I have just switched to Mac from Windows and am in the process of configuring Scrivener. I like to use the Outliner to keep track of word counts but when I was adding columns to it, the Title column was too wide, such that it pushed the other columns out of view. Yet I could not activate a horizontal scroll bar in order to adjust the width. It looks like it is there for just a split second and then disappears (both horizontal and vertical scroll bars disappear), and no amount of hovering or clicking get it back.
Rather than hovering or clicking, when the scroll bars are hidden, macOS responds to a scrolling motion (up or down or left or right), showing the scroll bars only if there is anything off the screen that can be scrolled to.
The default setting can be changed to always show the scroll bars:
Apple Menu > System Preferences > General > Show scroll bars > Always