question

 namespace OutlookAddIn1
 {
     using System;
     using System.Threading.Tasks;
     using System.Windows;
     using System.Windows.Controls;
    
     using System.Windows.Input;
     using System.Windows.Media;
    
     public partial class VSTOComboBoxTestUserControl : System.Windows.Controls.UserControl
     {
         public VSTOComboBoxTestUserControl()
         {
             InitializeComponent();
    
             var combo = new ComboBox();
             combo.DropDownOpened += ComboBox_DropDownOpened;
    
             var s = "1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ";
             foreach (char c in s)
             {
                 combo.Items.Add(new string(c, 20));
             }
    
             this.Content = combo;
         }
    
         [System.Runtime.InteropServices.DllImport("user32.dll")]
         private static extern short GetAsyncKeyState(int virtualKeyCode);
         private const int VK_LBUTTON = 1;
    
         private async void ComboBox_DropDownOpened(object sender, EventArgs e)
         {
             var combo = (System.Windows.Controls.ComboBox)sender;
             if (Mouse.Captured == combo)
             {
                 while (combo.IsDropDownOpen && Mouse.Captured == combo)
                 {
                     if (Mouse.LeftButton == MouseButtonState.Released)
                     {
                         combo.LostMouseCapture += Combo_LostMouseCapture;
                         return;
                     }
                     await Task.Delay(1);
                 }
             }
         }
    
         private void Combo_LostMouseCapture(object sender, MouseEventArgs e)
         {
             var combo = (ComboBox)sender;
             combo.LostMouseCapture -= Combo_LostMouseCapture;
    
             if (Mouse.Captured != null)
             {
                 return; //Not Supported
             }
    
             var esckey = (GetAsyncKeyState(System.Windows.Input.KeyInterop.VirtualKeyFromKey(Key.Escape)) & 0x8000) == 0x8000;
             var lbutton = (GetAsyncKeyState(VK_LBUTTON) & 0x8000) == 0x8000;
    
             if (esckey || !lbutton)
             {
                 return;
             }
    
             var d = e.MouseDevice.Target as DependencyObject;
             while (d != null)
             {
                 var comboItem = d as ComboBoxItem;
                 if (comboItem != null)
                 {
                     var index = combo.ItemContainerGenerator.IndexFromContainer(comboItem);
                     if (index >= 0)
                     {
                         if (comboItem.IsEnabled && comboItem.IsHitTestVisible && comboItem.IsMouseOver)
                         {
                             if (combo.SelectedIndex != index)
                             {
                                 combo.SelectedIndex = index;
    
                             }
                             return;
                         }
                     }
                 }
                 d = VisualTreeHelper.GetParent(d);
             }
    
         }
     }
 }