这里有一个解决方法，您可以在下拉框关闭之前保存文本框的值，并在下拉框关闭之后设置它。它使用三个事件LosingFocus、SelectionChanged和DropDownClosed。

<ComboBox x:Name="ComboTest" Height="80" Width="296" ItemsSource="{x:Bind items}" 
                  IsEditable="True" LosingFocus="ComboTest_LosingFocus" SelectionChanged="ComboTest_SelectionChanged" DropDownClosed="ComboTest_DropDownClosed"/>

public sealed partial class MainPage : Page
{
    ObservableCollection<string> items = new ObservableCollection<string>();

    private string comboText = "";
    public MainPage()
    {
        this.InitializeComponent();

        items.Add("A");
        items.Add("B");
        items.Add("C");
        items.Add("D");
    }

    private void ComboTest_LosingFocus(UIElement sender, LosingFocusEventArgs args)
    {
        ComboBox comboBox = sender as ComboBox;
        comboText = comboBox.Text;
    }

    private void ComboTest_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        ComboBox comboBox = sender as ComboBox;
        comboText = comboBox.SelectedValue.ToString();
    }

    private void ComboTest_DropDownClosed(object sender, object e)
    {
        ComboBox comboBox = sender as ComboBox;
        comboBox.Text = comboText;
    }
}