LibWeb: Dispatch mouseenter and mouseleave events when required

I've left a FIXME here about populating the events with mouse
coordinates, button states, etc. We also need to verify that the
dispatch order either doesn't matter or at least match other engines.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -936,10 +936,29 @@ void Document::set_hovered_node(Node* node)
     JS::GCPtr<Node> old_hovered_node = move(m_hovered_node);
     m_hovered_node = node;
 
-    if (auto* common_ancestor = find_common_ancestor(old_hovered_node, m_hovered_node))
+    auto* common_ancestor = find_common_ancestor(old_hovered_node, m_hovered_node);
+    if (common_ancestor)
         common_ancestor->invalidate_style();
     else
         invalidate_style();
+
+    // https://w3c.github.io/uievents/#mouseleave
+    if (old_hovered_node && (!m_hovered_node || !m_hovered_node->is_descendant_of(*old_hovered_node))) {
+        // FIXME: Check if we need to dispatch these events in a specific order.
+        for (auto target = old_hovered_node; target && target.ptr() != common_ancestor; target = target->parent()) {
+            // FIXME: Populate the event with mouse coordinates, etc.
+            target->dispatch_event(*UIEvents::MouseEvent::create(realm(), UIEvents::EventNames::mouseleave));
+        }
+    }
+
+    // https://w3c.github.io/uievents/#mouseenter
+    if (m_hovered_node && (!old_hovered_node || !m_hovered_node->is_ancestor_of(*old_hovered_node))) {
+        // FIXME: Check if we need to dispatch these events in a specific order.
+        for (auto target = m_hovered_node; target.ptr() != common_ancestor; target = target->parent()) {
+            // FIXME: Populate the event with mouse coordinates, etc.
+            target->dispatch_event(*UIEvents::MouseEvent::create(realm(), UIEvents::EventNames::mouseenter));
+        }
+    }
 }
 
 JS::NonnullGCPtr<HTMLCollection> Document::get_elements_by_name(String const& name)