--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -7212,16 +7212,20 @@ nsCSSFrameConstructor::CheckBitsForLazyF
 // Returns true if this operation can be lazy, false if not.
 //
 // FIXME(emilio, bug 1410020): This function assumes that the flattened tree
 // parent of all the appended children is the same, which, afaict, is not
 // necessarily true.
 //
 // But we disable lazy frame construction for shadow trees... We should fix
 // that, too.
+//
+// NOTE(emilio): The IsXULElement check is pretty unfortunate, but there's tons
+// of browser chrome code that rely on XBL bindings getting synchronously loaded
+// as soon as the elements get inserted in the DOM.
 bool
 nsCSSFrameConstructor::MaybeConstructLazily(Operation aOperation,
                                             nsIContent* aContainer,
                                             nsIContent* aChild)
 {
   if (!aContainer || aContainer->IsInNativeAnonymousSubtree() ||
       aContainer->IsXULElement()) {
     return false;