--- a/layout/painting/nsDisplayList.h
+++ b/layout/painting/nsDisplayList.h
@@ -2636,17 +2636,23 @@ public:
   /**
    * Returns the building rectangle used by nsDisplayListBuilder when
    * this item was constructed.
    */
   const nsRect& GetBuildingRect() const { return mBuildingRect; }
 
   void SetBuildingRect(const nsRect& aBuildingRect)
   {
+    if (aBuildingRect == mBuildingRect) {
+      // Avoid unnecessary paint rect recompution when the
+      // building rect is staying the same.
+      return;
+    }
     mPaintRect = mBuildingRect = aBuildingRect;
+    mPaintRectValid = false;
   }
 
   void SetPaintRect(const nsRect& aPaintRect) {
     mPaintRect = aPaintRect;
     mPaintRectValid = true;
   }
   bool HasPaintRect() const { return mPaintRectValid; }