LibWeb: Only allocate DOM::Node registered observer list on demand

Most DOM nodes don't have registered mutation observers, so let's put
the metadata about them behind an OwnPtr to save space in the common
case.

Saves 16 bytes per DOM node that doesn't have registered observers.

Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp

@@ -608,9 +608,11 @@ void queue_mutation_observer_microtask(DOM::Document const& document)
                 if (node.is_null())
                     continue;
 
-                node->registered_observers_list().remove_all_matching([&mutation_observer](DOM::RegisteredObserver& registered_observer) {
+                if (node->registered_observer_list()) {
-                    return is<DOM::TransientRegisteredObserver>(registered_observer) && static_cast<DOM::TransientRegisteredObserver&>(registered_observer).observer().ptr() == mutation_observer.ptr();
+                    node->registered_observer_list()->remove_all_matching([&mutation_observer](DOM::RegisteredObserver& registered_observer) {
-                });
+                        return is<DOM::TransientRegisteredObserver>(registered_observer) && static_cast<DOM::TransientRegisteredObserver&>(registered_observer).observer().ptr() == mutation_observer.ptr();
+                    });
+                }
             }
 
             // 4. If records is not empty, then invoke mo’s callback with « records, mo », and mo. If this throws an exception, catch it, and report the exception.

Userland/Libraries/LibWeb/DOM/MutationObserver.cpp

@@ -85,26 +85,30 @@ WebIDL::ExceptionOr<void> MutationObserver::observe(Node& target, MutationObserv
 
     // 7. For each registered of target’s registered observer list, if registered’s observer is this:
     bool updated_existing_observer = false;
-    for (auto& registered_observer : target.registered_observers_list()) {
+    if (target.registered_observer_list()) {
-        if (registered_observer->observer().ptr() != this)
+        for (auto& registered_observer : *target.registered_observer_list()) {
-            continue;
+            if (registered_observer->observer().ptr() != this)
-
-        updated_existing_observer = true;
-
-        // 1. For each node of this’s node list, remove all transient registered observers whose source is registered from node’s registered observer list.
-        for (auto& node : m_node_list) {
-            // FIXME: Is this correct?
-            if (node.is_null())
                 continue;
 
-            node->registered_observers_list().remove_all_matching([&registered_observer](RegisteredObserver& observer) {
+            updated_existing_observer = true;
-                return is<TransientRegisteredObserver>(observer) && verify_cast<TransientRegisteredObserver>(observer).source().ptr() == registered_observer;
-            });
-        }
 
-        // 2. Set registered’s options to options.
+            // 1. For each node of this’s node list, remove all transient registered observers whose source is registered from node’s registered observer list.
-        registered_observer->set_options(options);
+            for (auto& node : m_node_list) {
-        break;
+                // FIXME: Is this correct?
+                if (node.is_null())
+                    continue;
+
+                if (node->registered_observer_list()) {
+                    node->registered_observer_list()->remove_all_matching([&registered_observer](RegisteredObserver& observer) {
+                        return is<TransientRegisteredObserver>(observer) && verify_cast<TransientRegisteredObserver>(observer).source().ptr() == registered_observer;
+                    });
+                }
+            }
+
+            // 2. Set registered’s options to options.
+            registered_observer->set_options(options);
+            break;
+        }
     }
 
     // 8. Otherwise:
@@ -129,9 +133,11 @@ void MutationObserver::disconnect()
         if (node.is_null())
             continue;
 
-        node->registered_observers_list().remove_all_matching([this](RegisteredObserver& registered_observer) {
+        if (node->registered_observer_list()) {
-            return registered_observer.observer().ptr() == this;
+            node->registered_observer_list()->remove_all_matching([this](RegisteredObserver& registered_observer) {
-        });
+                return registered_observer.observer().ptr() == this;
+            });
+        }
     }
 
     // 2. Empty this’s record queue.

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -103,8 +103,10 @@ void Node::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_layout_node);
     visitor.visit(m_paintable);
 
-    for (auto& registered_observer : m_registered_observer_list)
+    if (m_registered_observer_list) {
-        visitor.visit(registered_observer);
+        for (auto& registered_observer : *m_registered_observer_list)
+            visitor.visit(registered_observer);
+    }
 }
 
 // https://dom.spec.whatwg.org/#dom-node-baseuri
@@ -686,10 +688,12 @@ void Node::remove(bool suppress_observers)
     //     if registered’s options["subtree"] is true, then append a new transient registered observer
     //     whose observer is registered’s observer, options is registered’s options, and source is registered to node’s registered observer list.
     for (auto* inclusive_ancestor = parent; inclusive_ancestor; inclusive_ancestor = inclusive_ancestor->parent()) {
-        for (auto& registered : inclusive_ancestor->m_registered_observer_list) {
+        if (!inclusive_ancestor->m_registered_observer_list)
+            continue;
+        for (auto& registered : *inclusive_ancestor->m_registered_observer_list) {
             if (registered->options().subtree) {
                 auto transient_observer = TransientRegisteredObserver::create(registered->observer(), registered->options(), registered);
-                m_registered_observer_list.append(move(transient_observer));
+                m_registered_observer_list->append(move(transient_observer));
             }
         }
     }
@@ -1546,7 +1550,9 @@ void Node::queue_mutation_record(FlyString const& type, Optional<FlyString> cons
     // 2. Let nodes be the inclusive ancestors of target.
     // 3. For each node in nodes, and then for each registered of node’s registered observer list:
     for (auto* node = this; node; node = node->parent()) {
-        for (auto& registered_observer : node->m_registered_observer_list) {
+        if (!node->m_registered_observer_list)
+            continue;
+        for (auto& registered_observer : *node->m_registered_observer_list) {
             // 1. Let options be registered’s options.
             auto& options = registered_observer->options();
 
@@ -1995,4 +2001,11 @@ ErrorOr<void> Node::prepend_with_space(StringBuilder x, StringView const& result
     return {};
 }
 
+void Node::add_registered_observer(RegisteredObserver& registered_observer)
+{
+    if (!m_registered_observer_list)
+        m_registered_observer_list = make<Vector<JS::NonnullGCPtr<RegisteredObserver>>>();
+    m_registered_observer_list->append(registered_observer);
+}
+
 }

Userland/Libraries/LibWeb/DOM/Node.h

@@ -254,10 +254,10 @@ public:
 
     size_t length() const;
 
-    auto& registered_observers_list() { return m_registered_observer_list; }
+    auto& registered_observer_list() { return m_registered_observer_list; }
-    auto const& registered_observers_list() const { return m_registered_observer_list; }
+    auto const& registered_observer_list() const { return m_registered_observer_list; }
 
-    void add_registered_observer(RegisteredObserver& registered_observer) { m_registered_observer_list.append(registered_observer); }
+    void add_registered_observer(RegisteredObserver&);
 
     void queue_mutation_record(FlyString const& type, Optional<FlyString> const& attribute_name, Optional<FlyString> const& attribute_namespace, Optional<String> const& old_value, Vector<JS::Handle<Node>> added_nodes, Vector<JS::Handle<Node>> removed_nodes, Node* previous_sibling, Node* next_sibling) const;
 
@@ -693,7 +693,7 @@ protected:
 
     // https://dom.spec.whatwg.org/#registered-observer-list
     // "Nodes have a strong reference to registered observers in their registered observer list." https://dom.spec.whatwg.org/#garbage-collection
-    Vector<JS::NonnullGCPtr<RegisteredObserver>> m_registered_observer_list;
+    OwnPtr<Vector<JS::NonnullGCPtr<RegisteredObserver>>> m_registered_observer_list;
 
     void build_accessibility_tree(AccessibilityTreeNode& parent);