LibHTML: Add a convenient way to get from any layout node to the root

Libraries/LibHTML/DOM/Document.cpp

@@ -239,6 +239,11 @@ const LayoutDocument* Document::layout_node() const
     return static_cast<const LayoutDocument*>(Node::layout_node());
 }
 
+LayoutDocument* Document::layout_node()
+{
+    return static_cast<LayoutDocument*>(Node::layout_node());
+}
+
 void Document::set_hovered_node(Node* node)
 {
     if (m_hovered_node == node)

Libraries/LibHTML/DOM/Document.h

@@ -78,6 +78,7 @@ public:
     virtual bool is_child_allowed(const Node&) const override;
 
     const LayoutDocument* layout_node() const;
+    LayoutDocument* layout_node();
 
     void schedule_style_update();
 

Libraries/LibHTML/Layout/LayoutNode.cpp

@@ -63,6 +63,26 @@ const Document& LayoutNode::document() const
     return node()->document();
 }
 
+Document& LayoutNode::document()
+{
+    if (is_anonymous())
+        return parent()->document();
+    // FIXME: Remove this const_cast once we give up on the idea of a const link from layout tree to DOM tree.
+    return const_cast<Node*>(node())->document();
+}
+
+const LayoutDocument& LayoutNode::root() const
+{
+    ASSERT(document().layout_node());
+    return *document().layout_node();
+}
+
+LayoutDocument& LayoutNode::root()
+{
+    ASSERT(document().layout_node());
+    return *document().layout_node();
+}
+
 void LayoutNode::split_into_lines(LayoutBlock& container)
 {
     for_each_child([&](auto& child) {

Libraries/LibHTML/Layout/LayoutNode.h

@@ -11,6 +11,7 @@
 class Document;
 class Element;
 class LayoutBlock;
+class LayoutDocument;
 class LayoutNode;
 class LayoutNodeWithStyle;
 class LineBoxFragment;
@@ -29,8 +30,12 @@ public:
     bool is_anonymous() const { return !m_node; }
     const Node* node() const { return m_node; }
 
+    Document& document();
     const Document& document() const;
 
+    const LayoutDocument& root() const;
+    LayoutDocument& root();
+
     template<typename Callback>
     inline void for_each_child(Callback callback) const
     {