this.mcb.SizeChanged += OnMcbSizeChanged;

private void OnMcbSizeChanged(object sender, SizeChangedEventArgs e)
{
    if (!e.HeightChanged)
        return;

    var dropDownButton = this.mcb.Template?.FindName("PART_DropDownButton", this.mcb) as RadDropDownButton;
    if (dropDownButton == null || !dropDownButton.IsOpen)
        return;

    dropDownButton.IsOpen = false;
    dropDownButton.IsOpen = true;
}