private void TvTest_ContextMenuOpening(object sender, TreeViewContextMenuOpeningEventArgs e)
 {
     (e.Menu.DropDown as RadDropDownMenu).DropDownOpened -= Form1_DropDownOpened;
     (e.Menu.DropDown as RadDropDownMenu).DropDownOpened += Form1_DropDownOpened;
 }

 private void Form1_DropDownOpened(object sender, EventArgs e)
 {
     var dropdownmenu = (sender as RadDropDownMenu);
     (dropdownmenu as RadContextMenuDropDown).RootElement.InvalidateMeasure(true);
     (dropdownmenu as RadContextMenuDropDown).RootElement.UpdateLayout();
     dropdownmenu.Size = (dropdownmenu as RadContextMenuDropDown).RootElement.DesiredSize.ToSize();
 }