WPF Popup Keypad spike

Window1.xaml

<Window x:Class="WpfApplication1.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Popup Tester" Height="300" Width="300">

    <Window.Resources>
        <Style TargetType="{x:Type TextBox}">
            <Setter Property="Margin" Value="10" />
            <EventSetter Event="GotKeyboardFocus" Handler="run_MouseEnter" />
        </Style>
        <Style x:Key="KeypadBtn" TargetType="{x:Type Button}">
            <EventSetter Event="Click" Handler="key_Click" />
        </Style>
    </Window.Resources>

    <StackPanel>
        <TextBox />
        <TextBox />
        <TextBox />
        <Popup Name="popLink" StaysOpen="True" Placement="Bottom"
                PopupAnimation="Slide" AllowsTransparency="True">
            <Border BorderBrush="Beige" BorderThickness="2" Background="White">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="50"  />
                        <ColumnDefinition Width="50" />
                        <ColumnDefinition Width="50" />
                        <ColumnDefinition Width="50" />
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="50" />
                        <RowDefinition Height="50" />
                        <RowDefinition Height="50" />
                        <RowDefinition Height="50" />
                    </Grid.RowDefinitions>
                    <Button Content="7" Style="{StaticResource KeypadBtn}" Grid.Row="0" Grid.Column="0" />
                    <Button Content="8" Style="{StaticResource KeypadBtn}" Grid.Row="0" Grid.Column="1" />
                    <Button Content="9" Style="{StaticResource KeypadBtn}" Grid.Row="0" Grid.Column="2" />
                    <Button Content="X" Style="{StaticResource KeypadBtn}" Grid.Row="0" Grid.Column="3" Grid.RowSpan="2" />
                    <Button Content="4" Style="{StaticResource KeypadBtn}" Grid.Row="1" Grid.Column="0" />
                    <Button Content="5" Style="{StaticResource KeypadBtn}" Grid.Row="1" Grid.Column="1" />
                    <Button Content="6" Style="{StaticResource KeypadBtn}" Grid.Row="1" Grid.Column="2" />
                    <Button Content="1" Style="{StaticResource KeypadBtn}" Grid.Row="2" Grid.Column="0" />
                    <Button Content="2" Style="{StaticResource KeypadBtn}" Grid.Row="2" Grid.Column="1" />
                    <Button Content="3" Style="{StaticResource KeypadBtn}" Grid.Row="2" Grid.Column="2" />
                    <Button Content="Save" Style="{StaticResource KeypadBtn}" Grid.Row="2" Grid.Column="3" Grid.RowSpan="2" />
                    <Button Content="0" Style="{StaticResource KeypadBtn}" Grid.Row="3" Grid.Column="0" Grid.ColumnSpan="2" />
                    <Button Content="." Style="{StaticResource KeypadBtn}" Grid.Row="3" Grid.Column="2" />
                </Grid>
            </Border>
        </Popup>
    </StackPanel>

</Window>

Window1.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Diagnostics;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }

        private TextBox targetTextbox;

        private void run_MouseEnter(object sender, KeyboardEventArgs e)
        {
            popLink.IsOpen = false;

            popLink.PlacementTarget = (UIElement)e.Source;
            popLink.Placement = System.Windows.Controls.Primitives.PlacementMode.Bottom;
            popLink.IsOpen = true;
            targetTextbox = sender as TextBox;
        }
        private void lnk_Click(object sender, RoutedEventArgs e)
        {
            popLink.IsOpen = false;
            targetTextbox = null;
        }

        private void key_Click(object sender, RoutedEventArgs e)
        {
            Button btn = (Button)e.OriginalSource;
            string s = btn.Content.ToString();
            if (s == "X" || s == "Save")
            {
                popLink.IsOpen = false;
                targetTextbox = null;
            }
            else
            {
                targetTextbox.Text += s;
            }
        }
    }
}