ShowDialog() can be bypassed?

Question

I accidentally found this while working with my project, but I was able to reproduce this in a base project:

I open a MainWindow, then with ShowDialog a child window, Owner set to the Main. When I click on the Main, the Child starts to flash, then I start clicking between the Main and the Child very fast, and sometimes the Main gets the focus, then I able to open another Child window.

MainWindow:

<Window x:Class="WpfShowDialog.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="200" Width="300" WindowStartupLocation="CenterScreen" >
    <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center" >
        <TextBlock Text="MainWindow" Margin="10" />
        <Button Content="Open child" Margin="10" Click="Button_Click" />
    </StackPanel>
</Window>

Code behind:

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

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        var childWindow = new ChildWindow()
        {
            Owner = this
        };
        childWindow.ShowDialog();
    }
}

ChildWindow:

<Window x:Class="WpfShowDialog.ChildWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="ChildWindow" Height="150" Width="200" WindowStartupLocation="CenterOwner">
    <Grid>
        <TextBlock Text="ChildWindow" HorizontalAlignment="Center" VerticalAlignment="Center" />
    </Grid>
</Window>

Code behind:

public partial class ChildWindow : Window
{
    public ChildWindow()
    {
        InitializeComponent();
    }
}

I click on between these red circles:

Is this some kind of bug, or I miss something?