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[DllImport("user32.dll")]
public static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

protected override void OnSourceInitialized(EventArgs e)
{
    base.OnSourceInitialized(e);
    var helper = new WindowInteropHelper(this);
    SetWindowLong(helper.Handle, -20, 0x80);
}