LibWeb: Act as if parsing stopped when generating an error document

The user agent now acts as if it has stopped parsing when creating a
document for inline content to display an error.

Libraries/LibWeb/HTML/Navigable.cpp

@@ -1171,10 +1171,18 @@ WebIDL::ExceptionOr<void> Navigable::populate_session_history_entry_document(
             auto error_message = navigation_params.get<NullOrError>().value_or("Unknown error"sv);
 
             auto error_html = load_error_page(entry->url(), error_message).release_value_but_fixme_should_propagate_errors();
-            entry->document_state()->set_document(create_document_for_inline_content(this, navigation_id, [error_html](auto& document) {
+            entry->document_state()->set_document(create_document_for_inline_content(this, navigation_id, [this, error_html](auto& document) {
                 auto parser = HTML::HTMLParser::create(document, error_html, "utf-8"sv);
                 document.set_url(URL::URL("about:error"));
                 parser->run();
+
+                // NOTE: Once the page has been set up, the user agent must act as if it had stopped parsing.
+                // FIXME: Directly calling parser->the_end results in a deadlock, because it waits for the warning image to load.
+                //        However the response is never processed when parser->the_end is called.
+                //        Queuing a global task is a workaround for now.
+                queue_a_task(Task::Source::Unspecified, HTML::main_thread_event_loop(), document, GC::create_function(heap(), [&document]() {
+                    HTMLParser::the_end(document);
+                }));
             }));
 
             // 2. Make document unsalvageable given entry's document state's document and "navigation-failure".