LibWeb: Rename Layout::InitialContainingBlock to Layout::Viewport

The name "initial containing block" was wrong for this, as it doesn't
correspond to the HTML element, and that's specifically what it's
supposed to do! :^)

Userland/Libraries/LibWeb/Layout/Viewport.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2018-2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/Layout/BlockContainer.h>
+#include <LibWeb/Selection/Selection.h>
+
+namespace Web::Layout {
+
+class Viewport final : public BlockContainer {
+    JS_CELL(Viewport, BlockContainer);
+
+public:
+    explicit Viewport(DOM::Document&, NonnullRefPtr<CSS::StyleProperties>);
+    virtual ~Viewport() override;
+
+    const DOM::Document& dom_node() const { return static_cast<const DOM::Document&>(*Node::dom_node()); }
+
+    void paint_all_phases(PaintContext&);
+
+    JS::GCPtr<Selection::Selection> selection() const;
+
+    void build_stacking_context_tree_if_needed();
+    void recompute_selection_states();
+
+private:
+    void build_stacking_context_tree();
+    virtual bool is_viewport() const override { return true; }
+};
+
+template<>
+inline bool Node::fast_is<Viewport>() const { return is_viewport(); }
+
+}