Bug 584494. Avoid creating opacity/transform wrappers for empty lists. r=tnikkel a2.0=blocking2.0:betaN

layout/generic/nsFrame.cpp

@@ -1527,8 +1527,10 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   } else
 #endif
 
-  /* If there is any opacity, wrap it up in an opacity list. */
-  if (disp->mOpacity < 1.0f) {
+  /* If there is any opacity, wrap it up in an opacity list.
+   * If there's nothing in the list, don't add anything.
+   */
+  if (disp->mOpacity < 1.0f && !resultList.IsEmpty()) {
     rv = resultList.AppendNewToTop(
         new (aBuilder) nsDisplayOpacity(aBuilder, this, &resultList));
     if (NS_FAILED(rv))
@@ -1536,10 +1538,10 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   }
 
   /* If we're going to apply a transformation, wrap everything in an
-   * nsDisplayTransform.
+   * nsDisplayTransform. If there's nothing in the list, don't add anything.
    */
   if ((mState & NS_FRAME_MAY_BE_TRANSFORMED) &&
-      disp->HasTransform()) {
+      disp->HasTransform() && !resultList.IsEmpty()) {
     rv = resultList.AppendNewToTop(
         new (aBuilder) nsDisplayTransform(aBuilder, this, &resultList));
     if (NS_FAILED(rv))