Drag window without title bar by mouse left button down.

WindowHelper.cs

public static class WindowHelper
{
    public static bool GetIsDragEnabled(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsDragEnabledProperty);
    }
    public static void SetIsDragEnabled(DependencyObject obj, bool value)
    {
        obj.SetValue(IsDragEnabledProperty, value);
    }
    public static readonly DependencyProperty IsDragEnabledProperty =
        DependencyProperty.RegisterAttached("IsDragEnabled", typeof(bool), typeof(WindowHelper), new PropertyMetadata(false, OnChanged));

    private static void OnChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var window = d as Window ?? Window.GetWindow(d);
        if (window is null)
            return;

        if ((bool)e.NewValue)
        {
            window.PreviewMouseLeftButtonDown += OnPreviewMouseLeftButtonDown;
            window.Closed += OnClosed;
        }
        else
        {
            window.PreviewMouseLeftButtonDown -= OnPreviewMouseLeftButtonDown;
            window.Closed -= OnClosed;
        }

        static void OnPreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            // Make sure left button is pressed.
            if (Mouse.LeftButton == MouseButtonState.Pressed)
                ((Window)sender).DragMove();
        }

        static void OnClosed(object? sender, EventArgs e)
        {
            if (sender is Window window)
            {
                window.PreviewMouseLeftButtonDown -= OnPreviewMouseLeftButtonDown;
                window.Closed -= OnClosed;
            }
        }
    }
}