LibWeb: Add HTML::TraversableNavigable

This is the "traversable navigable" concept from the HTML spec.

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

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -349,6 +349,7 @@ set(SOURCES
     HTML/Timer.cpp
     HTML/TimeRanges.cpp
     HTML/TrackEvent.cpp
+    HTML/TraversableNavigable.cpp
     HTML/VideoTrack.cpp
     HTML/VideoTrackList.cpp
     HTML/Window.cpp

Userland/Libraries/LibWeb/Forward.h

@@ -385,6 +385,7 @@ class TextMetrics;
 class Timer;
 class TimeRanges;
 class TrackEvent;
+class TraversableNavigable;
 class VideoTrack;
 class VideoTrackList;
 class Window;

Userland/Libraries/LibWeb/HTML/TraversableNavigable.cpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/HTML/SessionHistoryEntry.h>
+#include <LibWeb/HTML/TraversableNavigable.h>
+
+namespace Web::HTML {
+
+TraversableNavigable::TraversableNavigable() = default;
+
+TraversableNavigable::~TraversableNavigable() = default;
+
+void TraversableNavigable::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    for (auto& entry : m_session_history_entries)
+        visitor.visit(entry);
+}
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#top-level-traversable
+bool TraversableNavigable::is_top_level_traversable() const
+{
+    // A top-level traversable is a traversable navigable with a null parent.
+    return parent() == nullptr;
+}
+
+}

Userland/Libraries/LibWeb/HTML/TraversableNavigable.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibWeb/HTML/Navigable.h>
+#include <LibWeb/HTML/VisibilityState.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/document-sequences.html#traversable-navigable
+class TraversableNavigable final : public Navigable {
+    JS_CELL(TraversableNavigable, Navigable);
+
+public:
+    virtual ~TraversableNavigable() override;
+
+    bool is_top_level_traversable() const;
+
+    int current_session_history_step() const { return m_current_session_history_step; };
+    Vector<JS::NonnullGCPtr<SessionHistoryEntry>> const& session_history_entries() const { return m_session_history_entries; };
+    bool running_nested_apply_history_step() const { return m_running_nested_apply_history_step; };
+    VisibilityState system_visibility_state() const { return m_system_visibility_state; };
+
+private:
+    TraversableNavigable();
+
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    // https://html.spec.whatwg.org/multipage/document-sequences.html#tn-current-session-history-step
+    int m_current_session_history_step { 0 };
+
+    // https://html.spec.whatwg.org/multipage/document-sequences.html#tn-session-history-entries
+    Vector<JS::NonnullGCPtr<SessionHistoryEntry>> m_session_history_entries;
+
+    // FIXME: https://html.spec.whatwg.org/multipage/document-sequences.html#tn-session-history-traversal-queue
+
+    // https://html.spec.whatwg.org/multipage/document-sequences.html#tn-running-nested-apply-history-step
+    bool m_running_nested_apply_history_step { false };
+
+    // https://html.spec.whatwg.org/multipage/document-sequences.html#system-visibility-state
+    VisibilityState m_system_visibility_state { VisibilityState::Visible };
+};
+
+}