Bug 593361. Part 2. Don't bother trying to draw directly to the backbuffer instead of retaining ThebesLayer, if the backbuffer is transparent anyway. r=tnikkel

gfx/layers/basic/BasicLayers.cpp

@@ -378,9 +378,18 @@ ShouldRetainTransparentSurface(PRUint32 aContentFlags,
   case gfxASurface::TEXT_QUALITY_OK:
     return PR_TRUE;
   case gfxASurface::TEXT_QUALITY_OK_OVER_OPAQUE_PIXELS:
+    // Retain the buffer if all text is over opaque pixels. Otherwise,
+    // don't retain the buffer, in the hope that the backbuffer has
+    // opaque pixels where our layer does not.
     return (aContentFlags & Layer::CONTENT_NO_TEXT_OVER_TRANSPARENT) != 0;
+  case gfxASurface::TEXT_QUALITY_BAD:
+    // If the backbuffer is opaque, then draw directly into it to get
+    // subpixel AA. If the backbuffer is not an opaque format, then we won't get
+    // subpixel AA by drawing into it, so we might as well retain.
+    return aTargetSurface->GetContentType() != gfxASurface::CONTENT_COLOR;
   default:
-    return PR_FALSE;
+    NS_ERROR("Unknown quality type");
+    return PR_TRUE;
   }
 }