--- a/layout/base/GeometryUtils.cpp
+++ b/layout/base/GeometryUtils.cpp
@@ -147,21 +147,17 @@ GetBoxRectForFrame(nsIFrame** aFrame, CS
     return r;
   }
 
   f = *aFrame;
   switch (aType) {
   case CSSBoxType::Content: r = f->GetContentRectRelativeToSelf(); break;
   case CSSBoxType::Padding: r = f->GetPaddingRectRelativeToSelf(); break;
   case CSSBoxType::Border: r = nsRect(nsPoint(0, 0), f->GetSize()); break;
-  case CSSBoxType::Margin: {
-    r = nsRect(nsPoint(0, 0), f->GetSize());
-    r.Inflate(f->GetUsedMargin());
-    break;
-  }
+  case CSSBoxType::Margin: r = f->GetMarginRectRelativeToSelf(); break;
   default: MOZ_ASSERT(false, "unknown box type"); return r;
   }
 
   return r;
 }
 
 class AccumulateQuadCallback : public nsLayoutUtils::BoxCallback {
 public: