Bug 1078005 - Skip sending MozAfterPaint events to hidden PresShells. r=roc

layout/base/nsPresContext.cpp

@@ -2524,6 +2524,11 @@ nsPresContext::NotifyDidPaintForSubtree(uint32_t aFlags)
       return;
     }
   }
+
+  if (!PresShell()->IsVisible() && !mFireAfterPaintEvents) {
+    return;
+  }
+
   // Non-root prescontexts fire MozAfterPaint to all their descendants
   // unconditionally, even if no invalidations have been collected. This is
   // because we don't want to eat the cost of collecting invalidations for