Bug 779269 - Implement a TryMerge method for nsDisplayFixedPosition. r=roc

Allow nsDisplayFixedPosition items that share the same fixed-pos frame to be
merged. This avoids a bug where the Google PDF viewer document image gets
re-rendered offset on the initial frame of a scroll when layer acceleration is
enabled.

layout/base/nsDisplayList.cpp

@@ -2528,6 +2528,17 @@ nsDisplayFixedPosition::BuildLayer(nsDisplayListBuilder* aBuilder,
   return layer.forget();
 }
 
+bool nsDisplayFixedPosition::TryMerge(nsDisplayListBuilder* aBuilder, nsDisplayItem* aItem) {
+  if (aItem->GetType() != TYPE_FIXED_POSITION)
+    return false;
+  // Items with the same fixed position frame can be merged.
+  nsDisplayFixedPosition* other = static_cast<nsDisplayFixedPosition*>(aItem);
+  if (other->mFixedPosFrame != mFixedPosFrame)
+    return false;
+  MergeFromTrackingMergedFrames(other);
+  return true;
+}
+
 nsDisplayScrollLayer::nsDisplayScrollLayer(nsDisplayListBuilder* aBuilder,
                                            nsDisplayList* aList,
                                            nsIFrame* aForFrame,

layout/base/nsDisplayList.h

@@ -1948,6 +1948,8 @@ public:
   {
     return mozilla::LAYER_ACTIVE;
   }
+  virtual bool TryMerge(nsDisplayListBuilder* aBuilder, nsDisplayItem* aItem);
+
   NS_DISPLAY_DECL_NAME("FixedPosition", TYPE_FIXED_POSITION)
 
 protected: