Warn when scroll view gets accidentally scrolled and fix it

I want this code to go away once we track down the causes of any
unwanted autoscrolling by the browser
Position hidden input at 0,0 unless cursor is focused

The editor's scroll view is getting autoscrolled when the editor is
focused, so we won't position the hidden input until after the editor
is focused, and will always return it to 0,0 when the editor is blurred.
Prevent autoscrolling scroll-view when cursor is at end of longest line

If the longest line is longer than the width of the scroll view, we need
to allow 2px horizontally to prevent the hidden input from
autoscrolling.
Merge pull request #2370 from atom/ns-react-fix-scroll-corruption

Prevent React editor's scroll view from being auto-scrolled by Chromium