function getScrollableDistances() {
      return [
        el.scrollTop,
        el.scrollWidth - el.scrollLeft - containerDimensions.width,
        el.scrollHeight - el.scrollTop - containerDimensions.height,
        el.scrollLeft
      ];
    }
Returns the available distance the selection container viewport can
still be scrolled before reaching the selection container margins.

@return {!Array.<number>} Available scrolling distances, respectively
    from the following borders: top, right, bottom, left.