WPF - Remove focus when clicking outside of a textbox

To avoid code behind you can use this Behavior The behavior

 public class ClearFocusOnClickBehavior : Behavior<FrameworkElement>
 {
    protected override void OnAttached()
    {
        AssociatedObject.MouseDown += AssociatedObject_MouseDown;
        base.OnAttached();
    }

    private static void AssociatedObject_MouseDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
    {
        Keyboard.ClearFocus();
    }

    protected override void OnDetaching()
    {
        AssociatedObject.MouseDown -= AssociatedObject_MouseDown;
    }
}

Useing in XAML:

On any element outside the text box that you want him to clear the focus on click add:

    <i:Interaction.Behaviors>
        <behaviors:ClearFocusOnClickBehavior/>
    </i:Interaction.Behaviors>

Rather than adding new control to window, I think you should give your Grid a name and react to the MouseDown event on your window, moving the focus to the Grid itself. Something like this:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Title="Window1" 
    Height="412" Width="569" 
    MouseDown="Window_MouseDown" 
    Name="window1">

    <Grid ShowGridLines="False" 
          Background="#01FFFFFF"
          KeyDown="Grid_KeyDown" 
          Name="grid1" 
          Focusable="True">

          <TextBox Width="120" Margin="117,61,0,0" 
                   Name="textBox1" 
                   VerticalAlignment="Top" 
                   HorizontalAlignment="Left"/>
    </Grid>
</Window>

code behind:

private void window1_MouseDown(object sender, MouseButtonEventArgs e)
{
    grid1.Focus();
}

I think, better way to solve this problem is adding MouseDown event handler to window with code behind:

private void window_MouseDown(object sender, MouseButtonEventArgs e)
{
    Keyboard.ClearFocus();
}

Another way that worked for me was using

Mouse.AddPreviewMouseDownOutsideCapturedElementHandler

For example, say you had a TextBlock that when clicked, should become editable by showing a focused TextBox. Then when the user clicked outside the TextBox, it should be hidden again. Here's how you can do it:

private void YourTextBlock_OnMouseDown(object sender, MouseButtonEventArgs e)
{
    YourTextBox.Visibility = Visibility.Visible;
    YourTextBox.Focus();
    CaptureMouse();
    Mouse.AddPreviewMouseDownOutsideCapturedElementHandler(this, OnMouseDownOutsideElement);
}

private void OnMouseDownOutsideElement(object sender, MouseButtonEventArgs e)
{
    Mouse.RemovePreviewMouseDownOutsideCapturedElementHandler(this, OnMouseDownOutsideElement);
    ReleaseMouseCapture();
    YourTextBox.Visibility = Visibility.Hidden;
}