流文档阅读器滚动条在滚动查看模式下的可见性

I need to show FlowDocument with font size slider and search button. FlowDocumentReader in Scroll viewing mode does exactly what I want, except that it always shows vertical scroll bar, whereas I need to show it only when it will be enabled (internally ReaderScrollViewer's VerticalScrollBarVisibility is Visible, but I need Auto). How can I achieve such view?I need to show FlowDocument with font size slid