在TextBox焦点上打开WPF弹出窗口

<Window x:Class="WpfApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="WPF弹出窗口示例" Height="350" Width="500">
    <Grid>
        <TextBox x:Name="textBox" HorizontalAlignment="Left" VerticalAlignment="Top" Width="200" Margin="10"/>
        <Button Content="打开弹出窗口" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="10,30,0,0" Click="Button_Click"/>
    </Grid>
</Window>

using System.Windows;

namespace WpfApp
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            PopupWindow popupWindow = new PopupWindow();
            popupWindow.Owner = this;
            popupWindow.ShowDialog();
        }
    }
}

<Window x:Class="WpfApp.PopupWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="弹出窗口" Height="200" Width="300">
    <Grid>
        <TextBlock Text="这是一个弹出窗口" HorizontalAlignment="Center" VerticalAlignment="Center"/>
    </Grid>
</Window>

using System.Windows;

namespace WpfApp
{
    public partial class PopupWindow : Window
    {
        public PopupWindow()
        {
            InitializeComponent();
        }
    }
}