text: Title: Virtualized table error when navigating past the lower edge of the viewport
  Body: Issue first described in https://github.com/plotly/dash-table/issues/447.
  
  The bug seems fixed in `dash==1.0.2` but that can only have happened by random luck.
  
  There's no actual fix to be done here, just adding a test to make sure navigating with the keyboard past the edge of the viewport (top or bottom) does not trigger a console error / behaves correctly.