameConstructor.cpp
@@ -2033,6 +2033,11 @@ nsCSSFrameConstructor::ConstructRootFrame(nsIPresContext* aPresContext,
                                              nsnull, PR_FALSE,
                                              getter_AddRefs(viewportPseudoStyle));
 
+  { // ensure that the viewport thinks it is a block frame, layout goes pootsy if it doesn't
+    nsStyleDisplay* display = (nsStyleDisplay*)viewportPseudoStyle->GetMutableStyleData(eStyleStruct_Display);
+    display->mDisplay = NS_STYLE_DISPLAY_BLOCK;
+  }
+
   // Initialize the viewport frame. It has a NULL content object
   viewportFrame->Init(*aPresContext, nsnull, nsnull, viewportPseudoStyle, nsnull);