Question

I can not move the border control while the mouse is outside the border cotnrol

<Window
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication2"
  x:Class="WpfApplication2.MainWindow"
  x:Name="Window"
  Title="Window1"
  Width="346.5" Height="215" WindowStyle="SingleBorderWindow" >

    <Grid Name="stack" >

        <Border x:Name="btn" Width="50" Height="20" VerticalAlignment="Top" BorderThickness="1" BorderBrush="Black" Background="#FFF50000"
                MouseMove="btn_MouseMove" MouseDown="btn_MouseDown" MouseUp="btn_MouseUp"  />
    </Grid>
</Window> 

Code behind

bool state = false;
Point prePoint;

private void btn_MouseMove(object sender, MouseEventArgs e)
{
    if (state)
    {
        Point p = e.GetPosition(this);
        Point p2 = e.GetPosition(btn);
        btn.Margin = new Thickness(0, p.Y - p2.Y + p.Y - prePoint.Y, 0, 0);
        prePoint = e.GetPosition(this);
    }
}
private void btn_MouseDown(object sender, MouseButtonEventArgs e)
{
    if (sender == btn)
    {
        prePoint = e.GetPosition(this);
        state = true;
    }
}

private void btn_MouseUp(object sender, MouseButtonEventArgs e)
{
    state = false;
}

Answer 1

Just Capture the mouse when it is inside the border and release it after task is completed, else it will stick. OR acc. to your code it might be like this ->

bool state = false;
Point prePoint;

private void btn_MouseMove(object sender, MouseEventArgs e)
{
if (state)
{
    Point p = e.GetPosition(this);
    Point p2 = e.GetPosition(btn);
    btn.Margin = new Thickness(0, p.Y - p2.Y + p.Y - prePoint.Y, 0, 0);
    prePoint = e.GetPosition(this);
    // Capture Mouse here ! as far as i think. !!!
    Mouse.Capture(this.ColorPlane, CaptureMode.Element);
}
else
{
    // Release Mouse here ! as far as i think. !!!
    Mouse.Capture(null);
}
}