The visibility of the `ScrollBar` was used as a condition for judging whether it should scroll.
**Visible** and **scrollable** are currently not the same in some cases (`SCROLL_MODE_SHOW_NEVER`).
This makes `SCROLL_MODE_SHOW_NEVER` not working properly.
This patch will make `SCROLL_MODE_SHOW_NEVER` available, so that the external `SrollBar` can
share the internal `ScrollBar`'s data, we can achieve the purpose of replacing the internal
`ScrollBar` with a custom external one.