Bug 987005 - Add comments to a condition in PopThebesLayerData(). r=roc

layout/base/FrameLayerBuilder.cpp

@@ -1843,6 +1843,29 @@ AddTransformedBoundsToRegion(const nsIntRegion& aRegion,
   aDest->Or(*aDest, intRect);
 }
 
+static bool
+CanOptimizeAwayThebesLayer(ThebesLayerData* aData,
+                           FrameLayerBuilder* aLayerBuilder)
+{
+  bool isRetained = aData->mLayer->Manager()->IsWidgetLayerManager();
+  if (!isRetained) {
+    return false;
+  }
+
+  // If there's no thebes layer with valid content in it that we can reuse,
+  // always create a color or image layer (and potentially throw away an
+  // existing completely invalid thebes layer).
+  if (aData->mLayer->GetValidRegion().IsEmpty()) {
+    return true;
+  }
+
+  // There is an existing thebes layer we can reuse. Throwing it away can make
+  // compositing cheaper (see bug 946952), but it might cause us to re-allocate
+  // the thebes layer frequently due to an animation. So we only discard it if
+  // we're in tree compression mode, which is triggered at a low frequency.
+  return aLayerBuilder->CheckInLayerTreeCompressionMode();
+}
+
 void
 ContainerState::PopThebesLayerData()
 {
@@ -1858,9 +1881,8 @@ ContainerState::PopThebesLayerData()
   nsRefPtr<Layer> layer;
   nsRefPtr<ImageContainer> imageContainer = data->CanOptimizeImageLayer(mBuilder);
 
-  bool isRetained = data->mLayer->Manager()->IsWidgetLayerManager();
-  if (isRetained && (data->mIsSolidColorInVisibleRegion || imageContainer) &&
-      (data->mLayer->GetValidRegion().IsEmpty() || mLayerBuilder->CheckInLayerTreeCompressionMode())) {
+  if ((data->mIsSolidColorInVisibleRegion || imageContainer) &&
+      CanOptimizeAwayThebesLayer(data, mLayerBuilder)) {
     NS_ASSERTION(!(data->mIsSolidColorInVisibleRegion && imageContainer),
                  "Can't be a solid color as well as an image!");
     if (imageContainer) {