Added active state for title bar buttons a la the CSS pseudo class

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/Drawer/TitleBarButtonDrawer.cs

 
             m_clicker = new Clicker();
             m_clicker.onClick += OnClick;
+            m_clicker.onStateChange += OnClickStateChanged;
             AddManipulator(m_clicker);
 
             AddChild(new VisualElement() { classList = new ClassList("titleBarItemBorder") });
                 m_dataProvider.onClick();
         }
 
+        void OnClickStateChanged(ClickerState newState)
+        {
+            if (newState == ClickerState.Active)
+                AddToClassList("active");
+            else if (newState == ClickerState.Inactive)
+                RemoveFromClassList("active");
+            this.Touch(ChangeType.Repaint);
+        }
 
         protected override object toWatch
         {

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/Manipulators/Clicker.cs

 
 namespace UnityEditor.Graphing.Drawing
 {
-    public class Clicker : MouseManipulator
+    public enum ClickerState
+        Inactive,
+        Active
+    }
-        public delegate void StateChangeCallback();
+    public class Clicker : MouseManipulator
+    {
+        public delegate void StateChangeCallback(ClickerState newState);
+        public StateChangeCallback onStateChange { get; set; }
-        bool withinInitialTarget;
+        ClickerState state;
 
         public override EventPropagation HandleEvent(Event evt, VisualElement finalTarget)
         {
                     {
                         this.TakeCapture();
                         initialTarget = finalTarget;
+                        UpdateState(evt);
+                case EventType.mouseDrag:
+                    UpdateState(evt);
+                    break;
+
-                        withinInitialTarget = initialTarget != null && initialTarget.ContainsPoint(evt.mousePosition);
-                        if (withinInitialTarget && onClick != null)
-                        {
+                        // withinInitialTarget = initialTarget != null && initialTarget.ContainsPoint(evt.mousePosition);
+                        if (initialTarget != null && state == ClickerState.Active && onClick != null)
-                        }
+                        initialTarget = null;
+                        UpdateState(evt);
+        }
+
+        void UpdateState(Event evt)
+        {
+            ClickerState newState;
+            if (initialTarget != null && initialTarget.ContainsPoint(evt.mousePosition))
+                newState = ClickerState.Active;
+            else
+                newState = ClickerState.Inactive;
+            
+            if (onStateChange != null && state != newState)
+                onStateChange(newState);
+            state = newState;
         }
     }
 }

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/Styles/TitleBar.uss

     flex-direction: row;
 }
 
+.titleBarItem.active {
+    background-color: rgba(33, 33, 33, 0.5);
+}
+
 #left .titleBarItem {
     margin-left: 5;
 }