LibWeb: Add HTML::Navigable

This is the first step towards implementing the new "navigable" concept
from the HTML spec.

Co-authored-by: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/HTML/BrowsingContext.h>
+#include <LibWeb/HTML/DocumentState.h>
+#include <LibWeb/HTML/Navigable.h>
+#include <LibWeb/HTML/SessionHistoryEntry.h>
+
+namespace Web::HTML {
+
+Navigable::Navigable() = default;
+
+Navigable::~Navigable() = default;
+
+void Navigable::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_parent);
+    visitor.visit(m_current_session_history_entry);
+    visitor.visit(m_active_session_history_entry);
+    visitor.visit(m_container);
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-document
+JS::GCPtr<DOM::Document> Navigable::active_document()
+{
+    // A navigable's active document is its active session history entry's document.
+    return m_active_session_history_entry->document_state->document();
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-bc
+JS::GCPtr<BrowsingContext> Navigable::active_browsing_context()
+{
+    // A navigable's active browsing context is its active document's browsing context.
+    // If this navigable is a traversable navigable, then its active browsing context will be a top-level browsing context.
+    if (auto document = active_document())
+        return document->browsing_context();
+    return nullptr;
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-wp
+JS::GCPtr<HTML::WindowProxy> Navigable::active_window_proxy()
+{
+    // A navigable's active WindowProxy is its active browsing context's associated WindowProxy.
+    if (auto browsing_context = active_browsing_context())
+        return browsing_context->window_proxy();
+    return nullptr;
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-window
+JS::GCPtr<HTML::Window> Navigable::active_window()
+{
+    // A navigable's active window is its active WindowProxy's [[Window]].
+    if (auto window_proxy = active_window_proxy())
+        return window_proxy->window();
+    return nullptr;
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-target
+String Navigable::target_name() const
+{
+    // FIXME: A navigable's target name is its active session history entry's document state's navigable target name.
+    dbgln("FIXME: Implement Navigable::target_name()");
+    return {};
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-container
+JS::GCPtr<NavigableContainer> Navigable::container() const
+{
+    // The container of a navigable navigable is the navigable container whose nested navigable is navigable, or null if there is no such element.
+    return m_container;
+}
+
+void Navigable::set_container(JS::GCPtr<NavigableContainer> container)
+{
+    m_container = container;
+}
+
+}