HadrionClifton discovered this in a previous post, but that post has a title set before they realized what the cause was, so it's a bit misleading.
When UIScaling > 100%, then the window can get bigger than the screen, and so there's no scroll bar when it would really have been needed. See https://imgur.com/a/cgWAjFW for a screenshot, with UIScaling at 150%.
