Scrollable visual scrollbar bug on dragging

# Bug
## Affected Components
- Scrollable

## Description
The scrollbar of the `Scrollable` component doesn't stay in the *activated* visual state when dragging it while also moving the mouse outside the scrollbar element area.
![scrollable_scrollbar_bug](https://user-images.githubusercontent.com/9574743/40521336-5b47af70-5fcb-11e8-8dc7-a6e37f497a4b.gif)
