LibWeb: Generate layout nodes for shadow subtrees

Elements with shadow roots will now recurse into those shadow trees
while building the layout tree.

This is the first step towards basic Shadow DOM support. :^)

Userland/Libraries/LibWeb/DOM/ShadowRoot.cpp

@@ -26,6 +26,7 @@
 
 #include <LibWeb/DOM/Event.h>
 #include <LibWeb/DOM/ShadowRoot.h>
+#include <LibWeb/Layout/BlockBox.h>
 
 namespace Web::DOM {
 
@@ -46,4 +47,9 @@ EventTarget* ShadowRoot::get_parent(const Event& event)
     return host();
 }
 
+RefPtr<Layout::Node> ShadowRoot::create_layout_node()
+{
+    return adopt(*new Layout::BlockBox(document(), this, CSS::ComputedValues {}));
+}
+
 }

Userland/Libraries/LibWeb/DOM/ShadowRoot.h

@@ -49,6 +49,10 @@ public:
     String mode() const { return m_closed ? "closed" : "open"; }
 
 private:
+    // ^Node
+    virtual FlyString node_name() const override { return "#shadow-root"; }
+    virtual RefPtr<Layout::Node> create_layout_node() override;
+
     // NOTE: The specification doesn't seem to specify a default value for closed. Assuming false for now.
     bool m_closed { false };
     bool m_delegates_focus { false };

Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp

@@ -27,6 +27,7 @@
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/ParentNode.h>
+#include <LibWeb/DOM/ShadowRoot.h>
 #include <LibWeb/Dump.h>
 #include <LibWeb/Layout/InitialContainingBlockBox.h>
 #include <LibWeb/Layout/Node.h>
@@ -92,14 +93,14 @@ static Layout::Node& insertion_parent_for_block_node(Layout::Node& layout_parent
 void TreeBuilder::create_layout_tree(DOM::Node& dom_node)
 {
     // If the parent doesn't have a layout node, we don't need one either.
-    if (dom_node.parent() && !dom_node.parent()->layout_node())
+    if (dom_node.parent_or_shadow_host() && !dom_node.parent_or_shadow_host()->layout_node())
         return;
 
     auto layout_node = dom_node.create_layout_node();
     if (!layout_node)
         return;
 
-    if (!dom_node.parent()) {
+    if (!dom_node.parent_or_shadow_host()) {
         m_layout_root = layout_node;
     } else {
         if (layout_node->is_inline()) {
@@ -122,8 +123,12 @@ void TreeBuilder::create_layout_tree(DOM::Node& dom_node)
         }
     }
 
-    if (dom_node.has_children() && layout_node->can_have_children()) {
+    auto* shadow_root = is<DOM::Element>(dom_node) ? downcast<DOM::Element>(dom_node).shadow_root() : nullptr;
+
+    if ((dom_node.has_children() || shadow_root) && layout_node->can_have_children()) {
         push_parent(downcast<NodeWithStyle>(*layout_node));
+        if (shadow_root)
+            create_layout_tree(*shadow_root);
         downcast<DOM::ParentNode>(dom_node).for_each_child([&](auto& dom_child) {
             create_layout_tree(dom_child);
         });