1 files changed, 6 insertions, 4 deletions
diff --git a/dom/base/DOMIntersectionObserver.cpp b/dom/base/DOMIntersectionObserver.cpp
index 650ec83cd..2f3a341c5 100644
--- a/dom/base/DOMIntersectionObserver.cpp
+++ b/dom/base/DOMIntersectionObserver.cpp
@@ -413,13 +413,15 @@ DOMIntersectionObserver::Update(nsIDocument* aDocument, DOMHighResTimeStamp time
       }
     }
 
-    double targetArea = targetRect.width * targetRect.height;
-    double intersectionArea = !intersectionRect ?
-      0 : intersectionRect->width * intersectionRect->height;
+    int64_t targetArea =
+      (int64_t) targetRect.Width() * (int64_t) targetRect.Height();
+    int64_t intersectionArea = !intersectionRect ? 0 :
+      (int64_t) intersectionRect->Width() *
+      (int64_t) intersectionRect->Height();
     
     double intersectionRatio;
     if (targetArea > 0.0) {
-      intersectionRatio = intersectionArea / targetArea;
+      intersectionRatio = (double) intersectionArea / (double) targetArea;
     } else {
       intersectionRatio = intersectionRect.isSome() ? 1.0 : 0.0;
     }