updateSelectionBox = function(evt) {
    var data = $(this).data(pluginName);
    data.selectionBoxExtents = {
      // pageX, pageY positions are relative to the document, so they need
      // to be converted to the selection container reference frame.
      'top':
          Math.min(data.startPosition.pageY, evt.pageY) -
          data.containerDimensions.top +
          this.scrollTop,
      'left':
          Math.min(data.startPosition.pageX, evt.pageX) -
          data.containerDimensions.left +
          this.scrollLeft,
      'height': Math.abs(data.startPosition.pageY - evt.pageY),
      'width': Math.abs(data.startPosition.pageX - evt.pageX)
    };
    data.selectionBox.css(data.selectionBoxExtents);
  }
Updates the selection box position and sizing to match the distance
travelled from the position where the selection gesture started and
the current mouse position.