The scrollbar is always visible. The view size is independent of the content.
The scrollbar will appear and disappear as necessary. For example, when all of a Gtk.TreeView can not be seen.
Don't show a scrollbar, but don't force the size to follow the content. This can be used e.g. to make multiple scrolled windows share a scrollbar.
The scrollbar should never appear. In this mode the content determines the size.
Determines how the size should be computed to achieve the one of the visibility mode for the scrollbars.