LibWeb: Invalidate layout tree of parent of inserted node

f7a3f78 made the layout tree invalidate only the inserted nodes
themselves, but it turned out that CSS containment invalidation relies
on the parent being invalidated as well.

Libraries/LibWeb/DOM/Node.cpp

@@ -767,6 +767,10 @@ void Node::insert_before(GC::Ref<Node> node, GC::Ptr<Node> child, bool suppress_
             node->post_connection();
     }
 
+    if (is_connected()) {
+        set_needs_layout_tree_update(true);
+    }
+
     document().bump_dom_tree_version();
 }
 
@@ -1405,7 +1409,6 @@ void Node::set_needs_style_update(bool value)
 
 void Node::post_connection()
 {
-    set_needs_layout_tree_update(true);
 }
 
 void Node::inserted()