Bug 1596458 part 1. Fix CreateContextualFragment in XML to not accidentally trigger custom element constructors. r=hsivonen

Differential Revision: https://phabricator.services.mozilla.com/D53076

--HG--
extra : moz-landing-system : lando

dom/base/nsContentUtils.cpp

@@ -4616,7 +4616,22 @@ already_AddRefed<DocumentFragment> nsContentUtils::CreateContextualFragment(
 
   while (content && content->IsElement()) {
     nsString& tagName = *tagStack.AppendElement();
-    tagName = content->NodeInfo()->QualifiedName();
+    // It mostly doesn't actually matter what tag name we use here: XML doesn't
+    // have parsing that depends on the open tag stack, apart from namespace
+    // declarations.  So this whole tagStack bit is just there to get the right
+    // namespace declarations to the XML parser.  That said, the parser _is_
+    // going to create elements with the tag names we provide here, so we need
+    // to make sure they are not names that can trigger custom element
+    // constructors.  Just make up a name that is never going to be a valid
+    // custom element name.
+    //
+    // The principled way to do this would probably be to add a new FromParser
+    // value and make sure we use it when creating the context elements, then
+    // make sure we teach all FromParser consumers (and in particular the custom
+    // element code) about it as needed.  But right now the XML parser never
+    // actually uses FromParser values other than NOT_FROM_PARSER, and changing
+    // that is pretty complicated.
+    tagName.AssignLiteral("notacustomelement");
 
     // see if we need to add xmlns declarations
     uint32_t count = content->AsElement()->GetAttrCount();