This PR fixes scrollbar auto-show feature when there're not visible
cursors. The intial behavior was broken in #11147.

The problem is that `selections` only contains visible selections, so
the `if` with `non_visible_cursors |= true` is only visited in rare edge
cases when we have at least part of the selection still visible on the
screen. But when we scroll far enough from the cursor,
`non_visible_cursors` keeps its default `false` value, which is
incorrect.

Release Notes:

- N/A