Make sure the UIScrollView's content size is set up correctly. Here are some useful parameters.
scrollVIew.contentInset
scrollVIew.scrollIndicatorInsets
scrollView.contentSize
When the content view is taller than the scroll view, the scroll view enables vertical scrolling. When the content view is wider than the scroll view, the scroll view enables horizontal scrolling. Otherwise, scrolling is disabled by default. You must set your content view size dynamically so when they change the font, the content view gets wider than the scroll view width. You could wrap your stack view in another UIViewController and treat it as a content view.