--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -309,16 +309,20 @@ nsresult nsXULElement::Clone(mozilla::do
 
   nsresult rv = element->mAttrs.EnsureCapacityToClone(mAttrs);
   NS_ENSURE_SUCCESS(rv, rv);
 
   // Note that we're _not_ copying mControllers.
 
   uint32_t count = mAttrs.AttrCount();
   rv = NS_OK;
+  // FIXME(emilio): This setup looks somewhat error prone. Other than the
+  // AddListenerFor bit (what is that for?), everything else could be handled by
+  // the generic Element::CopyInnerTo, or the equivalent in
+  // nsGenericHTMLElement (somehow), both of which go through AfterSetAttr...
   for (uint32_t i = 0; i < count; ++i) {
     const nsAttrName* originalName = mAttrs.AttrNameAt(i);
     const nsAttrValue* originalValue = mAttrs.AttrAt(i);
     nsAttrValue attrValue;
 
     // Style rules need to be cloned.
     if (originalValue->Type() == nsAttrValue::eCSSDeclaration) {
       DeclarationBlock* decl = originalValue->GetCSSDeclarationValue();
@@ -347,16 +351,19 @@ nsresult nsXULElement::Clone(mozilla::do
       element->SetHasID();
     }
     if (originalName->Equals(nsGkAtoms::_class)) {
       element->SetMayHaveClass();
     }
     if (originalName->Equals(nsGkAtoms::style)) {
       element->SetMayHaveStyle();
     }
+    if (originalName->Equals(nsGkAtoms::part)) {
+      element->SetHasPartAttribute(true);
+    }
   }
 
   element.forget(aResult);
   return rv;
 }
 
 //----------------------------------------------------------------------