public class ComboBoxEx : ComboBox
    {
        private ToggleButton _toggleButton;
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            _toggleButton = FindChild<ToggleButton>(this);
            var desc = TypeDescriptor.GetProperties(typeof(UIElement))["IsMouseOver"];
            desc.AddValueChanged(this, OnChanged);
        }

        private void OnChanged(object sender, EventArgs e)
        {
            Debug.Print("1");
            if (_toggleButton == null) return;
            _toggleButton.Opacity = IsMouseOver ? 1.0 : 0.0;
        }


        public static T FindChild<T>(DependencyObject parent, string childName = null) where T : DependencyObject
        {
            // Confirm parent and childName are valid. 
            if (parent == null) return null;

            T foundChild = null;

            int childrenCount = VisualTreeHelper.GetChildrenCount(parent);
            for (int i = 0; i < childrenCount; i++)
            {
                var child = VisualTreeHelper.GetChild(parent, i);
                // If the child is not of the request child type child
                T childType = child as T;
                if (childType == null)
                {
                    // recursively drill down the tree
                    foundChild = FindChild<T>(child, childName);

                    // If the child is found, break so we do not overwrite the found child. 
                    if (foundChild != null) break;
                }
                else if (!string.IsNullOrEmpty(childName))
                {
                    var frameworkElement = child as FrameworkElement;
                    // If the child's name is set for search
                    if (frameworkElement != null && frameworkElement.Name == childName)
                    {
                        // if the child's name is of the request name
                        foundChild = (T)child;
                        break;
                    }
                }
                else
                {
                    // child element found.
                    foundChild = (T)child;
                    break;
                }
            }

            return foundChild;
        }
    }

    public class ComboBoxEx : ComboBox
    {
        private ToggleButton _toggleButton;
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            _toggleButton = FindChild<ToggleButton>(this);
            _toggleButton.Opacity = 0.0;
            var desc = TypeDescriptor.GetProperties(typeof(UIElement))["IsMouseOver"];
            desc.AddValueChanged(this, OnChanged);
        }

        private void OnChanged(object sender, EventArgs e)
        {
            if (_toggleButton == null) return;
            if (IsMouseOver)
                Show();
            else
                Hide();
        }

        private void Hide()
        {
            var animation = new DoubleAnimation
            {
                To = 0.0,
                Duration = TimeSpan.FromSeconds(0.5)
            };
            _toggleButton.BeginAnimation(OpacityProperty, animation);
        }
        private void Show()
        {
            var animation = new DoubleAnimation
            {
                To = 1.0,
                Duration = TimeSpan.FromSeconds(0.5)
            };
            _toggleButton.BeginAnimation(OpacityProperty, animation);
        }

        public static T FindChild<T>(DependencyObject parent, string childName = null) where T : DependencyObject
        {
            // Confirm parent and childName are valid. 
            if (parent == null) return null;

            T foundChild = null;

            int childrenCount = VisualTreeHelper.GetChildrenCount(parent);
            for (int i = 0; i < childrenCount; i++)
            {
                var child = VisualTreeHelper.GetChild(parent, i);
                // If the child is not of the request child type child
                T childType = child as T;
                if (childType == null)
                {
                    // recursively drill down the tree
                    foundChild = FindChild<T>(child, childName);

                    // If the child is found, break so we do not overwrite the found child. 
                    if (foundChild != null) break;
                }
                else if (!string.IsNullOrEmpty(childName))
                {
                    var frameworkElement = child as FrameworkElement;
                    // If the child's name is set for search
                    if (frameworkElement != null && frameworkElement.Name == childName)
                    {
                        // if the child's name is of the request name
                        foundChild = (T)child;
                        break;
                    }
                }
                else
                {
                    // child element found.
                    foundChild = (T)child;
                    break;
                }
            }

            return foundChild;
        }
    }