The overlayer is absolutely positioned to exactly fill the scroll-view.
If we can retain this strategy and never give the input a position that
exceeds the bounds of the overlayer, we can guarantee that it never
forces the scroll position of the scroll view to change when it is
focused due to the browsers default behavior.
The space-pen view is now a simple wrapper around the entire React
component to integrate it cleanly into our existing system. React
components can't adopt existing DOM nodes, otherwise I would just have
the react component take over the entire view instead of wrapping.
