MainWindow.xaml

<Window x:Class="TestDragWindow.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:TestDragWindow"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800"
        WindowStyle="None" ResizeMode="NoResize" Topmost="True">

    <WindowChrome.WindowChrome>
        <WindowChrome GlassFrameThickness="-1" CaptionHeight="0" />
    </WindowChrome.WindowChrome>

    <Window.Background>
        <SolidColorBrush Color="Black" Opacity="0" />
    </Window.Background>
    
    <Grid>
        <Border BorderThickness="3" BorderBrush="Red" />
        <Button Content="X" Width="80" Height="80"  />
    </Grid>
</Window>