Made sure computedOffsets is initialized to all zeros for the root frame

layout/generic/nsHTMLReflowState.cpp

@@ -611,6 +611,7 @@ nsHTMLReflowState::InitConstraints(nsIPresContext& aPresContext)
     computedWidth = availableWidth;
     computedHeight = availableHeight;
     computedMargin.SizeTo(0, 0, 0, 0);
+    computedOffsets.SizeTo(0, 0, 0, 0);
 
   } else {
     // Get the containing block reflow state

layout/html/base/src/nsHTMLReflowState.cpp

@@ -611,6 +611,7 @@ nsHTMLReflowState::InitConstraints(nsIPresContext& aPresContext)
     computedWidth = availableWidth;
     computedHeight = availableHeight;
     computedMargin.SizeTo(0, 0, 0, 0);
+    computedOffsets.SizeTo(0, 0, 0, 0);
 
   } else {
     // Get the containing block reflow state