Scroller disables its TranslateComposer if it doesn't need to scroll.

If there is nothing to scroll, we should disable the TranslateComposer so that it no longer claims pointers or prevents default, and the normal behavior occurs. 

There are a few use cases where this is desirable:

- Dragging over a digit list that doesn't need to scroll should bubble up to cause the window to scroll.
- A mouse wheel event while over a list that doesn't need a scroller should bubble up to cause the window to scroll.