function vertical (scrollEvent) {
      self.isScrollingVertically = false;
      self.api.core.raise.scrollEnd(scrollEvent);
      self.scrollDirection = uiGridConstants.scrollDirection.NONE;
    }
@ngdoc property
@name scrollDirection
@propertyOf ui.grid.class:Grid
@description set one of the {@link ui.grid.service:uiGridConstants#properties_scrollDirection uiGridConstants.scrollDirection}
 values (UP, DOWN, LEFT, RIGHT, NONE), which tells us which direction we are scrolling.
Set to NONE via debounced method