Here is a workaround for WPF. Setting the HitTestable attached property to True for the Header of RadWindow will prevent this issue from happening:

private void RadWindow_Loaded(object sender, RoutedEventArgs e)
{
    var window = sender as RadWindow;
    window.ResizeBorder = new Thickness(0);
    window.CaptionHeight = 0;
 
    var gridHeader = (sender as RadWindow).Template.FindName("Header", sender as RadWindow) as Grid;
    WindowHost.SetHitTestable(gridHeader, true);
}