Clicking a button on disconnected form should not fire submit event.

third_party/WebKit/Source/core/html/HTMLFormElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights
  * reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 268 matching lines @@
   return false;
 }
 
 void HTMLFormElement::PrepareForSubmission(
     Event* event,
     HTMLFormControlElement* submit_button) {
   LocalFrame* frame = GetDocument().GetFrame();
   if (!frame || is_submitting_ || in_user_js_submit_event_)
     return;
 
+  if (!isConnected()) {
+    GetDocument().AddConsoleMessage(ConsoleMessage::Create(
+        kJSMessageSource, kWarningMessageLevel,
+        "Form submission canceled because the form is not connected"));
+    return;
+  }
+
   if (GetDocument().IsSandboxed(kSandboxForms)) {
     GetDocument().AddConsoleMessage(ConsoleMessage::Create(
         kSecurityMessageSource, kErrorMessageLevel,
         "Blocked form submission to '" + attributes_.Action() +
             "' because the form's frame is sandboxed and the 'allow-forms' "
             "permission is not set."));
     return;
   }
 
   // https://github.com/whatwg/html/issues/2253
@@ skipping 543 matching lines @@
   for (const auto& control : ListedElements()) {
     if (!control->IsFormControlElement())
       continue;
     if (ToHTMLFormControlElement(control)->CanBeSuccessfulSubmitButton())
       ToHTMLFormControlElement(control)->PseudoStateChanged(
           CSSSelector::kPseudoDefault);
   }
 }
 
 }  // namespace blink