Bug 1000875 - Add invalidation debugging annotations for changes of a layer's forced background color. r=roc

layout/base/FrameLayerBuilder.cpp

@@ -1975,6 +1975,15 @@ ContainerState::PopThebesLayerData()
     if (userData->mForcedBackgroundColor != backgroundColor) {
       // Invalidate the entire target ThebesLayer since we're changing
       // the background color
+#ifdef MOZ_DUMP_PAINTING
+      if (nsLayoutUtils::InvalidationDebuggingIsEnabled()) {
+        printf_stderr("Forced background color has changed from #%08X to #%08X on layer %p\n",
+                      userData->mForcedBackgroundColor, backgroundColor, data->mLayer);
+        nsAutoCString str;
+        AppendToString(str, data->mLayer->GetValidRegion());
+        printf_stderr("Invalidating layer %p: %s\n", data->mLayer, str.get());
+      }
+#endif
       data->mLayer->InvalidateRegion(data->mLayer->GetValidRegion());
     }
     userData->mForcedBackgroundColor = backgroundColor;