Bug 1023677 - Don't flatten active transform layers. r=roc

layout/base/FrameLayerBuilder.cpp

@@ -530,7 +530,7 @@ public:
    * @param aTextContentFlags if any child layer has CONTENT_COMPONENT_ALPHA,
    * set *aTextContentFlags to CONTENT_COMPONENT_ALPHA
    */
-  void Finish(uint32_t *aTextContentFlags, LayerManagerData* aData);
+  void Finish(uint32_t *aTextContentFlags, LayerManagerData* aData, bool& aHasComponentAlphaChildren);
 
   nsRect GetChildrenBounds() { return mBounds; }
 
@@ -3065,7 +3065,7 @@ ContainerState::CollectOldLayers()
 }
 
 void
-ContainerState::Finish(uint32_t* aTextContentFlags, LayerManagerData* aData)
+ContainerState::Finish(uint32_t* aTextContentFlags, LayerManagerData* aData, bool& aHasComponentAlphaChildren)
 {
   while (!mThebesLayerDataStack.IsEmpty()) {
     PopThebesLayerData();
@@ -3084,6 +3084,16 @@ ContainerState::Finish(uint32_t* aTextContentFlags, LayerManagerData* aData)
       textContentFlags |=
         layer->GetContentFlags() & (Layer::CONTENT_COMPONENT_ALPHA |
                                     Layer::CONTENT_COMPONENT_ALPHA_DESCENDANT);
+
+      // Notify the parent of component alpha children unless it's coming from
+      // within a transformed child since we don't want flattening of component
+      // alpha layers to happen across transforms. Re-rendering the text during
+      // transform animations looks worse than losing subpixel-AA.
+      if ((layer->GetContentFlags() & Layer::CONTENT_COMPONENT_ALPHA) &&
+          (layer->GetType() != Layer::TYPE_CONTAINER ||
+           layer->GetBaseTransform().IsIdentity())) {
+        aHasComponentAlphaChildren = true;
+      }
     }
 
     if (!layer->GetParent()) {
@@ -3423,12 +3433,13 @@ FrameLayerBuilder::BuildContainerLayerFor(nsDisplayListBuilder* aBuilder,
     // Set CONTENT_COMPONENT_ALPHA if any of our children have it.
     // This is suboptimal ... a child could have text that's over transparent
     // pixels in its own layer, but over opaque parts of previous siblings.
-    state.Finish(&flags, data);
+    bool hasComponentAlphaChildren = false;
+    state.Finish(&flags, data, hasComponentAlphaChildren);
     bounds = state.GetChildrenBounds();
     pixBounds = state.ScaleToOutsidePixels(bounds, false);
     appUnitsPerDevPixel = state.GetAppUnitsPerDevPixel();
 
-    if ((flags & Layer::CONTENT_COMPONENT_ALPHA) &&
+    if (hasComponentAlphaChildren &&
         mRetainingManager &&
         !mRetainingManager->AreComponentAlphaLayersEnabled() &&
         containerLayer->HasMultipleChildren() &&