LibHTML: Implement naive hit testing

We don't have proper line boxes yet, so we can't easily hit test
inline text.

Libraries/LibHTML/HtmlView.cpp

@@ -1,5 +1,6 @@
 #include <LibGUI/GPainter.h>
 #include <LibGUI/GScrollBar.h>
+#include <LibHTML/DOM/Element.h>
 #include <LibHTML/Dump.h>
 #include <LibHTML/HtmlView.h>
 #include <LibHTML/Layout/LayoutNode.h>
@@ -78,3 +79,17 @@ void HtmlView::paint_event(GPaintEvent& event)
     RenderingContext context { painter };
     m_layout_root->render(context);
 }
+
+void HtmlView::mousemove_event(GMouseEvent& event)
+{
+    if (!m_layout_root)
+        return GScrollableWidget::mousemove_event(event);
+
+    auto result = m_layout_root->hit_test(event.position());
+    if (result.layout_node) {
+        if (auto* node = result.layout_node->node()) {
+            dbg() << "HtmlView: mousemove: " << node->tag_name() << "{" << node << "}";
+        }
+    }
+    event.accept();
+}

Libraries/LibHTML/HtmlView.h

@@ -17,6 +17,7 @@ protected:
 
     virtual void resize_event(GResizeEvent&) override;
     virtual void paint_event(GPaintEvent&) override;
+    virtual void mousemove_event(GMouseEvent&) override;
 
 private:
     void layout_and_sync_size();

Libraries/LibHTML/Layout/LayoutNode.cpp

@@ -1,3 +1,4 @@
+#include <LibHTML/DOM/Element.h>
 #include <LibHTML/Layout/LayoutBlock.h>
 #include <LibHTML/Layout/LayoutNode.h>
 
@@ -34,3 +35,16 @@ void LayoutNode::render(RenderingContext& context)
         child.render(context);
     });
 }
+
+HitTestResult LayoutNode::hit_test(const Point& position) const
+{
+    if (!m_rect.contains(position))
+        return {};
+    HitTestResult result { this };
+    for_each_child([&](auto& child) {
+        auto child_result = child.hit_test(position);
+        if (child_result.layout_node)
+            result = child_result;
+    });
+    return result;
+}

Libraries/LibHTML/Layout/LayoutNode.h

@@ -9,7 +9,13 @@
 #include <LibHTML/TreeNode.h>
 
 class Node;
+class Element;
 class LayoutBlock;
+class LayoutNode;
+
+struct HitTestResult {
+    RefPtr<LayoutNode> layout_node;
+};
 
 class LayoutNode : public TreeNode<LayoutNode> {
 public:
@@ -22,6 +28,8 @@ public:
     ComputedStyle& style() { return m_style; }
     const ComputedStyle& style() const { return m_style; }
 
+    virtual HitTestResult hit_test(const Point&) const;
+
     bool is_anonymous() const { return !m_node; }
     const Node* node() const { return m_node; }